Merge pull request #5 from eugenp/master

merge

.travis.yml

@@ -1,24 +0,0 @@
-language: java
-
-before_install: 
-  - echo "MAVEN_OPTS='-Xmx2048M -Xss128M -XX:+CMSClassUnloadingEnabled -XX:+UseG1GC -XX:-UseGCOverheadLimit'" > ~/.mavenrc
-
-install: skip
-script: travis_wait 60 mvn -q install -Pdefault-first,default-second -Dgib.enabled=true
-
-sudo: required
-
-jdk:
-  - oraclejdk8
-
-addons:
-  apt:
-    packages:
-      - oracle-java8-installer
-
-cache:
-  directories:
-  - .autoconf
-  - $HOME/.m2
-  
-

algorithms-miscellaneous-3/README.md

@@ -7,3 +7,4 @@
 - [Checking If a List Is Sorted in Java](https://www.baeldung.com/java-check-if-list-sorted)
 - [Checking if a Java Graph has a Cycle](https://www.baeldung.com/java-graph-has-a-cycle)
 - [A Guide to the Folding Technique in Java](https://www.baeldung.com/folding-hashing-technique)
+- [Creating a Triangle with for Loops in Java](https://www.baeldung.com/java-print-triangle)

algorithms-miscellaneous-3/pom.xml

@@ -31,6 +31,17 @@
             <version>${guava.version}</version>
         </dependency>
 
+        <dependency>
+            <groupId>com.squareup.retrofit2</groupId>
+            <artifactId>retrofit</artifactId>
+            <version>${retrofit.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.squareup.retrofit2</groupId>
+            <artifactId>converter-jackson</artifactId>
+            <version>${retrofit.version}</version>
+        </dependency>
+
         <dependency>
             <groupId>org.apache.commons</groupId>
             <artifactId>commons-lang3</artifactId>
@@ -61,5 +72,6 @@
         <org.assertj.core.version>3.9.0</org.assertj.core.version>
         <commons-collections4.version>4.3</commons-collections4.version>
         <guava.version>28.0-jre</guava.version>
+        <retrofit.version>2.6.0</retrofit.version>
     </properties>
 </project>

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/interpolationsearch/InterpolationSearch.java

@@ -0,0 +1,35 @@
+package com.baeldung.algorithms.interpolationsearch;
+
+public class InterpolationSearch {
+
+    public static int interpolationSearch(int[] data, int item) {
+
+        int highEnd = (data.length - 1);
+        int lowEnd = 0;
+
+        while (item >= data[lowEnd] && item <= data[highEnd] && lowEnd <= highEnd) {
+
+            int probe = lowEnd + (highEnd - lowEnd) * (item - data[lowEnd]) / (data[highEnd] - data[lowEnd]);
+
+            if (highEnd == lowEnd) {
+                if (data[lowEnd] == item) {
+                    return lowEnd;
+                } else {
+                    return -1;
+                }
+            }
+
+            if (data[probe] == item) {
+                return probe;
+            }
+
+            if (data[probe] < item) {
+                lowEnd = probe + 1;
+            } else {
+                highEnd = probe - 1;
+            }
+        }
+        return -1;
+    }
+
+}

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/kmeans/Centroid.java

@@ -0,0 +1,45 @@
+package com.baeldung.algorithms.kmeans;
+
+import java.util.Map;
+import java.util.Objects;
+
+/**
+ * Encapsulates all coordinates for a particular cluster centroid.
+ */
+public class Centroid {
+
+    /**
+     * The centroid coordinates.
+     */
+    private final Map<String, Double> coordinates;
+
+    public Centroid(Map<String, Double> coordinates) {
+        this.coordinates = coordinates;
+    }
+
+    public Map<String, Double> getCoordinates() {
+        return coordinates;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+        Centroid centroid = (Centroid) o;
+        return Objects.equals(getCoordinates(), centroid.getCoordinates());
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(getCoordinates());
+    }
+
+    @Override
+    public String toString() {
+        return "Centroid " + coordinates;
+    }
+}

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/kmeans/Distance.java

@@ -0,0 +1,20 @@
+package com.baeldung.algorithms.kmeans;
+
+import java.util.Map;
+
+/**
+ * Defines a contract to calculate distance between two feature vectors. The less the
+ * calculated distance, the more two items are similar to each other.
+ */
+public interface Distance {
+
+    /**
+     * Calculates the distance between two feature vectors.
+     *
+     * @param f1 The first set of features.
+     * @param f2 The second set of features.
+     * @return Calculated distance.
+     * @throws IllegalArgumentException If the given feature vectors are invalid.
+     */
+    double calculate(Map<String, Double> f1, Map<String, Double> f2);
+}

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/kmeans/Errors.java

@@ -0,0 +1,23 @@
+package com.baeldung.algorithms.kmeans;
+
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Encapsulates methods to calculates errors between centroid and the cluster members.
+ */
+public class Errors {
+
+    public static double sse(Map<Centroid, List<Record>> clustered, Distance distance) {
+        double sum = 0;
+        for (Map.Entry<Centroid, List<Record>> entry : clustered.entrySet()) {
+            Centroid centroid = entry.getKey();
+            for (Record record : entry.getValue()) {
+                double d = distance.calculate(centroid.getCoordinates(), record.getFeatures());
+                sum += Math.pow(d, 2);
+            }
+        }
+
+        return sum;
+    }
+}

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/kmeans/EuclideanDistance.java

@@ -0,0 +1,26 @@
+package com.baeldung.algorithms.kmeans;
+
+import java.util.Map;
+
+/**
+ * Calculates the distance between two items using the Euclidean formula.
+ */
+public class EuclideanDistance implements Distance {
+
+    @Override
+    public double calculate(Map<String, Double> f1, Map<String, Double> f2) {
+        if (f1 == null || f2 == null) {
+            throw new IllegalArgumentException("Feature vectors can't be null");
+        }
+
+        double sum = 0;
+        for (String key : f1.keySet()) {
+            Double v1 = f1.get(key);
+            Double v2 = f2.get(key);
+
+            if (v1 != null && v2 != null) sum += Math.pow(v1 - v2, 2);
+        }
+
+        return Math.sqrt(sum);
+    }
+}

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/kmeans/KMeans.java

@@ -0,0 +1,236 @@
+package com.baeldung.algorithms.kmeans;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Random;
+import java.util.Set;
+
+import static java.util.stream.Collectors.toList;
+import static java.util.stream.Collectors.toSet;
+
+/**
+ * Encapsulates an implementation of KMeans clustering algorithm.
+ *
+ * @author Ali Dehghani
+ */
+public class KMeans {
+
+    private KMeans() {
+        throw new IllegalAccessError("You shouldn't call this constructor");
+    }
+
+    /**
+     * Will be used to generate random numbers.
+     */
+    private static final Random random = new Random();
+
+    /**
+     * Performs the K-Means clustering algorithm on the given dataset.
+     *
+     * @param records       The dataset.
+     * @param k             Number of Clusters.
+     * @param distance      To calculate the distance between two items.
+     * @param maxIterations Upper bound for the number of iterations.
+     * @return K clusters along with their features.
+     */
+    public static Map<Centroid, List<Record>> fit(List<Record> records, int k, Distance distance, int maxIterations) {
+        applyPreconditions(records, k, distance, maxIterations);
+
+        List<Centroid> centroids = randomCentroids(records, k);
+        Map<Centroid, List<Record>> clusters = new HashMap<>();
+        Map<Centroid, List<Record>> lastState = new HashMap<>();
+
+        // iterate for a pre-defined number of times
+        for (int i = 0; i < maxIterations; i++) {
+            boolean isLastIteration = i == maxIterations - 1;
+
+            // in each iteration we should find the nearest centroid for each record
+            for (Record record : records) {
+                Centroid centroid = nearestCentroid(record, centroids, distance);
+                assignToCluster(clusters, record, centroid);
+            }
+
+            // if the assignment does not change, then the algorithm terminates
+            boolean shouldTerminate = isLastIteration || clusters.equals(lastState);
+            lastState = clusters;
+            if (shouldTerminate) {
+                break;
+            }
+
+            // at the end of each iteration we should relocate the centroids
+            centroids = relocateCentroids(clusters);
+            clusters = new HashMap<>();
+        }
+
+        return lastState;
+    }
+
+    /**
+     * Move all cluster centroids to the average of all assigned features.
+     *
+     * @param clusters The current cluster configuration.
+     * @return Collection of new and relocated centroids.
+     */
+    private static List<Centroid> relocateCentroids(Map<Centroid, List<Record>> clusters) {
+        return clusters
+          .entrySet()
+          .stream()
+          .map(e -> average(e.getKey(), e.getValue()))
+          .collect(toList());
+    }
+
+    /**
+     * Moves the given centroid to the average position of all assigned features. If
+     * the centroid has no feature in its cluster, then there would be no need for a
+     * relocation. Otherwise, for each entry we calculate the average of all records
+     * first by summing all the entries and then dividing the final summation value by
+     * the number of records.
+     *
+     * @param centroid The centroid to move.
+     * @param records  The assigned features.
+     * @return The moved centroid.
+     */
+    private static Centroid average(Centroid centroid, List<Record> records) {
+        // if this cluster is empty, then we shouldn't move the centroid
+        if (records == null || records.isEmpty()) {
+            return centroid;
+        }
+
+        // Since some records don't have all possible attributes, we initialize
+        // average coordinates equal to current centroid coordinates
+        Map<String, Double> average = centroid.getCoordinates();
+
+        // The average function works correctly if we clear all coordinates corresponding
+        // to present record attributes
+        records
+          .stream()
+          .flatMap(e -> e
+            .getFeatures()
+            .keySet()
+            .stream())
+          .forEach(k -> average.put(k, 0.0));
+
+        for (Record record : records) {
+            record
+              .getFeatures()
+              .forEach((k, v) -> average.compute(k, (k1, currentValue) -> v + currentValue));
+        }
+
+        average.forEach((k, v) -> average.put(k, v / records.size()));
+
+        return new Centroid(average);
+    }
+
+    /**
+     * Assigns a feature vector to the given centroid. If this is the first assignment for this centroid,
+     * first we should create the list.
+     *
+     * @param clusters The current cluster configuration.
+     * @param record   The feature vector.
+     * @param centroid The centroid.
+     */
+    private static void assignToCluster(Map<Centroid, List<Record>> clusters, Record record, Centroid centroid) {
+        clusters.compute(centroid, (key, list) -> {
+            if (list == null) {
+                list = new ArrayList<>();
+            }
+
+            list.add(record);
+            return list;
+        });
+    }
+
+    /**
+     * With the help of the given distance calculator, iterates through centroids and finds the
+     * nearest one to the given record.
+     *
+     * @param record    The feature vector to find a centroid for.
+     * @param centroids Collection of all centroids.
+     * @param distance  To calculate the distance between two items.
+     * @return The nearest centroid to the given feature vector.
+     */
+    private static Centroid nearestCentroid(Record record, List<Centroid> centroids, Distance distance) {
+        double minimumDistance = Double.MAX_VALUE;
+        Centroid nearest = null;
+
+        for (Centroid centroid : centroids) {
+            double currentDistance = distance.calculate(record.getFeatures(), centroid.getCoordinates());
+
+            if (currentDistance < minimumDistance) {
+                minimumDistance = currentDistance;
+                nearest = centroid;
+            }
+        }
+
+        return nearest;
+    }
+
+    /**
+     * Generates k random centroids. Before kicking-off the centroid generation process,
+     * first we calculate the possible value range for each attribute. Then when
+     * we're going to generate the centroids, we generate random coordinates in
+     * the [min, max] range for each attribute.
+     *
+     * @param records The dataset which helps to calculate the [min, max] range for
+     *                each attribute.
+     * @param k       Number of clusters.
+     * @return Collections of randomly generated centroids.
+     */
+    private static List<Centroid> randomCentroids(List<Record> records, int k) {
+        List<Centroid> centroids = new ArrayList<>();
+        Map<String, Double> maxs = new HashMap<>();
+        Map<String, Double> mins = new HashMap<>();
+
+        for (Record record : records) {
+            record
+              .getFeatures()
+              .forEach((key, value) -> {
+                  // compares the value with the current max and choose the bigger value between them
+                  maxs.compute(key, (k1, max) -> max == null || value > max ? value : max);
+
+                  // compare the value with the current min and choose the smaller value between them
+                  mins.compute(key, (k1, min) -> min == null || value < min ? value : min);
+              });
+        }
+
+        Set<String> attributes = records
+          .stream()
+          .flatMap(e -> e
+            .getFeatures()
+            .keySet()
+            .stream())
+          .collect(toSet());
+        for (int i = 0; i < k; i++) {
+            Map<String, Double> coordinates = new HashMap<>();
+            for (String attribute : attributes) {
+                double max = maxs.get(attribute);
+                double min = mins.get(attribute);
+                coordinates.put(attribute, random.nextDouble() * (max - min) + min);
+            }
+
+            centroids.add(new Centroid(coordinates));
+        }
+
+        return centroids;
+    }
+
+    private static void applyPreconditions(List<Record> records, int k, Distance distance, int maxIterations) {
+        if (records == null || records.isEmpty()) {
+            throw new IllegalArgumentException("The dataset can't be empty");
+        }
+
+        if (k <= 1) {
+            throw new IllegalArgumentException("It doesn't make sense to have less than or equal to 1 cluster");
+        }
+
+        if (distance == null) {
+            throw new IllegalArgumentException("The distance calculator is required");
+        }
+
+        if (maxIterations <= 0) {
+            throw new IllegalArgumentException("Max iterations should be a positive number");
+        }
+    }
+}

