Merge branch 'master' into bael-3090

README.md

@@ -1,12 +1,16 @@
 
-The "REST with Spring" Classes
+The Courses
 ==============================
 
-Here's the Master Class of REST With Spring (along with the newly announced Boot 2 material): <br/>
-**[>> THE REST WITH SPRING - MASTER CLASS](http://www.baeldung.com/rest-with-spring-course?utm_source=github&utm_medium=social&utm_content=tutorials&utm_campaign=rws#master-class)**
 
-And here's the Master Class of Learn Spring Security: <br/>
+Here's the new "Learn Spring" course: <br/>
-**[>> LEARN SPRING SECURITY - MASTER CLASS](http://www.baeldung.com/learn-spring-security-course?utm_source=github&utm_medium=social&utm_content=tutorials&utm_campaign=lss#master-class)**
+**[>> LEARN SPRING - THE MASTER CLASS](https://www.baeldung.com/learn-spring-course?utm_source=github&utm_medium=social&utm_content=tutorials&utm_campaign=ls#master-class)**
+
+Here's the Master Class of "REST With Spring" (along with the new announced Boot 2 material): <br/>
+**[>> THE REST WITH SPRING - MASTER CLASS](https://www.baeldung.com/rest-with-spring-course?utm_source=github&utm_medium=social&utm_content=tutorials&utm_campaign=rws#master-class)**
+
+And here's the Master Class of "Learn Spring Security": <br/>
+**[>> LEARN SPRING SECURITY - MASTER CLASS](https://www.baeldung.com/learn-spring-security-course?utm_source=github&utm_medium=social&utm_content=tutorials&utm_campaign=lss#master-class)**
 
 
 
@@ -15,7 +19,7 @@ Java and Spring Tutorials
 
 This project is **a collection of small and focused tutorials** - each covering a single and well defined area of development in the Java ecosystem. 
 A strong focus of these is, of course, the Spring Framework - Spring, Spring Boot and Spring Security. 
-In additional to Spring, the following technologies are in focus: `core Java`, `Jackson`, `HttpClient`, `Guava`. 
+In additional to Spring, the modules here are covering a number of aspects in Java. 
 
 
 Building the project
@@ -32,8 +36,15 @@ Running a Spring Boot module
 ====================
 To run a Spring Boot module run the command: `mvn spring-boot:run` in the module directory
 
-###Running Tests
 
+Working with the IDE
+====================
+This repo contains a large number of modules. 
+When you're working with an individual module, there's no need to import all of them (or build all of them) - you can simply import that particular module in either Eclipse or IntelliJ. 
+
+
+Running Tests
+=============
 The command `mvn clean install` will run the unit tests in a module.
 To run the integration tests, use the command `mvn clean install -Pintegration-lite-first`
 

algorithms-miscellaneous-3/README.md

@@ -1,4 +1,4 @@
-## Relevant articles:
+## Relevant Articles:
 
 - [Java Two Pointer Technique](https://www.baeldung.com/java-two-pointer-technique)
 - [Implementing Simple State Machines with Java Enums](https://www.baeldung.com/java-enum-simple-state-machine)

algorithms-miscellaneous-3/pom.xml

@@ -1,5 +1,5 @@
 <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">
+         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>algorithms-miscellaneous-3</artifactId>
     <version>0.0.1-SNAPSHOT</version>
@@ -20,15 +20,39 @@
         </dependency>
 
         <dependency>
-        	<groupId>org.apache.commons</groupId>
+            <groupId>org.apache.commons</groupId>
-        	<artifactId>commons-collections4</artifactId>
+            <artifactId>commons-collections4</artifactId>
-        	<version>${commons-collections4.version}</version>
+            <version>${commons-collections4.version}</version>
         </dependency>
 
         <dependency>
-        	<groupId>com.google.guava</groupId>
+            <groupId>com.google.guava</groupId>
-        	<artifactId>guava</artifactId>
+            <artifactId>guava</artifactId>
-        	<version>${guava.version}</version>
+            <version>${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>
+            <version>3.8.1</version>
+        </dependency>
+
+        <dependency>
+            <groupId>pl.pragmatists</groupId>
+            <artifactId>JUnitParams</artifactId>
+            <version>1.1.0</version>
+            <scope>test</scope>
         </dependency>
     </dependencies>
 
@@ -48,5 +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

@@ -0,0 +1,49 @@
+package com.baeldung.algorithms.printtriangles;
+
+import org.apache.commons.lang3.StringUtils;
+
+public class PrintTriangleExamples {
+
+    public static String printARightAngledTriangle(int N) {
+        StringBuilder result = new StringBuilder();
+        for (int r = 1; r <= N; r++) {
+            for (int j = 1; j <= r; j++) {
+                result.append("*");
+            }
+            result.append(System.lineSeparator());
+        }
+        return result.toString();
+    }
+
+    public static String printAnIsoscelesTriangle(int N) {
+        StringBuilder result = new StringBuilder();
+        for (int r = 1; r <= N; r++) {
+            for (int sp = 1; sp <= N - r; sp++) {
+                result.append(" ");
+            }
+            for (int c = 1; c <= (r * 2) - 1; c++) {
+                result.append("*");
+            }
+            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);
+
+        for (int r = 0; r < N; r++) {
+            result.append(helperString.substring(r, N + 2 * r));
+            result.append(System.lineSeparator());
+        }
+        return result.toString();
+    }
+
+    public static void main(String[] args) {
+        System.out.println(printARightAngledTriangle(5));
+        System.out.println(printAnIsoscelesTriangle(5));
+        System.out.println(printAnIsoscelesTriangleUsingSubstring(5));
+    }
+
+}

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/stringsortingbynumber/NaturalOrderComparators.java

@@ -0,0 +1,30 @@
+package com.baeldung.algorithms.stringsortingbynumber;
+
+import java.util.Comparator;
+
+public final class NaturalOrderComparators {
+
+    private static final String DIGIT_AND_DECIMAL_REGEX = "[^\\d.]";
+
+    private NaturalOrderComparators() {
+        throw new AssertionError("Let's keep this static");
+    }
+
+    public static Comparator<String> createNaturalOrderRegexComparator() {
+        return Comparator.comparingDouble(NaturalOrderComparators::parseStringToNumber);
+    }
+
+    private static double parseStringToNumber(String input){
+
+        final String digitsOnly = input.replaceAll(DIGIT_AND_DECIMAL_REGEX, "");
+
+        if("".equals(digitsOnly)) return 0;
+
+        try{
+            return Double.parseDouble(digitsOnly);
+        }catch (NumberFormatException nfe){
+            return 0;
+        }
+    }
+
+}