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/kmeans/LastFm.java

@@ -0,0 +1,144 @@
+package com.baeldung.algorithms.kmeans;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import okhttp3.OkHttpClient;
+import retrofit2.Retrofit;
+import retrofit2.converter.jackson.JacksonConverterFactory;
+
+import static java.util.stream.Collectors.toSet;
+
+public class LastFm {
+
+    private static OkHttpClient okHttp = new OkHttpClient.Builder()
+      .addInterceptor(new LastFmService.Authenticator("put your API key here"))
+      .build();
+
+    private static Retrofit retrofit = new Retrofit.Builder()
+      .client(okHttp)
+      .addConverterFactory(JacksonConverterFactory.create())
+      .baseUrl("http://ws.audioscrobbler.com/")
+      .build();
+
+    private static LastFmService lastFm = retrofit.create(LastFmService.class);
+
+    private static ObjectMapper mapper = new ObjectMapper();
+
+    public static void main(String[] args) throws IOException {
+        List<String> artists = getTop100Artists();
+        Set<String> tags = getTop100Tags();
+        List<Record> records = datasetWithTaggedArtists(artists, tags);
+
+        Map<Centroid, List<Record>> clusters = KMeans.fit(records, 7, new EuclideanDistance(), 1000);
+        // Print the cluster configuration
+        clusters.forEach((key, value) -> {
+            System.out.println("------------------------------ CLUSTER -----------------------------------");
+
+            System.out.println(sortedCentroid(key));
+            String members = String.join(", ", value
+              .stream()
+              .map(Record::getDescription)
+              .collect(toSet()));
+            System.out.print(members);
+
+            System.out.println();
+            System.out.println();
+        });
+
+        Map<String, Object> json = convertToD3CompatibleMap(clusters);
+        System.out.println(mapper.writeValueAsString(json));
+    }
+
+    private static Map<String, Object> convertToD3CompatibleMap(Map<Centroid, List<Record>> clusters) {
+        Map<String, Object> json = new HashMap<>();
+        json.put("name", "Musicians");
+        List<Map<String, Object>> children = new ArrayList<>();
+        clusters.forEach((key, value) -> {
+            Map<String, Object> child = new HashMap<>();
+            child.put("name", dominantGenre(sortedCentroid(key)));
+            List<Map<String, String>> nested = new ArrayList<>();
+            for (Record record : value) {
+                nested.add(Collections.singletonMap("name", record.getDescription()));
+            }
+            child.put("children", nested);
+
+            children.add(child);
+        });
+        json.put("children", children);
+        return json;
+    }
+
+    private static String dominantGenre(Centroid centroid) {
+        return centroid
+          .getCoordinates()
+          .keySet()
+          .stream()
+          .limit(2)
+          .collect(Collectors.joining(", "));
+    }
+
+    private static Centroid sortedCentroid(Centroid key) {
+        List<Map.Entry<String, Double>> entries = new ArrayList<>(key
+          .getCoordinates()
+          .entrySet());
+        entries.sort((e1, e2) -> e2
+          .getValue()
+          .compareTo(e1.getValue()));
+
+        Map<String, Double> sorted = new LinkedHashMap<>();
+        for (Map.Entry<String, Double> entry : entries) {
+            sorted.put(entry.getKey(), entry.getValue());
+        }
+
+        return new Centroid(sorted);
+    }
+
+    private static List<Record> datasetWithTaggedArtists(List<String> artists, Set<String> topTags) throws IOException {
+        List<Record> records = new ArrayList<>();
+        for (String artist : artists) {
+            Map<String, Double> tags = lastFm
+              .topTagsFor(artist)
+              .execute()
+              .body()
+              .all();
+
+            // Only keep popular tags.
+            tags
+              .entrySet()
+              .removeIf(e -> !topTags.contains(e.getKey()));
+
+            records.add(new Record(artist, tags));
+        }
+        return records;
+    }
+
+    private static Set<String> getTop100Tags() throws IOException {
+        return lastFm
+          .topTags()
+          .execute()
+          .body()
+          .all();
+    }
+
+    private static List<String> getTop100Artists() throws IOException {
+        List<String> artists = new ArrayList<>();
+        for (int i = 1; i <= 2; i++) {
+            artists.addAll(lastFm
+              .topArtists(i)
+              .execute()
+              .body()
+              .all());
+        }
+
+        return artists;
+    }
+}

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/kmeans/LastFmService.java

@@ -0,0 +1,118 @@
+package com.baeldung.algorithms.kmeans;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import com.fasterxml.jackson.annotation.JsonAutoDetect;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import okhttp3.HttpUrl;
+import okhttp3.Interceptor;
+import okhttp3.Request;
+import okhttp3.Response;
+import retrofit2.Call;
+import retrofit2.http.GET;
+import retrofit2.http.Query;
+
+import static com.fasterxml.jackson.annotation.JsonAutoDetect.Visibility.ANY;
+import static java.util.stream.Collectors.toList;
+
+public interface LastFmService {
+
+    @GET("/2.0/?method=chart.gettopartists&format=json&limit=50")
+    Call<Artists> topArtists(@Query("page") int page);
+
+    @GET("/2.0/?method=artist.gettoptags&format=json&limit=20&autocorrect=1")
+    Call<Tags> topTagsFor(@Query("artist") String artist);
+
+    @GET("/2.0/?method=chart.gettoptags&format=json&limit=100")
+    Call<TopTags> topTags();
+
+    /**
+     * HTTP interceptor to intercept all HTTP requests and add the API key to them.
+     */
+    class Authenticator implements Interceptor {
+
+        private final String apiKey;
+
+        Authenticator(String apiKey) {
+            this.apiKey = apiKey;
+        }
+
+        @Override
+        public Response intercept(Chain chain) throws IOException {
+            HttpUrl url = chain
+              .request()
+              .url()
+              .newBuilder()
+              .addQueryParameter("api_key", apiKey)
+              .build();
+            Request request = chain
+              .request()
+              .newBuilder()
+              .url(url)
+              .build();
+
+            return chain.proceed(request);
+        }
+    }
+
+    @JsonAutoDetect(fieldVisibility = ANY)
+    class TopTags {
+
+        private Map<String, Object> tags;
+
+        @SuppressWarnings("unchecked")
+        public Set<String> all() {
+            List<Map<String, Object>> topTags = (List<Map<String, Object>>) tags.get("tag");
+            return topTags
+              .stream()
+              .map(e -> ((String) e.get("name")))
+              .collect(Collectors.toSet());
+        }
+    }
+
+    @JsonAutoDetect(fieldVisibility = ANY)
+    class Tags {
+
+        @JsonProperty("toptags") private Map<String, Object> topTags;
+
+        @SuppressWarnings("unchecked")
+        public Map<String, Double> all() {
+            try {
+                Map<String, Double> all = new HashMap<>();
+                List<Map<String, Object>> tags = (List<Map<String, Object>>) topTags.get("tag");
+                for (Map<String, Object> tag : tags) {
+                    all.put(((String) tag.get("name")), ((Integer) tag.get("count")).doubleValue());
+                }
+
+                return all;
+            } catch (Exception e) {
+                return Collections.emptyMap();
+            }
+        }
+    }
+
+    @JsonAutoDetect(fieldVisibility = ANY)
+    class Artists {
+
+        private Map<String, Object> artists;
+
+        @SuppressWarnings("unchecked")
+        public List<String> all() {
+            try {
+                List<Map<String, Object>> artists = (List<Map<String, Object>>) this.artists.get("artist");
+                return artists
+                  .stream()
+                  .map(e -> ((String) e.get("name")))
+                  .collect(toList());
+            } catch (Exception e) {
+                return Collections.emptyList();
+            }
+        }
+    }
+}

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/kmeans/Record.java

@@ -0,0 +1,65 @@
+package com.baeldung.algorithms.kmeans;
+
+import java.util.Map;
+import java.util.Objects;
+
+/**
+ * Encapsulates all feature values for a few attributes. Optionally each record
+ * can be described with the {@link #description} field.
+ */
+public class Record {
+
+    /**
+     * The record description. For example, this can be the artist name for the famous musician
+     * example.
+     */
+    private final String description;
+
+    /**
+     * Encapsulates all attributes and their corresponding values, i.e. features.
+     */
+    private final Map<String, Double> features;
+
+    public Record(String description, Map<String, Double> features) {
+        this.description = description;
+        this.features = features;
+    }
+
+    public Record(Map<String, Double> features) {
+        this("", features);
+    }
+
+    public String getDescription() {
+        return description;
+    }
+
+    public Map<String, Double> getFeatures() {
+        return features;
+    }
+
+    @Override
+    public String toString() {
+        String prefix = description == null || description
+          .trim()
+          .isEmpty() ? "Record" : description;
+
+        return prefix + ": " + features;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+        Record record = (Record) o;
+        return Objects.equals(getDescription(), record.getDescription()) && Objects.equals(getFeatures(), record.getFeatures());
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(getDescription(), getFeatures());
+    }
+}

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/printtriangles/PrintTriangleExamples.java

@@ -4,7 +4,7 @@ import org.apache.commons.lang3.StringUtils;
 
 public class PrintTriangleExamples {
 
-    public static String printARightAngledTriangle(int N) {
+    public static String printARightTriangle(int N) {
         StringBuilder result = new StringBuilder();
         for (int r = 1; r <= N; r++) {
             for (int j = 1; j <= r; j++) {
@@ -29,6 +29,17 @@ public class PrintTriangleExamples {
         return result.toString();
     }
 
+    public static String printAnIsoscelesTriangleUsingStringUtils(int N) {
+        StringBuilder result = new StringBuilder();
+
+        for (int r = 1; r <= N; r++) {
+            result.append(StringUtils.repeat(' ', N - r));
+            result.append(StringUtils.repeat('*', 2 * r - 1));
+            result.append(System.lineSeparator());
+        }
+        return result.toString();
+    }
+
     public static String printAnIsoscelesTriangleUsingSubstring(int N) {
         StringBuilder result = new StringBuilder();
         String helperString = StringUtils.repeat(' ', N - 1) + StringUtils.repeat('*', N * 2 - 1);
@@ -41,8 +52,9 @@ public class PrintTriangleExamples {
     }
 
     public static void main(String[] args) {
-        System.out.println(printARightAngledTriangle(5));
+        System.out.println(printARightTriangle(5));
         System.out.println(printAnIsoscelesTriangle(5));
+        System.out.println(printAnIsoscelesTriangleUsingStringUtils(5));
         System.out.println(printAnIsoscelesTriangleUsingSubstring(5));
     }
 

algorithms-miscellaneous-3/src/main/resources/kmeans/lastfm.json

@@ -0,0 +1,490 @@
+{
+  "children": [
+    {
+      "children": [
+        {
+          "name": "Radiohead"
+        },
+        {
+          "name": "Red Hot Chili Peppers"
+        },
+        {
+          "name": "Coldplay"
+        },
+        {
+          "name": "Nirvana"
+        },
+        {
+          "name": "Panic! at the Disco"
+        },
+        {
+          "name": "The Cure"
+        },
+        {
+          "name": "Linkin Park"
+        },
+        {
+          "name": "Radiohead"
+        },
+        {
+          "name": "Red Hot Chili Peppers"
+        },
+        {
+          "name": "Coldplay"
+        },
+        {
+          "name": "Nirvana"
+        },
+        {
+          "name": "Panic! at the Disco"
+        },
+        {
+          "name": "The Cure"
+        },
+        {
+          "name": "Linkin Park"
+        },
+        {
+          "name": "Muse"
+        },
+        {
+          "name": "Maroon 5"
+        },
+        {
+          "name": "Foo Fighters"
+        },
+        {
+          "name": "Paramore"
+        },
+        {
+          "name": "Oasis"
+        },
+        {
+          "name": "Fall Out Boy"
+        },
+        {
+          "name": "OneRepublic"
+        },
+        {
+          "name": "Weezer"
+        },
+        {
+          "name": "System of a Down"
+        },
+        {
+          "name": "The White Stripes"
+        }
+      ],
+      "name": "rock, alternative"
+    },
+    {
+      "children": [
+        {
+          "name": "Lil Nas X"
+        },
+        {
+          "name": "Post Malone"
+        },
+        {
+          "name": "Drake"
+        },
+        {
+          "name": "Kanye West"
+        },
+        {
+          "name": "Kendrick Lamar"
+        },
+        {
+          "name": "Tyler, the Creator"
+        },
+        {
+          "name": "Eminem"
+        },
+        {
+          "name": "Childish Gambino"
+        },
+        {
+          "name": "Frank Ocean"
+        },
+        {
+          "name": "Lil Nas X"
+        },
+        {
+          "name": "Post Malone"
+        },
+        {
+          "name": "Drake"
+        },
+        {
+          "name": "Kanye West"
+        },
+        {
+          "name": "Kendrick Lamar"
+        },
+        {
+          "name": "Tyler, the Creator"
+        },
+        {
+          "name": "Eminem"
+        },
+        {
+          "name": "Childish Gambino"
+        },
+        {
+          "name": "Frank Ocean"
+        },
+        {
+          "name": "Lizzo"
+        },
+        {
+          "name": "Travi$ Scott"
+        },
+        {
+          "name": "A$AP Rocky"
+        },
+        {
+          "name": "Nicki Minaj"
+        },
+        {
+          "name": "xxxtentacion"
+        }
+      ],
+      "name": "Hip-Hop, rap"
+    },
+    {
+      "children": [
+        {
+          "name": "Arctic Monkeys"
+        },
+        {
+          "name": "Imagine Dragons"
+        },
+        {
+          "name": "The Killers"
+        },
+        {
+          "name": "Gorillaz"
+        },
+        {
+          "name": "The Black Keys"
+        },
+        {
+          "name": "Arctic Monkeys"
+        },
+        {
+          "name": "Imagine Dragons"
+        },
+        {
+          "name": "The Killers"
+        },
+        {
+          "name": "Gorillaz"
+        },
+        {
+          "name": "The Black Keys"
+        },
+        {
+          "name": "Twenty One Pilots"
+        },
+        {
+          "name": "Ellie Goulding"
+        },
+        {
+          "name": "Florence + the Machine"
+        },
+        {
+          "name": "Vampire Weekend"
+        },
+        {
+          "name": "The Smiths"
+        },
+        {
+          "name": "The Strokes"
+        },
+        {
+          "name": "MGMT"
+        },
+        {
+          "name": "Foster the People"
+        },
+        {
+          "name": "Two Door Cinema Club"
+        },
+        {
+          "name": "Cage the Elephant"
+        },
+        {
+          "name": "Arcade Fire"
+        },
+        {
+          "name": "The 1975"
+        }
+      ],
+      "name": "indie, alternative"
+    },
+    {
+      "children": [
+        {
+          "name": "Ed Sheeran"
+        },
+        {
+          "name": "Tame Impala"
+        },
+        {
+          "name": "Ed Sheeran"
+        },
+        {
+          "name": "Tame Impala"
+        },
+        {
+          "name": "Green Day"
+        },
+        {
+          "name": "Metallica"
+        },
+        {
+          "name": "blink-182"
+        },
+        {
+          "name": "Bon Iver"
+        },
+        {
+          "name": "The Clash"
+        }
+      ],
+      "name": "rock, punk rock"
+    },
+    {
+      "children": [
+        {
+          "name": "Calvin Harris"
+        },
+        {
+          "name": "The Weeknd"
+        },
+        {
+          "name": "The Chainsmokers"
+        },
+        {
+          "name": "Daft Punk"
+        },
+        {
+          "name": "Marshmello"
+        },
+        {
+          "name": "David Guetta"
+        },
+        {
+          "name": "Calvin Harris"
+        },
+        {
+          "name": "The Weeknd"
+        },
+        {
+          "name": "The Chainsmokers"
+        },
+        {
+          "name": "Daft Punk"
+        },
+        {
+          "name": "Marshmello"
+        },
+        {
+          "name": "David Guetta"
+        },
+        {
+          "name": "Avicii"
+        },
+        {
+          "name": "Kygo"
+        },
+        {
+          "name": "Martin Garrix"
+        },
+        {
+          "name": "Major Lazer"
+        },
+        {
+          "name": "Depeche Mode"
+        }
+      ],
+      "name": "electronic, dance"
+    },
+    {
+      "children": [
+        {
+          "name": "Queen"
+        },
+        {
+          "name": "The Beatles"
+        },
+        {
+          "name": "David Bowie"
+        },
+        {
+          "name": "Fleetwood Mac"
+        },
+        {
+          "name": "Pink Floyd"
+        },
+        {
+          "name": "The Rolling Stones"
+        },
+        {
+          "name": "Led Zeppelin"
+        },
+        {
+          "name": "Queen"
+        },
+        {
+          "name": "The Beatles"
+        },
+        {
+          "name": "David Bowie"
+        },
+        {
+          "name": "Fleetwood Mac"
+        },
+        {
+          "name": "Pink Floyd"
+        },
+        {
+          "name": "The Rolling Stones"
+        },
+        {
+          "name": "Led Zeppelin"
+        },
+        {
+          "name": "Elton John"
+        }
+      ],
+      "name": "classic rock, rock"
+    },
+    {
+      "children": [
+        {
+          "name": "Billie Eilish"
+        },
+        {
+          "name": "Ariana Grande"
+        },
+        {
+          "name": "Taylor Swift"
+        },
+        {
+          "name": "Beyoncé"
+        },
+        {
+          "name": "Shawn Mendes"
+        },
+        {
+          "name": "Rihanna"
+        },
+        {
+          "name": "Lana Del Rey"
+        },
+        {
+          "name": "Katy Perry"
+        },
+        {
+          "name": "Lady Gaga"
+        },
+        {
+          "name": "Miley Cyrus"
+        },
+        {
+          "name": "Mark Ronson"
+        },
+        {
+          "name": "Madonna"
+        },
+        {
+          "name": "Lorde"
+        },
+        {
+          "name": "Khalid"
+        },
+        {
+          "name": "Billie Eilish"
+        },
+        {
+          "name": "Ariana Grande"
+        },
+        {
+          "name": "Taylor Swift"
+        },
+        {
+          "name": "Beyoncé"
+        },
+        {
+          "name": "Shawn Mendes"
+        },
+        {
+          "name": "Rihanna"
+        },
+        {
+          "name": "Lana Del Rey"
+        },
+        {
+          "name": "Katy Perry"
+        },
+        {
+          "name": "Lady Gaga"
+        },
+        {
+          "name": "Miley Cyrus"
+        },
+        {
+          "name": "Mark Ronson"
+        },
+        {
+          "name": "Madonna"
+        },
+        {
+          "name": "Lorde"
+        },
+        {
+          "name": "Khalid"
+        },
+        {
+          "name": "Sia"
+        },
+        {
+          "name": "Sam Smith"
+        },
+        {
+          "name": "Halsey"
+        },
+        {
+          "name": "Michael Jackson"
+        },
+        {
+          "name": "Charli XCX"
+        },
+        {
+          "name": "Britney Spears"
+        },
+        {
+          "name": "Dua Lipa"
+        },
+        {
+          "name": "Jonas Brothers"
+        },
+        {
+          "name": "Bruno Mars"
+        },
+        {
+          "name": "Carly Rae Jepsen"
+        },
+        {
+          "name": "P!nk"
+        },
+        {
+          "name": "Adele"
+        }
+      ],
+      "name": "pop, female vocalists"
+    }
+  ],
+  "name": "Musicians"
+}

algorithms-miscellaneous-3/src/main/resources/kmeans/radial.html

@@ -0,0 +1,68 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<style>
+    .node circle {
+        fill: #fff;
+        stroke: steelblue;
+        stroke-width: 1.5px;
+    }
+
+    .node {
+        font: 10px sans-serif;
+    }
+
+    .link {
+        fill: none;
+        stroke: #ccc;
+        stroke-width: 1.5px;
+    }
+</style>
+<body>
+<script src="http://d3js.org/d3.v3.min.js"></script>
+<script>
+    var diameter = 1100;
+    var tree = d3.layout.tree()
+        .size([360, diameter / 2 - 300])
+        .separation(function (a, b) {
+            return (a.parent == b.parent ? 1 : 2) / a.depth;
+        });
+    var diagonal = d3.svg.diagonal.radial()
+        .projection(function (d) {
+            return [d.y, d.x / 180 * Math.PI];
+        });
+    var svg = d3.select("body").append("svg")
+        .attr("width", diameter)
+        .attr("height", diameter - 150)
+        .append("g")
+        .attr("transform", "translate(" + diameter / 2 + "," + diameter / 2 + ")");
+    d3.json("lastfm.json", function (error, root) {
+        var nodes = tree.nodes(root),
+            links = tree.links(nodes);
+        var link = svg.selectAll(".link")
+            .data(links)
+            .enter().append("path")
+            .attr("class", "link")
+            .attr("d", diagonal);
+        var node = svg.selectAll(".node")
+            .data(nodes)
+            .enter().append("g")
+            .attr("class", "node")
+            .attr("transform", function (d) {
+                return "rotate(" + (d.x - 90) + ")translate(" + d.y + ")";
+            })
+        node.append("circle")
+            .attr("r", 4.5);
+        node.append("text")
+            .attr("dy", ".31em")
+            .attr("text-anchor", function (d) {
+                return d.x < 180 ? "start" : "end";
+            })
+            .attr("transform", function (d) {
+                return d.x < 180 ? "translate(8)" : "rotate(180)translate(-8)";
+            })
+            .text(function (d) {
+                return d.name;
+            });
+    });
+    d3.select(self.frameElement).style("height", diameter - 150 + "px");
+</script>

algorithms-miscellaneous-3/src/test/java/com/baeldung/algorithms/interpolationsearch/InterpolationSearchUnitTest.java

@@ -0,0 +1,29 @@
+package com.baeldung.algorithms.interpolationsearch;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.Before;
+import org.junit.Test;
+
+public class InterpolationSearchUnitTest {
+    
+    private int[] myData;
+
+    @Before
+    public void setUp() {
+        myData = new int[]{13,21,34,55,69,73,84,101};
+    }
+    
+    @Test
+    public void givenSortedArray_whenLookingFor84_thenReturn6() {
+        int pos = InterpolationSearch.interpolationSearch(myData, 84);
+        assertEquals(6, pos);
+    }
+    
+    @Test
+    public void givenSortedArray_whenLookingFor19_thenReturnMinusOne() {
+        int pos = InterpolationSearch.interpolationSearch(myData, 19);
+        assertEquals(-1, pos);
+    }
+
+}

algorithms-miscellaneous-3/src/test/java/com/baeldung/algorithms/printtriangles/PrintTriangleExamplesUnitTest.java

@@ -10,7 +10,7 @@ import static org.junit.Assert.assertEquals;
 @RunWith(JUnitParamsRunner.class)
 public class PrintTriangleExamplesUnitTest {
 
-    private static Object[][] rightAngledTriangles() {
+    private static Object[][] rightTriangles() {
         String expected0 = "";
         
         String expected2 = "*" + System.lineSeparator() 
@@ -39,9 +39,9 @@ public class PrintTriangleExamplesUnitTest {
     }
 
     @Test
-    @Parameters(method = "rightAngledTriangles")
+    @Parameters(method = "rightTriangles")
-    public void whenPrintARightAngledTriangleIsCalled_ThenTheCorrectStringIsReturned(int nrOfRows, String expected) {
+    public void whenPrintARightTriangleIsCalled_ThenTheCorrectStringIsReturned(int nrOfRows, String expected) {
-        String actual = PrintTriangleExamples.printARightAngledTriangle(nrOfRows);
+        String actual = PrintTriangleExamples.printARightTriangle(nrOfRows);
 
         assertEquals(expected, actual);
     }
@@ -82,6 +82,14 @@ public class PrintTriangleExamplesUnitTest {
         assertEquals(expected, actual);
     }
 
+    @Test
+    @Parameters(method = "isoscelesTriangles")
+    public void whenPrintAnIsoscelesTriangleUsingStringUtilsIsCalled_ThenTheCorrectStringIsReturned(int nrOfRows, String expected) {
+        String actual = PrintTriangleExamples.printAnIsoscelesTriangleUsingStringUtils(nrOfRows);
+
+        assertEquals(expected, actual);
+    }
+    
     @Test
     @Parameters(method = "isoscelesTriangles")
     public void whenPrintAnIsoscelesTriangleUsingSubstringIsCalled_ThenTheCorrectStringIsReturned(int nrOfRows, String expected) {

algorithms-sorting/README.md

@@ -5,3 +5,5 @@
 - [Quicksort Algorithm Implementation in Java](https://www.baeldung.com/java-quicksort)
 - [Insertion Sort in Java](https://www.baeldung.com/java-insertion-sort)
 - [Heap Sort in Java](https://www.baeldung.com/java-heap-sort)
+- [Shell Sort in Java](https://www.baeldung.com/java-shell-sort)
+- [Counting Sort in Java](https://www.baeldung.com/java-counting-sort)

algorithms-sorting/src/main/java/com/baeldung/algorithms/counting/CountingSort.java

@@ -0,0 +1,48 @@
+package com.baeldung.algorithms.counting;
+
+import java.util.Arrays;
+import java.util.stream.IntStream;
+
+public class CountingSort {
+
+    public static int[] sort(int[] input, int k) {
+        verifyPreconditions(input, k);
+        if (input.length == 0) return input;
+
+        int[] c = countElements(input, k);
+        int[] sorted = new int[input.length];
+        for (int i = input.length - 1; i >= 0; i--) {
+            int current = input[i];
+            sorted[c[current] - 1] = current;
+            c[current] -= 1;
+        }
+
+        return sorted;
+    }
+
+    static int[] countElements(int[] input, int k) {
+        int[] c = new int[k + 1];
+        Arrays.fill(c, 0);
+        for (int i : input) {
+            c[i] += 1;
+        }
+
+        for (int i = 1; i < c.length; i++) {
+            c[i] += c[i - 1];
+        }
+        return c;
+    }
+
+    private static void verifyPreconditions(int[] input, int k) {
+        if (input == null) {
+            throw new IllegalArgumentException("Input is required");
+        }
+
+        int min = IntStream.of(input).min().getAsInt();
+        int max = IntStream.of(input).max().getAsInt();
+
+        if (min < 0 || max > k) {
+            throw new IllegalArgumentException("The input numbers should be between zero and " + k);
+        }
+    }
+}

algorithms-sorting/src/main/java/com/baeldung/algorithms/sort/bynumber/NaturalOrderComparators.java

@@ -1,4 +1,4 @@
-package com.baeldung.algorithms.stringsortingbynumber;
+package com.baeldung.algorithms.sort.bynumber;
 
 import java.util.Comparator;
 

algorithms-sorting/src/test/java/com/baeldung/algorithms/counting/CountingSortUnitTest.java

@@ -0,0 +1,32 @@
+package com.baeldung.algorithms.counting;
+
+import java.util.Arrays;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+import org.junit.jupiter.api.Test;
+
+class CountingSortUnitTest {
+
+    @Test
+    void countElements_GivenAnArray_ShouldCalculateTheFrequencyArrayAsExpected() {
+        int k = 5;
+        int[] input = { 4, 3, 2, 5, 4, 3, 5, 1, 0, 2, 5 };
+
+        int[] c = CountingSort.countElements(input, k);
+        int[] expected = { 1, 2, 4, 6, 8, 11 };
+        assertArrayEquals(expected, c);
+    }
+
+    @Test
+    void sort_GivenAnArray_ShouldSortTheInputAsExpected() {
+        int k = 5;
+        int[] input = { 4, 3, 2, 5, 4, 3, 5, 1, 0, 2, 5 };
+
+        int[] sorted = CountingSort.sort(input, k);
+
+        // Our sorting algorithm and Java's should return the same result
+        Arrays.sort(input);
+        assertArrayEquals(input, sorted);
+    }
+}

algorithms-sorting/src/test/java/com/baeldung/algorithms/sort/bynumber/NaturalOrderComparatorsUnitTest.java

@@ -1,6 +1,6 @@
-package com.baeldung.algorithms.stringsortingbynumber;
+package com.baeldung.algorithms.sort.bynumber;
 
-import com.baeldung.algorithms.stringsortingbynumber.NaturalOrderComparators;
+import com.baeldung.algorithms.sort.bynumber.NaturalOrderComparators;
 import org.junit.Test;
 
 import java.util.ArrayList;

apache-olingo/olingo2/pom.xml

@@ -4,9 +4,9 @@
 	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 	<modelVersion>4.0.0</modelVersion>
 	<groupId>org.baeldung.examples.olingo2</groupId>
-	<artifactId>olingo2-sample</artifactId>
+	<artifactId>olingo2</artifactId>
 	<version>0.0.1-SNAPSHOT</version>
-	<name>olingo2-sample</name>
+	<name>olingo2</name>
 	<description>Sample Olingo 2 Project</description>
 