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

@@ -0,0 +1,93 @@
+package com.baeldung.algorithms.printtriangles;
+
+import junitparams.JUnitParamsRunner;
+import junitparams.Parameters;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import static org.junit.Assert.assertEquals;
+
+@RunWith(JUnitParamsRunner.class)
+public class PrintTriangleExamplesUnitTest {
+    
+    private static Object[][] rightAngledTriangles() {
+        String expected0 = ""; 
+        
+        String expected2 = "*" + System.lineSeparator() 
+                         + "**" + System.lineSeparator();
+        
+        String expected5 = "*" + System.lineSeparator() 
+                         + "**" + System.lineSeparator()
+                         + "***" + System.lineSeparator()
+                         + "****" + System.lineSeparator()
+                         + "*****" + System.lineSeparator();
+        
+        String expected7 = "*" + System.lineSeparator() 
+                         + "**" + System.lineSeparator()
+                         + "***" + System.lineSeparator()
+                         + "****" + System.lineSeparator()
+                         + "*****" + System.lineSeparator()
+                         + "******" + System.lineSeparator()
+                         + "*******" + System.lineSeparator();
+        
+        return new Object[][] {
+            { 0, expected0 },
+            { 2, expected2 },
+            { 5, expected5 },
+            { 7, expected7 }
+        };
+    }
+
+    @Test
+    @Parameters(method = "rightAngledTriangles")
+    public void whenPrintARightAngledTriangleIsCalled_ThenTheCorrectStringIsReturned(int nrOfRows, String expected) {
+        String actual = PrintTriangleExamples.printARightAngledTriangle(nrOfRows);
+
+        assertEquals(expected, actual);
+    }
+    
+    private static Object[][] isoscelesTriangles() {
+        String expected0 = ""; 
+        
+        String expected2 = " *" + System.lineSeparator() 
+                         + "***" + System.lineSeparator();
+        
+        String expected5 = "    *" + System.lineSeparator() 
+                         + "   ***" + System.lineSeparator()
+                         + "  *****" + System.lineSeparator()
+                         + " *******" + System.lineSeparator()
+                         + "*********" + System.lineSeparator();
+        
+        String expected7 = "      *" + System.lineSeparator() 
+                         + "     ***" + System.lineSeparator()
+                         + "    *****" + System.lineSeparator()
+                         + "   *******" + System.lineSeparator()
+                         + "  *********" + System.lineSeparator()
+                         + " ***********" + System.lineSeparator()
+                         + "*************" + System.lineSeparator();
+        
+        return new Object[][] {
+            { 0, expected0 },
+            { 2, expected2 },
+            { 5, expected5 },
+            { 7, expected7 }
+        };
+    }
+
+    @Test
+    @Parameters(method = "isoscelesTriangles")
+    public void whenPrintAnIsoscelesTriangleIsCalled_ThenTheCorrectStringIsReturned(int nrOfRows, String expected) {
+        String actual = PrintTriangleExamples.printAnIsoscelesTriangle(nrOfRows);
+
+        assertEquals(expected, actual);
+    }
+    
+    @Test
+    @Parameters(method = "isoscelesTriangles")
+    public void whenPrintAnIsoscelesTriangleUsingSubstringIsCalled_ThenTheCorrectStringIsReturned(int nrOfRows, String expected) {
+        String actual = PrintTriangleExamples.printAnIsoscelesTriangleUsingSubstring(nrOfRows);
+
+        assertEquals(expected, actual);
+    }
+
+}

algorithms-miscellaneous-3/src/test/java/com/baeldung/algorithms/stringsortingbynumber/NaturalOrderComparatorsUnitTest.java

@@ -0,0 +1,79 @@
+package com.baeldung.algorithms.stringsortingbynumber;
+
+import com.baeldung.algorithms.stringsortingbynumber.NaturalOrderComparators;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+import static org.junit.Assert.*;
+
+public class NaturalOrderComparatorsUnitTest {
+
+    @Test
+    public void givenSimpleStringsContainingIntsAndDoubles_whenSortedByRegex_checkSortingCorrect() {
+
+        List<String> testStrings = Arrays.asList("a1", "b3", "c4", "d2.2", "d2.4", "d2.3d");
+
+        testStrings.sort(NaturalOrderComparators.createNaturalOrderRegexComparator());
+
+        List<String> expected = Arrays.asList("a1", "d2.2", "d2.3d", "d2.4", "b3", "c4");
+
+        assertEquals(expected, testStrings);
+
+
+    }
+
+    @Test
+    public void givenSimpleStringsContainingIntsAndDoublesWithAnInvalidNumber_whenSortedByRegex_checkSortingCorrect() {
+
+        List<String> testStrings = Arrays.asList("a1", "b3", "c4", "d2.2", "d2.4", "d2.3.3d");
+
+        testStrings.sort(NaturalOrderComparators.createNaturalOrderRegexComparator());
+
+        List<String> expected = Arrays.asList("d2.3.3d", "a1", "d2.2", "d2.4", "b3", "c4");
+
+        assertEquals(expected, testStrings);
+
+
+    }
+
+    @Test
+    public void givenAllForseenProblems_whenSortedByRegex_checkSortingCorrect() {
+
+        List<String> testStrings = Arrays.asList("a1", "b3", "c4", "d2.2", "d2.f4", "d2.3.3d");
+
+        testStrings.sort(NaturalOrderComparators.createNaturalOrderRegexComparator());
+
+        List<String> expected = Arrays.asList("d2.3.3d", "a1", "d2.2", "d2.f4", "b3", "c4");
+
+        assertEquals(expected, testStrings);
+
+
+    }
+
+    @Test
+    public void givenComplexStringsContainingSeparatedNumbers_whenSortedByRegex_checkNumbersCondensedAndSorted() {
+
+        List<String> testStrings = Arrays.asList("a1b2c5", "b3ght3.2", "something65.thensomething5"); //125, 33.2, 65.5
+
+        List<String> expected = Arrays.asList("b3ght3.2", "something65.thensomething5", "a1b2c5" );
+
+        testStrings.sort(NaturalOrderComparators.createNaturalOrderRegexComparator());
+
+        assertEquals(expected, testStrings);
+
+    }
+
+    @Test
+    public void givenStringsNotContainingNumbers_whenSortedByRegex_checkOrderNotChanged() {
+
+        List<String> testStrings = Arrays.asList("a", "c", "d", "e");
+        List<String> expected = new ArrayList<>(testStrings);
+
+        testStrings.sort(NaturalOrderComparators.createNaturalOrderRegexComparator());
+
+        assertEquals(expected, testStrings);
+    }
+}