     <parent>

apache-spark/data/iris.data

@@ -0,0 +1,150 @@
+5.1,3.5,1.4,0.2,Iris-setosa
+4.9,3.0,1.4,0.2,Iris-setosa
+4.7,3.2,1.3,0.2,Iris-setosa
+4.6,3.1,1.5,0.2,Iris-setosa
+5.0,3.6,1.4,0.2,Iris-setosa
+5.4,3.9,1.7,0.4,Iris-setosa
+4.6,3.4,1.4,0.3,Iris-setosa
+5.0,3.4,1.5,0.2,Iris-setosa
+4.4,2.9,1.4,0.2,Iris-setosa
+4.9,3.1,1.5,0.1,Iris-setosa
+5.4,3.7,1.5,0.2,Iris-setosa
+4.8,3.4,1.6,0.2,Iris-setosa
+4.8,3.0,1.4,0.1,Iris-setosa
+4.3,3.0,1.1,0.1,Iris-setosa
+5.8,4.0,1.2,0.2,Iris-setosa
+5.7,4.4,1.5,0.4,Iris-setosa
+5.4,3.9,1.3,0.4,Iris-setosa
+5.1,3.5,1.4,0.3,Iris-setosa
+5.7,3.8,1.7,0.3,Iris-setosa
+5.1,3.8,1.5,0.3,Iris-setosa
+5.4,3.4,1.7,0.2,Iris-setosa
+5.1,3.7,1.5,0.4,Iris-setosa
+4.6,3.6,1.0,0.2,Iris-setosa
+5.1,3.3,1.7,0.5,Iris-setosa
+4.8,3.4,1.9,0.2,Iris-setosa
+5.0,3.0,1.6,0.2,Iris-setosa
+5.0,3.4,1.6,0.4,Iris-setosa
+5.2,3.5,1.5,0.2,Iris-setosa
+5.2,3.4,1.4,0.2,Iris-setosa
+4.7,3.2,1.6,0.2,Iris-setosa
+4.8,3.1,1.6,0.2,Iris-setosa
+5.4,3.4,1.5,0.4,Iris-setosa
+5.2,4.1,1.5,0.1,Iris-setosa
+5.5,4.2,1.4,0.2,Iris-setosa
+4.9,3.1,1.5,0.1,Iris-setosa
+5.0,3.2,1.2,0.2,Iris-setosa
+5.5,3.5,1.3,0.2,Iris-setosa
+4.9,3.1,1.5,0.1,Iris-setosa
+4.4,3.0,1.3,0.2,Iris-setosa
+5.1,3.4,1.5,0.2,Iris-setosa
+5.0,3.5,1.3,0.3,Iris-setosa
+4.5,2.3,1.3,0.3,Iris-setosa
+4.4,3.2,1.3,0.2,Iris-setosa
+5.0,3.5,1.6,0.6,Iris-setosa
+5.1,3.8,1.9,0.4,Iris-setosa
+4.8,3.0,1.4,0.3,Iris-setosa
+5.1,3.8,1.6,0.2,Iris-setosa
+4.6,3.2,1.4,0.2,Iris-setosa
+5.3,3.7,1.5,0.2,Iris-setosa
+5.0,3.3,1.4,0.2,Iris-setosa
+7.0,3.2,4.7,1.4,Iris-versicolor
+6.4,3.2,4.5,1.5,Iris-versicolor
+6.9,3.1,4.9,1.5,Iris-versicolor
+5.5,2.3,4.0,1.3,Iris-versicolor
+6.5,2.8,4.6,1.5,Iris-versicolor
+5.7,2.8,4.5,1.3,Iris-versicolor
+6.3,3.3,4.7,1.6,Iris-versicolor
+4.9,2.4,3.3,1.0,Iris-versicolor
+6.6,2.9,4.6,1.3,Iris-versicolor
+5.2,2.7,3.9,1.4,Iris-versicolor
+5.0,2.0,3.5,1.0,Iris-versicolor
+5.9,3.0,4.2,1.5,Iris-versicolor
+6.0,2.2,4.0,1.0,Iris-versicolor
+6.1,2.9,4.7,1.4,Iris-versicolor
+5.6,2.9,3.6,1.3,Iris-versicolor
+6.7,3.1,4.4,1.4,Iris-versicolor
+5.6,3.0,4.5,1.5,Iris-versicolor
+5.8,2.7,4.1,1.0,Iris-versicolor
+6.2,2.2,4.5,1.5,Iris-versicolor
+5.6,2.5,3.9,1.1,Iris-versicolor
+5.9,3.2,4.8,1.8,Iris-versicolor
+6.1,2.8,4.0,1.3,Iris-versicolor
+6.3,2.5,4.9,1.5,Iris-versicolor
+6.1,2.8,4.7,1.2,Iris-versicolor
+6.4,2.9,4.3,1.3,Iris-versicolor
+6.6,3.0,4.4,1.4,Iris-versicolor
+6.8,2.8,4.8,1.4,Iris-versicolor
+6.7,3.0,5.0,1.7,Iris-versicolor
+6.0,2.9,4.5,1.5,Iris-versicolor
+5.7,2.6,3.5,1.0,Iris-versicolor
+5.5,2.4,3.8,1.1,Iris-versicolor
+5.5,2.4,3.7,1.0,Iris-versicolor
+5.8,2.7,3.9,1.2,Iris-versicolor
+6.0,2.7,5.1,1.6,Iris-versicolor
+5.4,3.0,4.5,1.5,Iris-versicolor
+6.0,3.4,4.5,1.6,Iris-versicolor
+6.7,3.1,4.7,1.5,Iris-versicolor
+6.3,2.3,4.4,1.3,Iris-versicolor
+5.6,3.0,4.1,1.3,Iris-versicolor
+5.5,2.5,4.0,1.3,Iris-versicolor
+5.5,2.6,4.4,1.2,Iris-versicolor
+6.1,3.0,4.6,1.4,Iris-versicolor
+5.8,2.6,4.0,1.2,Iris-versicolor
+5.0,2.3,3.3,1.0,Iris-versicolor
+5.6,2.7,4.2,1.3,Iris-versicolor
+5.7,3.0,4.2,1.2,Iris-versicolor
+5.7,2.9,4.2,1.3,Iris-versicolor
+6.2,2.9,4.3,1.3,Iris-versicolor
+5.1,2.5,3.0,1.1,Iris-versicolor
+5.7,2.8,4.1,1.3,Iris-versicolor
+6.3,3.3,6.0,2.5,Iris-virginica
+5.8,2.7,5.1,1.9,Iris-virginica
+7.1,3.0,5.9,2.1,Iris-virginica
+6.3,2.9,5.6,1.8,Iris-virginica
+6.5,3.0,5.8,2.2,Iris-virginica
+7.6,3.0,6.6,2.1,Iris-virginica
+4.9,2.5,4.5,1.7,Iris-virginica
+7.3,2.9,6.3,1.8,Iris-virginica
+6.7,2.5,5.8,1.8,Iris-virginica
+7.2,3.6,6.1,2.5,Iris-virginica
+6.5,3.2,5.1,2.0,Iris-virginica
+6.4,2.7,5.3,1.9,Iris-virginica
+6.8,3.0,5.5,2.1,Iris-virginica
+5.7,2.5,5.0,2.0,Iris-virginica
+5.8,2.8,5.1,2.4,Iris-virginica
+6.4,3.2,5.3,2.3,Iris-virginica
+6.5,3.0,5.5,1.8,Iris-virginica
+7.7,3.8,6.7,2.2,Iris-virginica
+7.7,2.6,6.9,2.3,Iris-virginica
+6.0,2.2,5.0,1.5,Iris-virginica
+6.9,3.2,5.7,2.3,Iris-virginica
+5.6,2.8,4.9,2.0,Iris-virginica
+7.7,2.8,6.7,2.0,Iris-virginica
+6.3,2.7,4.9,1.8,Iris-virginica
+6.7,3.3,5.7,2.1,Iris-virginica
+7.2,3.2,6.0,1.8,Iris-virginica
+6.2,2.8,4.8,1.8,Iris-virginica
+6.1,3.0,4.9,1.8,Iris-virginica
+6.4,2.8,5.6,2.1,Iris-virginica
+7.2,3.0,5.8,1.6,Iris-virginica
+7.4,2.8,6.1,1.9,Iris-virginica
+7.9,3.8,6.4,2.0,Iris-virginica
+6.4,2.8,5.6,2.2,Iris-virginica
+6.3,2.8,5.1,1.5,Iris-virginica
+6.1,2.6,5.6,1.4,Iris-virginica
+7.7,3.0,6.1,2.3,Iris-virginica
+6.3,3.4,5.6,2.4,Iris-virginica
+6.4,3.1,5.5,1.8,Iris-virginica
+6.0,3.0,4.8,1.8,Iris-virginica
+6.9,3.1,5.4,2.1,Iris-virginica
+6.7,3.1,5.6,2.4,Iris-virginica
+6.9,3.1,5.1,2.3,Iris-virginica
+5.8,2.7,5.1,1.9,Iris-virginica
+6.8,3.2,5.9,2.3,Iris-virginica
+6.7,3.3,5.7,2.5,Iris-virginica
+6.7,3.0,5.2,2.3,Iris-virginica
+6.3,2.5,5.0,1.9,Iris-virginica
+6.5,3.0,5.2,2.0,Iris-virginica
+6.2,3.4,5.4,2.3,Iris-virginica
+5.9,3.0,5.1,1.8,Iris-virginica

apache-spark/model/logistic-regression/metadata/part-00000

@@ -0,0 +1 @@
+{"class":"org.apache.spark.mllib.classification.LogisticRegressionModel","version":"1.0","numFeatures":4,"numClasses":3}

apache-spark/pom.xml

@@ -1,4 +1,5 @@
-<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 	<modelVersion>4.0.0</modelVersion>
 	<groupId>com.baeldung</groupId>
@@ -33,6 +34,12 @@
 			<version>${org.apache.spark.spark-streaming.version}</version>
 			<scope>provided</scope>
 		</dependency>
+		<dependency>
+			<groupId>org.apache.spark</groupId>
+			<artifactId>spark-mllib_2.11</artifactId>
+			<version>${org.apache.spark.spark-mllib.version}</version>
+			<scope>provided</scope>
+		</dependency>
 		<dependency>
 			<groupId>org.apache.spark</groupId>
 			<artifactId>spark-streaming-kafka-0-10_2.11</artifactId>
@@ -84,6 +91,7 @@
 		<org.apache.spark.spark-core.version>2.3.0</org.apache.spark.spark-core.version>
 		<org.apache.spark.spark-sql.version>2.3.0</org.apache.spark.spark-sql.version>
 		<org.apache.spark.spark-streaming.version>2.3.0</org.apache.spark.spark-streaming.version>
+		<org.apache.spark.spark-mllib.version>2.3.0</org.apache.spark.spark-mllib.version>
 		<org.apache.spark.spark-streaming-kafka.version>2.3.0</org.apache.spark.spark-streaming-kafka.version>
 		<com.datastax.spark.spark-cassandra-connector.version>2.3.0</com.datastax.spark.spark-cassandra-connector.version>
 		<com.datastax.spark.spark-cassandra-connector-java.version>1.5.2</com.datastax.spark.spark-cassandra-connector-java.version>

apache-spark/src/main/java/com/baeldung/ml/MachineLearningApp.java

@@ -0,0 +1,111 @@
+package com.baeldung.ml;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import org.apache.log4j.Level;
+import org.apache.log4j.Logger;
+import org.apache.spark.SparkConf;
+import org.apache.spark.api.java.JavaPairRDD;
+import org.apache.spark.api.java.JavaRDD;
+import org.apache.spark.api.java.JavaSparkContext;
+import org.apache.spark.mllib.classification.LogisticRegressionModel;
+import org.apache.spark.mllib.classification.LogisticRegressionWithLBFGS;
+import org.apache.spark.mllib.evaluation.MulticlassMetrics;
+import org.apache.spark.mllib.linalg.Matrix;
+import org.apache.spark.mllib.linalg.Vector;
+import org.apache.spark.mllib.linalg.Vectors;
+import org.apache.spark.mllib.regression.LabeledPoint;
+import org.apache.spark.mllib.stat.MultivariateStatisticalSummary;
+import org.apache.spark.mllib.stat.Statistics;
+
+import scala.Tuple2;
+
+public class MachineLearningApp {
+
+    public static void main(String[] args) {
+
+        // 1. Setting the Spark Context
+        SparkConf conf = new SparkConf().setAppName("Main")
+            .setMaster("local[2]")
+            .set("spark.executor.memory", "3g")
+            .set("spark.driver.memory", "3g");
+        JavaSparkContext sc = new JavaSparkContext(conf);
+        Logger.getLogger("org")
+            .setLevel(Level.OFF);
+        Logger.getLogger("akka")
+            .setLevel(Level.OFF);
+
+        // 2. Loading the Data-set
+        String dataFile = "data\\iris.data";
+        JavaRDD<String> data = sc.textFile(dataFile);
+
+        // 3. Exploratory Data Analysis
+        // 3.1. Creating Vector of Input Data
+        JavaRDD<Vector> inputData = data.map(line -> {
+            String[] parts = line.split(",");
+            double[] v = new double[parts.length - 1];
+            for (int i = 0; i < parts.length - 1; i++) {
+                v[i] = Double.parseDouble(parts[i]);
+            }
+            return Vectors.dense(v);
+        });
+        // 3.2. Performing Statistical Analysis
+        MultivariateStatisticalSummary summary = Statistics.colStats(inputData.rdd());
+        System.out.println("Summary Mean:");
+        System.out.println(summary.mean());
+        System.out.println("Summary Variance:");
+        System.out.println(summary.variance());
+        System.out.println("Summary Non-zero:");
+        System.out.println(summary.numNonzeros());
+        // 3.3. Performing Correlation Analysis
+        Matrix correlMatrix = Statistics.corr(inputData.rdd(), "pearson");
+        System.out.println("Correlation Matrix:");
+        System.out.println(correlMatrix.toString());
+
+        // 4. Data Preparation
+        // 4.1. Creating Map for Textual Output Labels
+        Map<String, Integer> map = new HashMap<String, Integer>();
+        map.put("Iris-setosa", 0);
+        map.put("Iris-versicolor", 1);
+        map.put("Iris-virginica", 2);
+        // 4.2. Creating LabeledPoint of Input and Output Data
+        JavaRDD<LabeledPoint> parsedData = data.map(line -> {
+            String[] parts = line.split(",");
+            double[] v = new double[parts.length - 1];
+            for (int i = 0; i < parts.length - 1; i++) {
+                v[i] = Double.parseDouble(parts[i]);
+            }
+            return new LabeledPoint(map.get(parts[parts.length - 1]), Vectors.dense(v));
+        });
+
+        // 5. Data Splitting into 80% Training and 20% Test Sets
+        JavaRDD<LabeledPoint>[] splits = parsedData.randomSplit(new double[] { 0.8, 0.2 }, 11L);
+        JavaRDD<LabeledPoint> trainingData = splits[0].cache();
+        JavaRDD<LabeledPoint> testData = splits[1];
+
+        // 6. Modeling
+        // 6.1. Model Training
+        LogisticRegressionModel model = new LogisticRegressionWithLBFGS().setNumClasses(3)
+            .run(trainingData.rdd());
+        // 6.2. Model Evaluation
+        JavaPairRDD<Object, Object> predictionAndLabels = testData.mapToPair(p -> new Tuple2<>(model.predict(p.features()), p.label()));
+        MulticlassMetrics metrics = new MulticlassMetrics(predictionAndLabels.rdd());
+        double accuracy = metrics.accuracy();
+        System.out.println("Model Accuracy on Test Data: " + accuracy);
+
+        // 7. Model Saving and Loading
+        // 7.1. Model Saving
+        model.save(sc.sc(), "model\\logistic-regression");
+        // 7.2. Model Loading
+        LogisticRegressionModel sameModel = LogisticRegressionModel.load(sc.sc(), "model\\logistic-regression");
+        // 7.3. Prediction on New Data
+        Vector newData = Vectors.dense(new double[] { 1, 1, 1, 1 });
+        double prediction = sameModel.predict(newData);
+        System.out.println("Model Prediction on New Data = " + prediction);
+
+        // 8. Clean-up
+        sc.close();
+    }
+
+}

bazel/README.md

@@ -0,0 +1,3 @@
+## Relevant Articles:
+
+- [Building Java Applications with Bazel](https://www.baeldung.com/bazel-build-tool)

bazel/WORKSPACE

@@ -0,0 +1,31 @@
+
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_jar")
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+
+RULES_JVM_EXTERNAL_TAG = "2.0.1"
+RULES_JVM_EXTERNAL_SHA = "55e8d3951647ae3dffde22b4f7f8dee11b3f70f3f89424713debd7076197eaca"
+
+http_archive(
+    name = "rules_jvm_external",
+    strip_prefix = "rules_jvm_external-%s" % RULES_JVM_EXTERNAL_TAG,
+    sha256 = RULES_JVM_EXTERNAL_SHA,
+    url = "https://github.com/bazelbuild/rules_jvm_external/archive/%s.zip" % RULES_JVM_EXTERNAL_TAG,
+)
+
+load("@rules_jvm_external//:defs.bzl", "maven_install")
+
+maven_install(
+    artifacts = [
+        "org.apache.commons:commons-lang3:3.9"
+    ],
+    repositories = [
+        "https://repo1.maven.org/maven2",
+    ]
+)
+
+http_jar (
+    name = "apache-commons-lang",
+    url = "https://repo1.maven.org/maven2/org/apache/commons/commons-lang3/3.9/commons-lang3-3.9.jar"
+)
+