algorithms-sorting/README.md

@@ -5,3 +5,4 @@
 - [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)

algorithms-sorting/src/main/java/com/baeldung/algorithms/inoutsort/InOutSort.java

@@ -0,0 +1,25 @@
+package com.baeldung.algorithms.inoutsort;
+
+public class InOutSort {
+
+    public static int[] reverseInPlace(int A[]) {
+        int n = A.length;
+        for (int i = 0; i < n / 2; i++) {
+            int temp = A[i];
+            A[i] = A[n - 1 - i];
+            A[n - 1 - i] = temp;
+        }
+
+        return A;
+    }
+
+    public static int[] reverseOutOfPlace(int A[]) {
+        int n = A.length;
+        int[] B = new int[n];
+        for (int i = 0; i < n; i++) {
+            B[n - i - 1] = A[i];
+        }
+
+        return B;
+    }
+}

algorithms-sorting/src/main/java/com/baeldung/algorithms/selectionsort/SelectionSort.java

@@ -0,0 +1,38 @@
+package com.baeldung.algorithms.selectionsort;
+
+public class SelectionSort {
+
+    public static void sortAscending(final int[] arr) {
+        for (int i = 0; i < arr.length - 1; i++) {
+            int minElementIndex = i;
+            for (int j = i + 1; j < arr.length; j++) {
+                if (arr[minElementIndex] > arr[j]) {
+                    minElementIndex = j;
+                }
+            }
+
+            if (minElementIndex != i) {
+                int temp = arr[i];
+                arr[i] = arr[minElementIndex];
+                arr[minElementIndex] = temp;
+            }
+        }
+    }
+
+    public static void sortDescending(final int[] arr) {
+        for (int i = 0; i < arr.length - 1; i++) {
+            int maxElementIndex = i;
+            for (int j = i + 1; j < arr.length; j++) {
+                if (arr[maxElementIndex] < arr[j]) {
+                    maxElementIndex = j;
+                }
+            }
+
+            if (maxElementIndex != i) {
+                int temp = arr[i];
+                arr[i] = arr[maxElementIndex];
+                arr[maxElementIndex] = temp;
+            }
+        }
+    }
+}

algorithms-sorting/src/test/java/com/baeldung/algorithms/inoutsort/InOutSortUnitTest.java