bazel/bazelapp/BUILD

@@ -0,0 +1,7 @@
+
+java_binary (
+    name = "BazelApp",
+    srcs = glob(["src/main/java/com/baeldung/*.java"]),
+    main_class = "com.baeldung.BazelApp",
+    deps = ["//bazelgreeting:greeter", "@maven//:org_apache_commons_commons_lang3"]
+)

bazel/bazelapp/pom.xml

@@ -0,0 +1,29 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>bazelapp</artifactId>
+    <name>bazelapp</name>
+	
+    <parent>
+        <artifactId>bazel</artifactId>
+        <groupId>com.baeldung</groupId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>com.baeldung</groupId>
+            <artifactId>bazelgreeting</artifactId>
+            <version>1.0.0-SNAPSHOT</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+            <version>3.9</version>
+        </dependency>
+    </dependencies>
+
+
+</project>

bazel/bazelapp/src/main/java/com/baeldung/BazelApp.java

@@ -0,0 +1,15 @@
+package com.baeldung;
+
+import com.baeldung.Greetings;
+import org.apache.commons.lang3.StringUtils;
+
+public class BazelApp {
+
+    public static void main(String ... args) {
+        Greetings greetings = new Greetings();
+
+        System.out.println(greetings.greet("Bazel"));
+
+        System.out.println(StringUtils.lowerCase("Bazel"));
+    }
+}

bazel/bazelgreeting/BUILD

@@ -0,0 +1,6 @@
+
+java_library (
+    name = "greeter",
+    srcs = glob(["src/main/java/com/baeldung/*.java"]),
+    visibility = ["//bazelapp:__pkg__"]
+)

bazel/bazelgreeting/pom.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>bazelgreeting</artifactId>
+    <name>bazelgreeting</name>
+	
+    <parent>
+        <artifactId>bazel</artifactId>
+        <groupId>com.baeldung</groupId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+
+</project>

bazel/bazelgreeting/src/main/java/com/baeldung/Greetings.java

@@ -0,0 +1,8 @@
+package com.baeldung;
+
+public class Greetings {
+
+    public String greet(String name) {
+        return "Hello ".concat(name);
+    }
+}

bazel/pom.xml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>bazel</artifactId>
+    <packaging>pom</packaging>
+    <name>bazel</name>
+	
+    <parent>
+        <artifactId>parent-modules</artifactId>
+        <groupId>com.baeldung</groupId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+	
+    <modules>
+        <module>bazelgreeting</module>
+        <module>bazelapp</module>
+    </modules>
+
+
+</project>

core-groovy-2/README.md

@@ -8,3 +8,4 @@
 - [Pattern Matching in Strings in Groovy](https://www.baeldung.com/groovy-pattern-matching)
 - [Working with XML in Groovy](https://www.baeldung.com/groovy-xml)
 - [Integrating Groovy into Java Applications](https://www.baeldung.com/groovy-java-applications)
+- [Concatenate Strings with Groovy](https://www.baeldung.com/groovy-concatenate-strings)

core-java-modules/core-java-arrays-2/README.MD

@@ -1,3 +1,5 @@
 ## Relevant Articles
 
 - [Extending an Array’s Length](https://www.baeldung.com/java-array-add-element-at-the-end)
+- [Checking If an Array Is Sorted in Java](https://www.baeldung.com/java-check-sorted-array)
+- [Looping Diagonally Through a 2d Java Array](https://www.baeldung.com/java-loop-diagonal-array)

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGenerator.java

@@ -0,0 +1,12 @@
+package com.baeldung.concurrent.mutex;
+
+public class SequenceGenerator {
+
+    private int currentValue = 0;
+
+    public int getNextSequence() {
+        currentValue = currentValue + 1;
+        return currentValue;
+    }
+
+}

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGeneratorUsingMonitor.java

@@ -0,0 +1,19 @@
+package com.baeldung.concurrent.mutex;
+
+import com.google.common.util.concurrent.Monitor;
+
+public class SequenceGeneratorUsingMonitor extends SequenceGenerator {
+
+    private Monitor mutex = new Monitor();
+
+    @Override
+    public int getNextSequence() {
+        mutex.enter();
+        try {
+            return super.getNextSequence();
+        } finally {
+            mutex.leave();
+        }
+    }
+
+}

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGeneratorUsingReentrantLock.java

@@ -0,0 +1,19 @@
+package com.baeldung.concurrent.mutex;
+
+import java.util.concurrent.locks.ReentrantLock;
+
+public class SequenceGeneratorUsingReentrantLock extends SequenceGenerator {
+
+    private ReentrantLock mutex = new ReentrantLock();
+
+    @Override
+    public int getNextSequence() {
+        try {
+            mutex.lock();
+            return super.getNextSequence();
+        } finally {
+            mutex.unlock();
+        }
+    }
+
+}

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGeneratorUsingSemaphore.java

@@ -0,0 +1,21 @@
+package com.baeldung.concurrent.mutex;
+
+import java.util.concurrent.Semaphore;
+
+public class SequenceGeneratorUsingSemaphore extends SequenceGenerator {
+
+    private Semaphore mutex = new Semaphore(1);
+
+    @Override
+    public int getNextSequence() {
+        try {
+            mutex.acquire();
+            return super.getNextSequence();
+        } catch (InterruptedException e) {
+            throw new RuntimeException("Exception in critical section.", e);
+        } finally {
+            mutex.release();
+        }
+    }
+
+}

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGeneratorUsingSynchronizedBlock.java

@@ -0,0 +1,14 @@
+package com.baeldung.concurrent.mutex;
+
+public class SequenceGeneratorUsingSynchronizedBlock extends SequenceGenerator {
+
+    private Object mutex = new Object();
+
+    @Override
+    public int getNextSequence() {
+        synchronized (mutex) {
+            return super.getNextSequence();
+        }
+    }
+
+}

core-java-modules/core-java-concurrency-basic/src/main/java/com/baeldung/concurrent/mutex/SequenceGeneratorUsingSynchronizedMethod.java

@@ -0,0 +1,10 @@
+package com.baeldung.concurrent.mutex;
+
+public class SequenceGeneratorUsingSynchronizedMethod extends SequenceGenerator {
+
+    @Override
+    public synchronized int getNextSequence() {
+        return super.getNextSequence();
+    }
+
+}

core-java-modules/core-java-concurrency-basic/src/test/java/com/baeldung/concurrent/executorservice/Java8ExecutorServiceIntegrationTest.java

@@ -1,4 +1,4 @@
-package com.baeldung.java8;
+package com.baeldung.concurrent.executorservice;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;

core-java-modules/core-java-concurrency-basic/src/test/java/com/baeldung/concurrent/mutex/MutexUnitTest.java

@@ -0,0 +1,80 @@
+package com.baeldung.concurrent.mutex;
+
+import java.util.ArrayList;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class MutexUnitTest {
+
+    // @Test
+    // This test verifies the race condition use case, it may pass or fail based on execution environment
+    // Uncomment @Test to run it
+    public void givenUnsafeSequenceGenerator_whenRaceCondition_thenUnexpectedBehavior() throws Exception {
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGenerator(), count);
+        Assert.assertNotEquals(count, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingSynchronizedMethod_whenRaceCondition_thenSuccess() throws Exception {
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGeneratorUsingSynchronizedMethod(), count);
+        Assert.assertEquals(count, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingSynchronizedBlock_whenRaceCondition_thenSuccess() throws Exception {
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGeneratorUsingSynchronizedBlock(), count);
+        Assert.assertEquals(count, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingReentrantLock_whenRaceCondition_thenSuccess() throws Exception {
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGeneratorUsingReentrantLock(), count);
+        Assert.assertEquals(count, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingSemaphore_whenRaceCondition_thenSuccess() throws Exception {
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGeneratorUsingSemaphore(), count);
+        Assert.assertEquals(count, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingMonitor_whenRaceCondition_thenSuccess() throws Exception {
+        int count = 1000;
+        Set<Integer> uniqueSequences = getUniqueSequences(new SequenceGeneratorUsingMonitor(), count);
+        Assert.assertEquals(count, uniqueSequences.size());
+    }
+
+    private Set<Integer> getUniqueSequences(SequenceGenerator generator, int count) throws Exception {
+        ExecutorService executor = Executors.newFixedThreadPool(3);
+        Set<Integer> uniqueSequences = new LinkedHashSet<>();
+        List<Future<Integer>> futures = new ArrayList<>();
+
+        for (int i = 0; i < count; i++) {
+            futures.add(executor.submit(generator::getNextSequence));
+        }
+
+        for (Future<Integer> future : futures) {
+            uniqueSequences.add(future.get());
+        }
+
+        executor.awaitTermination(1, TimeUnit.SECONDS);
+        executor.shutdown();
+
+        return uniqueSequences;
+    }
+
+}

core-java-modules/core-java-exceptions/src/main/java/com/baeldung/exceptions/CheckedUncheckedExceptions.java

@@ -0,0 +1,43 @@
+package com.baeldung.exceptions;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileNotFoundException;
+
+public class CheckedUncheckedExceptions {
+    public static void checkedExceptionWithThrows() throws FileNotFoundException {
+        File file = new File("not_existing_file.txt");
+        FileInputStream stream = new FileInputStream(file);
+    }
+
+    public static void checkedExceptionWithTryCatch() {
+        File file = new File("not_existing_file.txt");
+        try {
+            FileInputStream stream = new FileInputStream(file);
+        } catch (FileNotFoundException e) {
+            e.printStackTrace();
+        }
+    }
+
+    public static int divideByZero() {
+        int numerator = 1;
+        int denominator = 0;
+        return numerator / denominator;
+    }
+
+    public static void checkFile(String fileName) throws IncorrectFileNameException {
+        if (fileName == null || fileName.isEmpty()) {
+            throw new NullOrEmptyException("The filename is null.");
+        }
+        if (!isCorrectFileName(fileName)) {
+            throw new IncorrectFileNameException("Incorrect filename : " + fileName);
+        }
+    }
+
+    private static boolean isCorrectFileName(String fileName) {
+        if (fileName.equals("wrongFileName.txt"))
+            return false;
+        else
+            return true;
+    }
+}

core-java-modules/core-java-exceptions/src/main/java/com/baeldung/exceptions/IncorrectFileNameException.java

@@ -0,0 +1,13 @@
+package com.baeldung.exceptions;
+
+public class IncorrectFileNameException extends Exception {
+    private static final long serialVersionUID = 1L;
+
+    public IncorrectFileNameException(String errorMessage) {
+        super(errorMessage);
+    }
+
+    public IncorrectFileNameException(String errorMessage, Throwable thr) {
+        super(errorMessage, thr);
+    }
+}

core-java-modules/core-java-exceptions/src/main/java/com/baeldung/exceptions/NullOrEmptyException.java

@@ -0,0 +1,14 @@
+package com.baeldung.exceptions;
+
+public class NullOrEmptyException extends RuntimeException {
+
+    private static final long serialVersionUID = 1L;
+
+    public NullOrEmptyException(String errorMessage) {
+        super(errorMessage);
+    }
+
+    public NullOrEmptyException(String errorMessage, Throwable thr) {
+        super(errorMessage, thr);
+    }
+}

core-java-modules/core-java-exceptions/src/test/java/com/baeldung/exceptions/CheckedUncheckedExceptionsUnitTest.java

@@ -0,0 +1,55 @@
+package com.baeldung.exceptions;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.io.FileNotFoundException;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests the {@link CheckedUncheckedExceptions}.
+ */
+public class CheckedUncheckedExceptionsUnitTest {
+
+    @Test
+    public void whenFileNotExist_thenThrowException() {
+        assertThrows(FileNotFoundException.class, () -> {
+            CheckedUncheckedExceptions.checkedExceptionWithThrows();
+        });
+    }
+
+    @Test
+    public void whenTryCatchExcetpion_thenSuccess() {
+        try {
+            CheckedUncheckedExceptions.checkedExceptionWithTryCatch();
+        } catch (Exception e) {
+            Assertions.fail(e.getMessage());
+        }
+    }
+
+    @Test
+    public void whenDivideByZero_thenThrowException() {
+        assertThrows(ArithmeticException.class, () -> {
+            CheckedUncheckedExceptions.divideByZero();
+        });
+    }
+
+    @Test
+    public void whenInvalidFile_thenThrowException() {
+
+        assertThrows(IncorrectFileNameException.class, () -> {
+            CheckedUncheckedExceptions.checkFile("wrongFileName.txt");
+        });
+    }
+
+    @Test
+    public void whenNullOrEmptyFile_thenThrowException() {
+        assertThrows(NullOrEmptyException.class, () -> {
+            CheckedUncheckedExceptions.checkFile(null);
+        });
+        assertThrows(NullOrEmptyException.class, () -> {
+            CheckedUncheckedExceptions.checkFile("");
+        });
+    }
+}

core-java-modules/core-java-io-2/.gitignore

@@ -0,0 +1,3 @@
+# Intellij
+.idea/
+*.iml

core-java-modules/core-java-io-2/pom.xml

@@ -0,0 +1,279 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-io-2</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-io-2</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-java</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>../../parent-java</relativePath>
+    </parent>
+
+    <dependencies>
+        <!-- utils -->
+        <dependency>
+            <groupId>net.sourceforge.collections</groupId>
+            <artifactId>collections-generic</artifactId>
+            <version>${collections-generic.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-collections4</artifactId>
+            <version>${commons-collections4.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>commons-io</groupId>
+            <artifactId>commons-io</artifactId>
+            <version>${commons-io.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+            <version>${commons-lang3.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-math3</artifactId>
+            <version>${commons-math3.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.decimal4j</groupId>
+            <artifactId>decimal4j</artifactId>
+            <version>${decimal4j.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.bouncycastle</groupId>
+            <artifactId>bcprov-jdk15on</artifactId>
+            <version>${bouncycastle.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.unix4j</groupId>
+            <artifactId>unix4j-command</artifactId>
+            <version>${unix4j.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.googlecode.grep4j</groupId>
+            <artifactId>grep4j</artifactId>
+            <version>${grep4j.version}</version>
+        </dependency>
+        <!-- web -->
+        <!-- marshalling -->
+        <dependency>
+            <groupId>com.fasterxml.jackson.core</groupId>
+            <artifactId>jackson-databind</artifactId>
+            <version>${jackson.version}</version>
+        </dependency>
+        <!-- logging -->
+        <dependency>
+            <groupId>log4j</groupId>
+            <artifactId>log4j</artifactId>
+            <version>${log4j.version}</version>
+        </dependency>
+        <dependency> <!-- needed to bridge to slf4j for projects that use the log4j APIs directly -->
+            <groupId>org.slf4j</groupId>
+            <artifactId>log4j-over-slf4j</artifactId>
+            <version>${org.slf4j.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.projectlombok</groupId>
+            <artifactId>lombok</artifactId>
+            <version>${lombok.version}</version>
+            <scope>provided</scope>
+        </dependency>
+        <!-- test scoped -->
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.jayway.awaitility</groupId>
+            <artifactId>awaitility</artifactId>
+            <version>${avaitility.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>commons-codec</groupId>
+            <artifactId>commons-codec</artifactId>
+            <version>${commons-codec.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.javamoney</groupId>
+            <artifactId>moneta</artifactId>
+            <version>${moneta.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.owasp.esapi</groupId>
+            <artifactId>esapi</artifactId>
+            <version>${esapi.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.sun.messaging.mq</groupId>
+            <artifactId>fscontext</artifactId>
+            <version>${fscontext.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.codepoetics</groupId>
+            <artifactId>protonpack</artifactId>
+            <version>${protonpack.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>one.util</groupId>
+            <artifactId>streamex</artifactId>
+            <version>${streamex.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>io.vavr</groupId>
+            <artifactId>vavr</artifactId>
+            <version>${vavr.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.openjdk.jmh</groupId>
+            <artifactId>jmh-core</artifactId>
+            <version>${jmh-core.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.openjdk.jmh</groupId>
+            <artifactId>jmh-generator-annprocess</artifactId>
+            <version>${jmh-generator-annprocess.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.hsqldb</groupId>
+            <artifactId>hsqldb</artifactId>
+            <version>${hsqldb.version}</version>
+            <scope>runtime</scope>
+        </dependency>
+        <!-- https://mvnrepository.com/artifact/org.asynchttpclient/async-http-client -->
+        <dependency>
+            <groupId>org.asynchttpclient</groupId>
+            <artifactId>async-http-client</artifactId>
+            <version>${async-http-client.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.opencsv</groupId>
+            <artifactId>opencsv</artifactId>
+            <version>${opencsv.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <!-- Mime Type Resolution Libraries -->
+        <dependency>
+            <groupId>org.apache.tika</groupId>
+            <artifactId>tika-core</artifactId>
+            <version>${tika.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>net.sf.jmimemagic</groupId>
+            <artifactId>jmimemagic</artifactId>
+            <version>${jmime-magic.version}</version>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <finalName>core-java-io-2</finalName>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+        <plugins>
+            <plugin>
+                <groupId>org.codehaus.mojo</groupId>
+                <artifactId>exec-maven-plugin</artifactId>
+                <version>${exec-maven-plugin.version}</version>
+                <configuration>
+                    <executable>java</executable>
+                    <mainClass>com.baeldung.outofmemoryerror.OutOfMemoryGCLimitExceed</mainClass>
+                    <arguments>
+                        <argument>-Xmx300m</argument>
+                        <argument>-XX:+UseParallelGC</argument>
+                        <argument>-classpath</argument>
+                        <classpath />
+                        <argument>com.baeldung.outofmemoryerror.OutOfMemoryGCLimitExceed</argument>
+                    </arguments>
+                </configuration>
+            </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-javadoc-plugin</artifactId>
+                <version>${maven-javadoc-plugin.version}</version>
+                <configuration>
+                    <source>${maven.compiler.source}</source>
+                    <target>${maven.compiler.target}</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <profiles>
+        <profile>
+            <id>integration</id>
+            <build>
+                <plugins>
+                    <plugin>
+                        <groupId>org.codehaus.mojo</groupId>
+                        <artifactId>exec-maven-plugin</artifactId>
+
+                        <executions>
+                            <execution>
+                                <id>run-benchmarks</id>
+                                <!-- <phase>integration-test</phase> -->
+                                <phase>none</phase>
+                                <goals>
+                                    <goal>exec</goal>
+                                </goals>
+                                <configuration>
+                                    <classpathScope>test</classpathScope>
+                                    <executable>java</executable>
+                                    <arguments>
+                                        <argument>-classpath</argument>
+                                        <classpath />
+                                        <argument>org.openjdk.jmh.Main</argument>
+                                        <argument>.*</argument>
+                                    </arguments>
+                                </configuration>
+                            </execution>
+                        </executions>
+                    </plugin>
+                </plugins>
+            </build>
+        </profile>
+    </profiles>
+
+    <properties>
+
+        <!-- util -->
+        <bouncycastle.version>1.55</bouncycastle.version>
+        <commons-codec.version>1.10</commons-codec.version>
+        <commons-math3.version>3.6.1</commons-math3.version>
+        <decimal4j.version>1.0.3</decimal4j.version>
+        <commons-collections4.version>4.1</commons-collections4.version>
+        <collections-generic.version>4.01</collections-generic.version>
+        <unix4j.version>0.4</unix4j.version>
+        <grep4j.version>1.8.7</grep4j.version>
+        <fscontext.version>4.6-b01</fscontext.version>
+        <protonpack.version>1.13</protonpack.version>
+        <streamex.version>0.6.5</streamex.version>
+        <vavr.version>0.9.0</vavr.version>
+        <opencsv.version>4.1</opencsv.version>
+        <!-- testing -->
+        <assertj.version>3.6.1</assertj.version>
+        <avaitility.version>1.7.0</avaitility.version>
+
+        <!-- maven plugins -->
+        <maven-javadoc-plugin.version>3.0.0-M1</maven-javadoc-plugin.version>
+        <hsqldb.version>2.4.0</hsqldb.version>
+        <esapi.version>2.1.0.1</esapi.version>
+        <jmh-generator-annprocess.version>1.19</jmh-generator-annprocess.version>
+        <async-http-client.version>2.4.5</async-http-client.version>
+        <!-- Mime Type Libraries -->
+        <tika.version>1.18</tika.version>
+        <jmime-magic.version>0.1.5</jmime-magic.version>
+    </properties>
+
+</project>

core-java-modules/core-java-io-2/src/main/java/com/baeldung/filereader/FileReaderExample.java

@@ -0,0 +1,57 @@
+package com.baeldung.filereader;
+
+import java.io.*;
+
+public class FileReaderExample {
+
+    public static String readAllCharactersOneByOne(Reader reader) throws IOException {
+        StringBuilder content = new StringBuilder();
+        int nextChar;
+        while ((nextChar = reader.read()) != -1) {
+            content.append((char) nextChar);
+        }
+        return String.valueOf(content);
+    }
+
+    public static String readMultipleCharacters(Reader reader, int length) throws IOException {
+        char[] buffer = new char[length];
+        int charactersRead = reader.read(buffer, 0, length);
+
+
+        if (charactersRead != -1) {
+            return new String(buffer, 0, charactersRead);
+        } else {
+            return "";
+        }
+    }
+
+    public static String readFile(String path) {
+        FileReader fileReader = null;
+        try {
+            fileReader = new FileReader(path);
+            return readAllCharactersOneByOne(fileReader);
+        } catch (IOException e) {
+            e.printStackTrace();
+        } finally {
+            if (fileReader != null) {
+                try {
+                    fileReader.close();
+                } catch (IOException e) {
+                    e.printStackTrace();
+                }
+            }
+        }
+        return null;
+    }
+
+    public static String readFileUsingTryWithResources(String path) {
+        try (FileReader fileReader = new FileReader(path)) {
+            return readAllCharactersOneByOne(fileReader);
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+        return null;
+    }
+
+
+}

core-java-modules/core-java-io-2/src/test/java/com/baeldung/filereader/FileReaderExampleUnitTest.java

@@ -0,0 +1,62 @@
+package com.baeldung.filereader;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.io.File;
+import java.io.FileReader;
+import java.io.IOException;
+
+public class FileReaderExampleUnitTest {
+
+
+    private static final String FILE_PATH = "src/test/resources/HelloWorld.txt";
+
+
+    @Test
+    public void givenFileReader_whenReadAllCharacters_thenReturnsContent() throws IOException {
+        String expectedText = "Hello, World!";
+        File file = new File(FILE_PATH);
+        FileReader fileReader = null;
+        try {
+            fileReader = new FileReader(file);
+            String content = FileReaderExample.readAllCharactersOneByOne(fileReader);
+            Assert.assertEquals(expectedText, content);
+        } finally {
+            if (fileReader != null) {
+                fileReader.close();
+            }
+        }
+    }
+
+    @Test
+    public void givenFileReader_whenReadMultipleCharacters_thenReturnsContent() throws IOException {
+        String expectedText = "Hello";
+        File file = new File(FILE_PATH);
+        FileReader fileReader = null;
+        try {
+            fileReader = new FileReader(file);
+            String content = FileReaderExample.readMultipleCharacters(fileReader, 5);
+            Assert.assertEquals(expectedText, content);
+        } finally {
+            if (fileReader != null) {
+                fileReader.close();
+            }
+        }
+    }
+
+    @Test
+    public void whenReadFile_thenReturnsContent() {
+        String expectedText = "Hello, World!";
+        String content = FileReaderExample.readFile(FILE_PATH);
+        Assert.assertEquals(expectedText, content);
+    }
+
+    @Test
+    public void whenReadFileUsingTryWithResources_thenReturnsContent() {
+        String expectedText = "Hello, World!";
+        String content = FileReaderExample.readFileUsingTryWithResources(FILE_PATH);
+        Assert.assertEquals(expectedText, content);
+    }
+
+}

core-java-modules/core-java-io-2/src/test/resources/HelloWorld.txt

@@ -0,0 +1 @@
+Hello, World!

core-java-modules/core-java-io/src/test/java/org/baeldung/writetofile/JavaWriteToFileUnitTest.java

@@ -1,4 +1,4 @@
-package org.baeldung.java.io;
+package org.baeldung.writetofile;
 
 import static org.junit.Assert.assertEquals;
 

core-java-modules/core-java-jndi/pom.xml

@@ -0,0 +1,66 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>com.baeldung.jndi</groupId>
+    <artifactId>core-java-jndi</artifactId>
+    <version>1.0-SNAPSHOT</version>
+    <name>core-java-jndi</name>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-modules</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+        <relativePath>../../</relativePath>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+            <version>5.5.1</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-core</artifactId>
+            <version>5.0.9.RELEASE</version>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-context</artifactId>
+            <version>5.0.9.RELEASE</version>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-jdbc</artifactId>
+            <version>5.0.9.RELEASE</version>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-test</artifactId>
+            <version>5.0.9.RELEASE</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.h2database</groupId>
+            <artifactId>h2</artifactId>
+            <version>1.4.199</version>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>1.8</source>
+                    <target>1.8</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+</project>

core-java-modules/core-java-jndi/src/test/java/com/baeldung/jndi/JndiUnitTest.java

@@ -0,0 +1,60 @@
+package com.baeldung.jndi;
+
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+import org.springframework.jdbc.datasource.DriverManagerDataSource;
+import org.springframework.jndi.JndiTemplate;
+import org.springframework.mock.jndi.SimpleNamingContextBuilder;
+
+import javax.naming.*;
+import javax.sql.DataSource;
+
+import java.util.Enumeration;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class JndiUnitTest {
+
+    private static InitialContext ctx;
+    private static DriverManagerDataSource ds;
+
+    @BeforeAll
+    static void setUp() throws Exception {
+        SimpleNamingContextBuilder builder = new SimpleNamingContextBuilder();
+        ds = new DriverManagerDataSource("jdbc:h2:mem:mydb");
+        builder.activate();
+
+        JndiTemplate jndiTemplate = new JndiTemplate();
+        ctx = (InitialContext) jndiTemplate.getContext();
+    }
+
+    @Test
+    void givenACompositeName_whenAddingAnElement_thenNameIncludesIt() throws Exception {
+        Name objectName = new CompositeName("java:comp/env/jdbc");
+
+        Enumeration<String> elements = objectName.getAll();
+        while(elements.hasMoreElements()) {
+            System.out.println(elements.nextElement());
+        }
+
+        objectName.add("example");
+
+        assertEquals("env", objectName.get(1));
+        assertEquals("example", objectName.get(objectName.size() - 1));
+    }
+
+    @Test
+    void givenADataSource_whenAddingDriver_thenBind() throws Exception {
+        ds.setDriverClassName("org.h2.Driver");
+        ctx.bind("java:comp/env/jdbc/datasource", ds);
+    }
+
+    @Test
+    void givenContext_whenLookupByName_thenValidDataSource() throws Exception {
+        DataSource ds = (DataSource) ctx.lookup("java:comp/env/jdbc/datasource");
+
+        assertNotNull(ds);
+        assertNotNull(ds.getConnection());
+    }
+
+}

core-java-modules/core-java-jndi/src/test/java/com/baeldung/jndi/exceptions/JndiExceptionsUnitTest.java

@@ -0,0 +1,42 @@
+package com.baeldung.jndi.exceptions;
+
+import org.junit.jupiter.api.MethodOrderer;
+import org.junit.jupiter.api.Order;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.TestMethodOrder;
+import org.springframework.jndi.JndiTemplate;
+import org.springframework.mock.jndi.SimpleNamingContextBuilder;
+
+import javax.naming.InitialContext;
+import javax.naming.NameNotFoundException;
+import javax.naming.NoInitialContextException;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+@TestMethodOrder(MethodOrderer.OrderAnnotation.class)
+public class JndiExceptionsUnitTest {
+
+    @Test
+    @Order(1)
+    void givenNoContext_whenLookupObject_thenThrowNoInitialContext() {
+        assertThrows(NoInitialContextException.class, () -> {
+            JndiTemplate jndiTemplate = new JndiTemplate();
+            InitialContext ctx = (InitialContext) jndiTemplate.getContext();
+            ctx.lookup("java:comp/env/jdbc/datasource");
+        }).printStackTrace();
+    }
+
+    @Test
+    @Order(2)
+    void givenEmptyContext_whenLookupNotBounds_thenThrowNameNotFound() {
+        assertThrows(NameNotFoundException.class, () -> {
+            SimpleNamingContextBuilder builder = new SimpleNamingContextBuilder();
+            builder.activate();
+
+            JndiTemplate jndiTemplate = new JndiTemplate();
+            InitialContext ctx = (InitialContext) jndiTemplate.getContext();
+            ctx.lookup("badJndiName");
+        }).printStackTrace();
+    }
+
+}

core-java-modules/core-java-lambdas/pom.xml

@@ -5,7 +5,7 @@
   <modelVersion>4.0.0</modelVersion>
   <artifactId>core-java-lambdas</artifactId>
   <version>0.1.0-SNAPSHOT</version>
-  <name>core-java</name>
+  <name>core-java-lambdas</name>
   <packaging>jar</packaging>
 
   <parent>

core-java-modules/core-java-lang-oop-2/pom.xml

@@ -14,6 +14,19 @@
         <relativePath>../../parent-java</relativePath>
     </parent>
 
+	<dependencies>
+		<dependency>
+			<groupId>com.h2database</groupId>
+			<artifactId>h2</artifactId>
+			<version>${h2.version}</version>
+			<scope>test</scope>
+		</dependency>
+	</dependencies>
+
+    <properties>
+        <h2.version>1.4.199</h2.version>
+    </properties>
+    
     <build>
         <finalName>core-java-lang-oop-2</finalName>
         <resources>

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/accessmodifiers/publicmodifier/ListOfThree.java