@@ -0,0 +1,23 @@
+package com.baeldung.algorithms.inoutsort;
+
+import static org.junit.Assert.*;
+import static org.junit.Assert.assertArrayEquals;
+
+import org.junit.Test;
+
+public class InOutSortUnitTest {
+
+    @Test
+    public void givenArray_whenInPlaceSort_thenReversed() {
+        int[] input = {1, 2, 3, 4, 5, 6, 7};
+        int[] expected = {7, 6, 5, 4, 3, 2, 1};
+        assertArrayEquals("the two arrays are not equal", expected, InOutSort.reverseInPlace(input));
+    }
+
+    @Test
+    public void givenArray_whenOutOfPlaceSort_thenReversed() {
+        int[] input = {1, 2, 3, 4, 5, 6, 7};
+        int[] expected = {7, 6, 5, 4, 3, 2, 1};
+        assertArrayEquals("the two arrays are not equal", expected, InOutSort.reverseOutOfPlace(input));
+    }
+}

algorithms-sorting/src/test/java/com/baeldung/algorithms/selectionsort/SelectionSortUnitTest.java

@@ -0,0 +1,25 @@
+package com.baeldung.algorithms.selectionsort;
+
+import static org.junit.Assert.*;
+import static org.junit.Assert.assertArrayEquals;
+
+import org.junit.Test;
+
+public class SelectionSortUnitTest {
+
+    @Test
+    public void givenUnsortedArray_whenSelectionSort_SortAscending_thenSortedAsc() {
+        int[] input = { 5, 4, 1, 6, 2 };
+        SelectionSort.sortAscending(input);
+        int[] expected = {1, 2, 4, 5, 6};
+        assertArrayEquals("the two arrays are not equal", expected, input);
+    }
+    
+    @Test
+    public void givenUnsortedArray_whenSelectionSort_SortDescending_thenSortedDesc() {
+        int[] input = { 5, 4, 1, 6, 2 };
+        SelectionSort.sortDescending(input);
+        int[] expected = {6, 5, 4, 2, 1};
+        assertArrayEquals("the two arrays are not equal", expected, input);
+    }
+}

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-groovy-2/src/main/groovy/com/baeldung/metaprogramming/Employee.groovy

@@ -0,0 +1,45 @@
+package com.baeldung.metaprogramming
+
+import groovy.transform.AutoClone
+import groovy.transform.Canonical
+import groovy.transform.EqualsAndHashCode
+import groovy.transform.ToString
+import groovy.transform.TupleConstructor
+import groovy.util.logging.*
+
+@Canonical
+@TupleConstructor
+@EqualsAndHashCode
+@ToString(includePackage=false, excludes=['id'])
+@Log 
+@AutoClone
+class Employee {
+
+    long id
+    String firstName
+    String lastName
+    int age
+
+    //method to catch missing property's getter
+    def propertyMissing(String propertyName) {
+        log.info "$propertyName is not available"
+        "property '$propertyName' is not available"
+    }
+
+    //method to catch missing property's setter
+    def propertyMissing(String propertyName, propertyValue) {
+        println "property '$propertyName' is not available"
+        log.info "$propertyName is not available"
+        "property '$propertyName' is not available"
+    }
+
+    def methodMissing(String methodName, def methodArgs) {
+        log.info "$methodName is not defined"
+        "method '$methodName' is not defined"
+    }
+    
+    def logEmp() {
+        log.info "Employee: $lastName, $firstName is of $age years age"
+    }
+
+}

core-groovy-2/src/main/groovy/com/baeldung/metaprogramming/extension/BasicExtensions.groovy

@@ -0,0 +1,30 @@
+package com.baeldung.metaprogramming.extension
+
+import com.baeldung.metaprogramming.Employee
+
+class BasicExtensions {
+    
+    static int getYearOfBirth(Employee self) {
+        return (new Date().getYear() + 1900) - self.age;
+    }
+    
+    static String capitalize(String self) {
+        return self.substring(0, 1).toUpperCase() + self.substring(1)
+    }
+
+    static void printCounter(Integer self) {
+        while (self>0) {
+            println self
+            self--
+        }
+    }
+    
+    static Long square(Long self) {
+        return self*self
+    }
+    
+    static BigDecimal cube(BigDecimal self) {
+        return self*self*self
+    }
+    
+}

core-groovy-2/src/main/groovy/com/baeldung/metaprogramming/extension/StaticEmployeeExtension.groovy