@@ -0,0 +1,31 @@
+package com.baeldung.accessmodifiers.publicmodifier;
+
+import java.util.AbstractList;
+import java.util.Arrays;
+
+public class ListOfThree<E> extends AbstractList<E> {
+
+    private static final int LENGTH = 3;
+    private Object[] elements;
+    
+    public ListOfThree(E[] data) {
+        if(data == null 
+            || data.length != LENGTH)
+            throw new IllegalArgumentException();
+        
+        this.elements = Arrays.copyOf(data, data.length); //shallow copy
+        
+    }
+    
+    @Override
+    @SuppressWarnings("unchecked")
+    public E get(int index) {      
+        return (E)elements[index];
+    }
+    
+    @Override
+    public int size() {
+        return LENGTH;
+    }
+
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/accessmodifiers/publicmodifier/SpecialCharacters.java

@@ -0,0 +1,7 @@
+package com.baeldung.accessmodifiers.publicmodifier;
+
+public class SpecialCharacters {
+
+    public static final String SLASH = "/";
+    
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/accessmodifiers/publicmodifier/Student.java

@@ -0,0 +1,67 @@
+package com.baeldung.accessmodifiers.publicmodifier;
+
+import java.math.BigDecimal;
+import java.sql.Connection;
+import java.sql.DriverManager;
+import java.sql.SQLException;
+import java.util.Date;
+
+public class Student {
+
+    private StudentGrade grade; //new data representation
+//    private int grade; //old data representation
+    private String name;   
+    private int age;
+    
+    public void setGrade(int grade) {        
+        this.grade = new StudentGrade(grade);
+    }
+
+    public int getGrade() {
+        return this.grade.getGrade().intValue(); //int is returned for backward compatibility
+    }
+    
+    public Connection getConnection() throws SQLException {
+        
+        final String URL = "jdbc:h2:~/test";
+        return DriverManager.getConnection(URL, "sa", "");
+
+    }
+        
+    public void setAge(int age) {
+        if (age < 0 || age > 150) {
+            throw new IllegalArgumentException();
+        }
+        
+        this.age = age;
+    }
+
+    public int getAge() {
+        return age;
+    }
+    
+    @Override
+    public String toString() {
+        return this.name;
+    }
+
+    private class StudentGrade {
+        private BigDecimal grade = BigDecimal.ZERO;
+        private Date updatedAt;
+        
+        public StudentGrade(int grade) {
+            this.grade = new BigDecimal(grade);
+            this.updatedAt = new Date();
+        }
+        
+        public BigDecimal getGrade() {
+            return grade;
+        }
+        
+        public Date getDate() {
+            return updatedAt;
+        }
+        
+     }
+    
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/core/modifiers/Employee.java

@@ -0,0 +1,59 @@
+package com.baeldung.core.modifiers;
+
+public class Employee {
+
+    private String privateId;
+    private String name;
+    private boolean manager;
+
+    public Employee(String id, String name) {
+        setPrivateId(id);
+        setName(name);
+    }
+
+    private Employee(String id, String name, boolean managerAttribute) {
+        this.privateId = id;
+        this.name = name;
+        this.privateId = id + "_ID-MANAGER";
+    }
+
+    public void setPrivateId(String customId) {
+        if (customId.endsWith("_ID")) {
+            this.privateId = customId;
+        } else {
+            this.privateId = customId + "_ID";
+        }
+    }
+
+    public String getPrivateId() {
+        return privateId;
+    }
+
+    public boolean isManager() {
+        return manager;
+    }
+
+    public void elevateToManager() {
+        if ("Carl".equals(this.name)) {
+            setManager(true);
+        }
+    }
+
+    private void setManager(boolean manager) {
+        this.manager = manager;
+    }
+
+    
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+    
+    public static Employee buildManager(String id, String name) {
+        return new Employee(id, name, true);
+    }
+    
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/core/modifiers/ExampleClass.java

@@ -0,0 +1,10 @@
+package com.baeldung.core.modifiers;
+
+public class ExampleClass {
+
+    public static void main(String[] args) {
+        Employee employee = new Employee("Bob","ABC123");
+        employee.setPrivateId("BCD234");
+        System.out.println(employee.getPrivateId());
+    }
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/core/modifiers/PublicOuterClass.java

@@ -0,0 +1,16 @@
+package com.baeldung.core.modifiers;
+
+public class PublicOuterClass {
+
+    public PrivateInnerClass getInnerClassInstance() {
+        PrivateInnerClass myPrivateClassInstance = this.new PrivateInnerClass();
+        myPrivateClassInstance.id = "ID1";
+        myPrivateClassInstance.name = "Bob";
+        return myPrivateClassInstance;
+    }
+
+    private class PrivateInnerClass {
+        public String name;
+        public String id;
+    }
+}

core-java-modules/core-java-lang-oop-2/src/test/java/com/baeldung/accessmodifiers/PublicAccessModifierUnitTest.java

@@ -0,0 +1,93 @@
+package com.baeldung.accessmodifiers;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.TestInstance;
+import org.junit.jupiter.api.TestInstance.Lifecycle;
+import com.baeldung.accessmodifiers.publicmodifier.ListOfThree;
+import com.baeldung.accessmodifiers.publicmodifier.Student;
+
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import java.math.BigDecimal;
+import java.sql.Connection;
+import java.sql.DriverManager;
+import java.sql.SQLException;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+@TestInstance(Lifecycle.PER_CLASS)
+public class PublicAccessModifierUnitTest {
+
+    @Test
+    public void whenUsingBigDecimalIntValueMethod_correspondingIntIsReturned() {
+        assertEquals(0, new BigDecimal(0).intValue()); //instance member
+    }
+    
+    @Test
+    public void whenUsingIntegerMaxValueField_maxPossibleIntValueIsReturned() {
+        assertEquals(2147483647, Integer.MAX_VALUE); //static field
+    }
+    
+    @Test
+    public void whenChangingStudentInternalRepresentation_clientCodeWillNotBreak() {
+        
+        Student student = new Student();
+        student.setGrade(100);
+        
+        assertEquals(100, student.getGrade());
+    }
+    
+    @Test
+    public void whenUsingEntrySet_keyValuePairsAreReturned() {
+        
+        Map<String, String> mapObject = new HashMap<String, String>();
+        mapObject.put("name", "Alex");
+       
+        for(Map.Entry<String, String> entry : mapObject.entrySet()) {
+            assertEquals("name", entry.getKey());
+            assertEquals("Alex", entry.getValue());
+        }
+        
+    }
+    
+    @Test
+    public void whenUsingStringToLowerCase_stringTurnsToLowerCase() {
+        assertEquals("alex", "ALEX".toLowerCase());
+    }
+
+    @Test
+    public void whenParsingStringOne_parseIntReturns1() {
+        assertEquals(1, Integer.parseInt("1"));
+    }
+    
+    @Test
+    public void whenConnectingToH2_connectionInstanceIsReturned() throws SQLException {
+
+        final String url = "jdbc:h2:~/test";
+        Connection conn = DriverManager.getConnection(url, "sa", "");
+        assertNotNull(conn);
+    }
+
+    @Test
+    public void whenCreatingCustomList_concreteAndInheritedMethodsWork() {
+
+        String[] dataSet1 = new String[] {"zero", "one", "two"};
+        
+        List<String> list1 = new ListOfThree<String>(dataSet1);
+        
+        //our implemented methods
+        assertEquals("one", list1.get(1));
+        assertEquals(3, list1.size());
+
+        //inherited implementations
+        assertEquals(1, list1.indexOf("one"));
+        
+        String[] dataSet2 = new String[] {"two", "zero", "one"};
+        List<String> list2 = new ListOfThree<String>(dataSet2);
+        
+        assertTrue(list1.containsAll(list2));
+    }
+    
+}

core-java-modules/core-java-networking-2/README.md

@@ -0,0 +1,3 @@
+### Relevant Articles
+
+- [Checking if a URL Exists in Java](https://www.baeldung.com/java-check-url-exists)

core-java-modules/core-java-networking/src/main/java/com/baeldung/httprequest/FullResponseBuilder.java

@@ -1,4 +1,4 @@
-package com.baeldung.http;
+package com.baeldung.httprequest;
 
 import java.io.BufferedReader;
 import java.io.IOException;

core-java-modules/core-java-networking/src/main/java/com/baeldung/httprequest/ParameterStringBuilder.java

@@ -1,4 +1,4 @@
-package com.baeldung.http;
+package com.baeldung.httprequest;
 
 import java.io.UnsupportedEncodingException;
 import java.net.URLEncoder;

core-java-modules/core-java-networking/src/test/java/com/baeldung/httprequest/HttpRequestLiveTest.java

@@ -1,8 +1,11 @@
-package com.baeldung.http;
+package com.baeldung.httprequest;
 
 import org.apache.commons.lang3.StringUtils;
 import org.junit.Test;
 
+import com.baeldung.httprequest.FullResponseBuilder;
+import com.baeldung.httprequest.ParameterStringBuilder;
+
 import java.io.BufferedReader;
 import java.io.DataOutputStream;
 import java.io.IOException;

core-java-modules/core-java-optional/pom.xml

@@ -3,6 +3,7 @@
     <artifactId>core-java-optional</artifactId>
     <version>0.1.0-SNAPSHOT</version>
     <packaging>jar</packaging>
+    <name>core-java-optional</name>
 	
     <parent>
         <groupId>com.baeldung.core-java-modules</groupId>

core-java-modules/core-java-os/README.md

@@ -7,4 +7,4 @@ This module uses Java 9, so make sure to have the JDK 9 installed to run it.
 - [Java 9 Process API Improvements](http://www.baeldung.com/java-9-process-api)
 - [Guide to java.lang.Process API](https://www.baeldung.com/java-process-api)
 - [Guide to java.lang.ProcessBuilder API](https://www.baeldung.com/java-lang-processbuilder-api)
-
+- [Get the Current Working Directory in Java](https://www.baeldung.com/java-current-directory)

core-java-modules/core-java-os/src/main/java/com/baeldung/system/exit/SystemExitExample.java

@@ -0,0 +1,20 @@
+package com.baeldung.system.exit;
+
+import java.io.BufferedReader;
+import java.io.FileReader;
+import java.io.IOException;
+
+public class SystemExitExample {
+
+    public void readFile() {
+        try {
+            BufferedReader br = new BufferedReader(new FileReader("file.txt"));
+            System.out.println(br.readLine());
+            br.close();
+        } catch (IOException e) {
+            System.exit(2);
+        } finally {
+            System.out.println("Exiting the program");
+        }
+    }
+}

core-java-modules/core-java-os/src/test/java/com/baeldung/system/exit/SystemExitUnitTest.java

@@ -0,0 +1,63 @@
+package com.baeldung.system.exit;
+
+import static org.junit.Assert.assertEquals;
+
+import java.security.Permission;
+
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+public class SystemExitUnitTest {
+
+    protected static class ExitException extends SecurityException {
+
+        private static final long serialVersionUID = 1L;
+        public final int status;
+
+        public ExitException(int status) {
+            this.status = status;
+        }
+    }
+
+    private static class NoExitSecurityManager extends SecurityManager {
+        @Override
+        public void checkPermission(Permission perm) {
+        }
+
+        @Override
+        public void checkPermission(Permission perm, Object context) {
+        }
+
+        @Override
+        public void checkExit(int status) {
+            super.checkExit(status);
+            throw new ExitException(status);
+        }
+    }
+
+    private SecurityManager securityManager;
+
+    private SystemExitExample example;
+
+    @Before
+    public void setUp() throws Exception {
+        example = new SystemExitExample();
+        securityManager = System.getSecurityManager();
+        System.setSecurityManager(new NoExitSecurityManager());
+    }
+
+    @After
+    public void tearDown() throws Exception {
+        System.setSecurityManager(securityManager);
+    }
+
+    @Test
+    public void testExit() throws Exception {
+        try {
+            example.readFile();
+        } catch (ExitException e) {
+            assertEquals("Exit status", 2, e.status);
+        }
+    }
+}

gradle-5/build.gradle

@@ -0,0 +1,54 @@
+plugins {
+    id "application"
+}
+apply plugin :"java"
+
+description = "Java MainClass execution examples"
+group = "com.baeldung"
+version = "0.0.1"
+sourceCompatibility = "1.8"
+targetCompatibility = "1.8"
+
+ext {
+   javaMainClass = "com.baeldung.gradle.exec.MainClass"
+}
+
+jar {
+    manifest {
+        attributes(
+            "Main-Class": javaMainClass
+        )
+    }
+}
+
+application {
+    mainClassName = javaMainClass
+}
+
+task runWithJavaExec(type: JavaExec) {
+    group = "Execution"
+    description = "Run the main class with JavaExecTask"
+    classpath = sourceSets.main.runtimeClasspath
+    main = javaMainClass
+}
+
+task runWithExec(type: Exec) {
+    dependsOn build
+    group = "Execution"
+    description = "Run the main class with ExecTask"
+    commandLine "java", "-classpath", sourceSets.main.runtimeClasspath.getAsPath(), javaMainClass
+}
+
+task runWithExecJarExecutable(type: Exec) {
+    dependsOn jar
+    group = "Execution"
+    description = "Run the output executable jar with ExecTask"
+    commandLine "java", "-jar", jar.archiveFile.get()
+}
+
+task runWithExecJarOnClassPath(type: Exec) {
+    dependsOn jar
+    group = "Execution"
+    description = "Run the mainClass from the output jar in classpath with ExecTask"
+    commandLine "java", "-classpath", jar.archiveFile.get() , javaMainClass
+}

gradle-5/gradle/wrapper/gradle-wrapper.properties

@@ -0,0 +1,5 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.5.1-bin.zip

gradle-5/gradlew

@@ -0,0 +1,164 @@
+#!/usr/bin/env bash
+
+##############################################################################
+##
+##  Gradle start up script for UN*X
+##
+##############################################################################
+
+# Attempt to set APP_HOME
+# Resolve links: $0 may be a link
+PRG="$0"
+# Need this for relative symlinks.
+while [ -h "$PRG" ] ; do
+    ls=`ls -ld "$PRG"`
+    link=`expr "$ls" : '.*-> \(.*\)$'`
+    if expr "$link" : '/.*' > /dev/null; then
+        PRG="$link"
+    else
+        PRG=`dirname "$PRG"`"/$link"
+    fi
+done
+SAVED="`pwd`"
+cd "`dirname \"$PRG\"`/" >/dev/null
+APP_HOME="`pwd -P`"
+cd "$SAVED" >/dev/null
+
+APP_NAME="Gradle"
+APP_BASE_NAME=`basename "$0"`
+
+# Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+DEFAULT_JVM_OPTS=""
+
+# Use the maximum available, or set MAX_FD != -1 to use that value.
+MAX_FD="maximum"
+
+warn ( ) {
+    echo "$*"
+}
+
+die ( ) {
+    echo
+    echo "$*"
+    echo
+    exit 1
+}
+
+# OS specific support (must be 'true' or 'false').
+cygwin=false
+msys=false
+darwin=false
+nonstop=false
+case "`uname`" in
+  CYGWIN* )
+    cygwin=true
+    ;;
+  Darwin* )
+    darwin=true
+    ;;
+  MINGW* )
+    msys=true
+    ;;
+  NONSTOP* )
+    nonstop=true
+    ;;
+esac
+
+CLASSPATH=$APP_HOME/gradle/wrapper/gradle-wrapper.jar
+
+# Determine the Java command to use to start the JVM.
+if [ -n "$JAVA_HOME" ] ; then
+    if [ -x "$JAVA_HOME/jre/sh/java" ] ; then
+        # IBM's JDK on AIX uses strange locations for the executables
+        JAVACMD="$JAVA_HOME/jre/sh/java"
+    else
+        JAVACMD="$JAVA_HOME/bin/java"
+    fi
+    if [ ! -x "$JAVACMD" ] ; then
+        die "ERROR: JAVA_HOME is set to an invalid directory: $JAVA_HOME
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+    fi
+else
+    JAVACMD="java"
+    which java >/dev/null 2>&1 || die "ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+fi
+
+# Increase the maximum file descriptors if we can.
+if [ "$cygwin" = "false" -a "$darwin" = "false" -a "$nonstop" = "false" ] ; then
+    MAX_FD_LIMIT=`ulimit -H -n`
+    if [ $? -eq 0 ] ; then
+        if [ "$MAX_FD" = "maximum" -o "$MAX_FD" = "max" ] ; then
+            MAX_FD="$MAX_FD_LIMIT"
+        fi
+        ulimit -n $MAX_FD
+        if [ $? -ne 0 ] ; then
+            warn "Could not set maximum file descriptor limit: $MAX_FD"
+        fi
+    else
+        warn "Could not query maximum file descriptor limit: $MAX_FD_LIMIT"
+    fi
+fi
+
+# For Darwin, add options to specify how the application appears in the dock
+if $darwin; then
+    GRADLE_OPTS="$GRADLE_OPTS \"-Xdock:name=$APP_NAME\" \"-Xdock:icon=$APP_HOME/media/gradle.icns\""
+fi
+
+# For Cygwin, switch paths to Windows format before running java
+if $cygwin ; then
+    APP_HOME=`cygpath --path --mixed "$APP_HOME"`
+    CLASSPATH=`cygpath --path --mixed "$CLASSPATH"`
+    JAVACMD=`cygpath --unix "$JAVACMD"`
+
+    # We build the pattern for arguments to be converted via cygpath
+    ROOTDIRSRAW=`find -L / -maxdepth 1 -mindepth 1 -type d 2>/dev/null`
+    SEP=""
+    for dir in $ROOTDIRSRAW ; do
+        ROOTDIRS="$ROOTDIRS$SEP$dir"
+        SEP="|"
+    done
+    OURCYGPATTERN="(^($ROOTDIRS))"
+    # Add a user-defined pattern to the cygpath arguments
+    if [ "$GRADLE_CYGPATTERN" != "" ] ; then
+        OURCYGPATTERN="$OURCYGPATTERN|($GRADLE_CYGPATTERN)"
+    fi
+    # Now convert the arguments - kludge to limit ourselves to /bin/sh
+    i=0
+    for arg in "$@" ; do
+        CHECK=`echo "$arg"|egrep -c "$OURCYGPATTERN" -`
+        CHECK2=`echo "$arg"|egrep -c "^-"`                                 ### Determine if an option
+
+        if [ $CHECK -ne 0 ] && [ $CHECK2 -eq 0 ] ; then                    ### Added a condition
+            eval `echo args$i`=`cygpath --path --ignore --mixed "$arg"`
+        else
+            eval `echo args$i`="\"$arg\""
+        fi
+        i=$((i+1))
+    done
+    case $i in
+        (0) set -- ;;
+        (1) set -- "$args0" ;;
+        (2) set -- "$args0" "$args1" ;;
+        (3) set -- "$args0" "$args1" "$args2" ;;
+        (4) set -- "$args0" "$args1" "$args2" "$args3" ;;
+        (5) set -- "$args0" "$args1" "$args2" "$args3" "$args4" ;;
+        (6) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" ;;
+        (7) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" ;;
+        (8) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" "$args7" ;;
+        (9) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" "$args7" "$args8" ;;
+    esac
+fi
+
+# Split up the JVM_OPTS And GRADLE_OPTS values into an array, following the shell quoting and substitution rules
+function splitJvmOpts() {
+    JVM_OPTS=("$@")
+}
+eval splitJvmOpts $DEFAULT_JVM_OPTS $JAVA_OPTS $GRADLE_OPTS
+JVM_OPTS[${#JVM_OPTS[*]}]="-Dorg.gradle.appname=$APP_BASE_NAME"
+
+exec "$JAVACMD" "${JVM_OPTS[@]}" -classpath "$CLASSPATH" org.gradle.wrapper.GradleWrapperMain "$@"

gradle-5/gradlew.bat

@@ -0,0 +1,90 @@
+@if "%DEBUG%" == "" @echo off
+@rem ##########################################################################
+@rem
+@rem  Gradle startup script for Windows
+@rem
+@rem ##########################################################################
+
+@rem Set local scope for the variables with windows NT shell
+if "%OS%"=="Windows_NT" setlocal
+
+set DIRNAME=%~dp0
+if "%DIRNAME%" == "" set DIRNAME=.
+set APP_BASE_NAME=%~n0
+set APP_HOME=%DIRNAME%
+
+@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+set DEFAULT_JVM_OPTS=
+
+@rem Find java.exe
+if defined JAVA_HOME goto findJavaFromJavaHome
+
+set JAVA_EXE=java.exe
+%JAVA_EXE% -version >NUL 2>&1
+if "%ERRORLEVEL%" == "0" goto init
+
+echo.
+echo ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
+echo.
+echo Please set the JAVA_HOME variable in your environment to match the
+echo location of your Java installation.
+
+goto fail
+
+:findJavaFromJavaHome
+set JAVA_HOME=%JAVA_HOME:"=%
+set JAVA_EXE=%JAVA_HOME%/bin/java.exe
+
+if exist "%JAVA_EXE%" goto init
+
+echo.
+echo ERROR: JAVA_HOME is set to an invalid directory: %JAVA_HOME%
+echo.
+echo Please set the JAVA_HOME variable in your environment to match the
+echo location of your Java installation.
+
+goto fail
+
+:init
+@rem Get command-line arguments, handling Windows variants
+
+if not "%OS%" == "Windows_NT" goto win9xME_args
+if "%@eval[2+2]" == "4" goto 4NT_args
+
+:win9xME_args
+@rem Slurp the command line arguments.
+set CMD_LINE_ARGS=
+set _SKIP=2
+
+:win9xME_args_slurp
+if "x%~1" == "x" goto execute
+
+set CMD_LINE_ARGS=%*
+goto execute
+
+:4NT_args
+@rem Get arguments from the 4NT Shell from JP Software
+set CMD_LINE_ARGS=%$
+
+:execute
+@rem Setup the command line
+
+set CLASSPATH=%APP_HOME%\gradle\wrapper\gradle-wrapper.jar
+
+@rem Execute Gradle
+"%JAVA_EXE%" %DEFAULT_JVM_OPTS% %JAVA_OPTS% %GRADLE_OPTS% "-Dorg.gradle.appname=%APP_BASE_NAME%" -classpath "%CLASSPATH%" org.gradle.wrapper.GradleWrapperMain %CMD_LINE_ARGS%
+
+:end
+@rem End local scope for the variables with windows NT shell
+if "%ERRORLEVEL%"=="0" goto mainEnd
+
+:fail
+rem Set variable GRADLE_EXIT_CONSOLE if you need the _script_ return code instead of
+rem the _cmd.exe /c_ return code!
+if  not "" == "%GRADLE_EXIT_CONSOLE%" exit 1
+exit /b 1
+
+:mainEnd
+if "%OS%"=="Windows_NT" endlocal
+
+:omega

gradle-5/src/main/java/com/baeldung/gradle/exec/MainClass.java

@@ -0,0 +1,8 @@
+package com.baeldung.gradle.exec;
+
+public class MainClass {
+
+    public static void main(String[] args) {
+        System.out.println("Goodbye cruel world ...");
+    }
+}

guava-collections/src/test/java/org/baeldung/guava/maps/initialize/GuavaMapInitializeUnitTest.java

@@ -1,4 +1,4 @@
-package org.baeldung.guava;
+package org.baeldung.guava.maps.initialize;
 
 import static org.hamcrest.core.IsEqual.equalTo;
 import static org.junit.Assert.*;

httpclient-simple/pom.xml

@@ -21,12 +21,12 @@
         <dependency>
             <groupId>org.springframework.security</groupId>
             <artifactId>spring-security-web</artifactId>
-            <version>${spring.version}</version>
+            <version>${spring-security.version}</version>
         </dependency>
         <dependency>
             <groupId>org.springframework.security</groupId>
             <artifactId>spring-security-config</artifactId>
-            <version>${spring.version}</version>
+            <version>${spring-security.version}</version>
         </dependency>
 
         <!-- Spring -->

jackson-2/README.md

@@ -11,3 +11,4 @@ The "REST With Spring" Classes: http://bit.ly/restwithspring
 - [Working with Tree Model Nodes in Jackson](https://www.baeldung.com/jackson-json-node-tree-model)
 - [Converting JSON to CSV in Java](https://www.baeldung.com/java-converting-json-to-csv)
 - [Compare Two JSON Objects with Jackson](https://www.baeldung.com/jackson-compare-two-json-objects)
+- [Calling Default Serializer from Custom Serializer in Jackson](https://www.baeldung.com/jackson-call-default-serializer-from-custom-serializer)

java-blockchain/.gitignore

@@ -0,0 +1,26 @@
+*.class
+
+0.*
+
+#folders#
+/target
+/neoDb*
+/data
+/src/main/webapp/WEB-INF/classes
+*/META-INF/*
+.resourceCache
+
+# Packaged files #
+*.jar
+*.war
+*.ear
+
+# Files generated by integration tests
+*.txt
+backup-pom.xml
+/bin/
+/temp
+
+#IntelliJ specific
+.idea/
+*.iml

java-blockchain/README.md

@@ -0,0 +1,6 @@
+=========
+
+## Basic Implementation of Blockchian in Java
+
+### Relevant Articles: 
+- []()

java-blockchain/pom.xml

@@ -0,0 +1,40 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <groupId>com.baeldung.blockchain</groupId>
+    <artifactId>java-blockchain</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>java-blockchain</name>
+    <packaging>jar</packaging>
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-java</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>../parent-java</relativePath>
+    </parent>
+    <build>
+        <finalName>java-blockchain</finalName>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>${maven-compiler-plugin.version}</version>
+                <configuration>
+                    <source>${maven.compiler.source}</source>
+                    <target>${maven.compiler.target}</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+    <properties>
+        <maven.compiler.source>1.8</maven.compiler.source>
+        <maven.compiler.target>1.8</maven.compiler.target>
+    </properties>
+</project>

java-blockchain/src/main/java/com/baeldung/blockchain/Block.java

@@ -0,0 +1,65 @@
+package com.baeldung.blockchain;
+
+import java.io.UnsupportedEncodingException;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.util.Date;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+public class Block {
+
+    private static Logger logger = Logger.getLogger(Block.class.getName());
+
+    private String hash;
+    private String previousHash;
+    private String data;
+    private long timeStamp;
+    private int nonce;
+
+    public Block(String data, String previousHash) {
+        this.data = data;
+        this.previousHash = previousHash;
+        this.timeStamp = new Date().getTime();
+        this.hash = calculateBlockHash();
+    }
+
+    public String mineBlock(int prefix) {
+        String prefixString = new String(new char[prefix]).replace('\0', '0');
+        while (!hash.substring(0, prefix)
+            .equals(prefixString)) {
+            nonce++;
+            hash = calculateBlockHash();
+        }
+        return hash;
+    }
+
+    public String calculateBlockHash() {
+        String dataToHash = previousHash + Long.toString(timeStamp) + Integer.toString(nonce) + data;
+        MessageDigest digest = null;
+        byte[] bytes = null;
+        try {
+            digest = MessageDigest.getInstance("SHA-256");
+            bytes = digest.digest(dataToHash.getBytes("UTF-8"));
+        } catch (NoSuchAlgorithmException | UnsupportedEncodingException ex) {
+            logger.log(Level.SEVERE, ex.getMessage());
+        }
+        StringBuffer buffer = new StringBuffer();
+        for (byte b : bytes) {
+            buffer.append(String.format("%02x", b));
+        }
+        return buffer.toString();
+    }
+
+    public String getHash() {
+        return this.hash;
+    }
+
+    public String getPreviousHash() {
+        return this.previousHash;
+    }
+
+    public void setData(String data) {
+        this.data = data;
+    }
+}

java-blockchain/src/test/java/com/baeldung/blockchain/BlockchainUnitTest.java

@@ -0,0 +1,67 @@
+package com.baeldung.blockchain;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+public class BlockchainUnitTest {
+
+    public static List<Block> blockchain = new ArrayList<Block>();
+    public static int prefix = 4;
+    public static String prefixString = new String(new char[prefix]).replace('\0', '0');
+
+    @BeforeClass
+    public static void setUp() {
+        Block genesisBlock = new Block("The is the Genesis Block.", "0");
+        genesisBlock.mineBlock(prefix);
+        blockchain.add(genesisBlock);
+        Block firstBlock = new Block("The is the First Block.", genesisBlock.getHash());
+        firstBlock.mineBlock(prefix);
+        blockchain.add(firstBlock);
+    }
+
+    @Test
+    public void givenBlockchain_whenNewBlockAdded_thenSuccess() {
+        Block newBlock = new Block("The is a New Block.", blockchain.get(blockchain.size() - 1)
+            .getHash());
+        newBlock.mineBlock(prefix);
+        assertTrue(newBlock.getHash()
+            .substring(0, prefix)
+            .equals(prefixString));
+        blockchain.add(newBlock);
+    }
+
+    @Test
+    public void givenBlockchain_whenValidated_thenSuccess() {
+        boolean flag = true;
+        for (int i = 0; i < blockchain.size(); i++) {
+            String previousHash = i == 0 ? "0"
+                : blockchain.get(i - 1)
+                    .getHash();
+            flag = blockchain.get(i)
+                .getHash()
+                .equals(blockchain.get(i)
+                    .calculateBlockHash())
+                && previousHash.equals(blockchain.get(i)
+                    .getPreviousHash())
+                && blockchain.get(i)
+                    .getHash()
+                    .substring(0, prefix)
+                    .equals(prefixString);
+            if (!flag)
+                break;
+        }
+        assertTrue(flag);
+    }
+
+    @AfterClass
+    public static void tearDown() {
+        blockchain.clear();
+    }
+
+}

java-blockchain/src/test/resources/.gitignore

@@ -0,0 +1,13 @@
+*.class
+
+#folders#
+/target
+/neoDb*
+/data
+/src/main/webapp/WEB-INF/classes
+*/META-INF/*
+
+# Packaged files #
+*.jar
+*.war
+*.ear