@@ -0,0 +1,10 @@
+package com.baeldung.metaprogramming.extension
+
+import com.baeldung.metaprogramming.Employee
+
+class StaticEmployeeExtension {
+    
+    static Employee getDefaultObj(Employee self) {
+        return new Employee(firstName: "firstName", lastName: "lastName", age: 20)
+    }
+}

core-groovy-2/src/main/resources/META-INF/services/org.codehaus.groovy.runtime.ExtensionModule

@@ -0,0 +1,4 @@
+moduleName=core-groovy-2
+moduleVersion=1.0-SNAPSHOT
+extensionClasses=com.baeldung.metaprogramming.extension.BasicExtensions
+staticExtensionClasses=com.baeldung.metaprogramming.extension.StaticEmployeeExtension

core-groovy-2/src/test/groovy/com/baeldung/metaprogramming/MetaprogrammingUnitTest.groovy

@@ -0,0 +1,118 @@
+package com.baeldung.metaprogramming
+
+import groovy.time.TimeCategory
+
+class MetaprogrammingUnitTest extends GroovyTestCase {
+
+    Employee emp = new Employee(firstName: "Norman", lastName: "Lewis")
+
+    void testPropertyMissing() {
+        assert emp.address == "property 'address' is not available"
+    }
+
+    void testMethodMissing() {
+        Employee emp = new Employee()
+        try {
+            emp.getFullName()
+        } catch(MissingMethodException e) {
+            println "method is not defined"
+        }
+        assert emp.getFullName() == "method 'getFullName' is not defined"
+    }
+
+    void testMetaClassProperty() {
+        Employee.metaClass.address = ""
+        emp = new Employee(firstName: "Norman", lastName: "Lewis", address: "US")
+        assert emp.address == "US"
+    }
+
+    void testMetaClassMethod() {
+        emp.metaClass.getFullName = {
+            "$lastName, $firstName"
+        }
+        assert emp.getFullName() == "Lewis, Norman"
+    }
+
+    void testMetaClassConstructor() {
+        try {
+            Employee emp = new Employee("Norman")
+        } catch(GroovyRuntimeException e) {
+            assert e.message == "Could not find matching constructor for: com.baeldung.metaprogramming.Employee(String)"
+        }
+
+        Employee.metaClass.constructor = { String firstName ->
+            new Employee(firstName: firstName)
+        }
+
+        Employee norman = new Employee("Norman")
+        assert norman.firstName == "Norman"
+        assert norman.lastName == null
+    }
+
+    void testJavaMetaClass() {
+        String.metaClass.capitalize = { String str ->
+            str.substring(0, 1).toUpperCase() + str.substring(1);
+        }
+        assert "norman".capitalize() == "Norman"
+    }
+
+    void testEmployeeExtension() {
+        Employee emp = new Employee(age: 28)
+        assert emp.getYearOfBirth() == 1991
+    }
+
+    void testJavaClassesExtensions() {
+        5.printCounter()
+
+        assert 40l.square() == 1600l
+
+        assert (2.98).cube() == 26.463592
+    }
+
+    void testStaticEmployeeExtension() {
+        assert Employee.getDefaultObj().firstName == "firstName"
+        assert Employee.getDefaultObj().lastName == "lastName"
+        assert Employee.getDefaultObj().age == 20
+    }
+
+    void testToStringAnnotation() {
+        Employee employee = new Employee()
+        employee.id = 1
+        employee.firstName = "norman"
+        employee.lastName = "lewis"
+        employee.age = 28
+
+        assert employee.toString() == "Employee(norman, lewis, 28)"
+    }
+
+    void testTupleConstructorAnnotation() {
+        Employee norman = new Employee(1, "norman", "lewis", 28)
+        assert norman.toString() == "Employee(norman, lewis, 28)"
+
+        Employee snape = new Employee(2, "snape")
+        assert snape.toString() == "Employee(snape, null, 0)"
+
+    }
+    
+    void testEqualsAndHashCodeAnnotation() {
+        Employee norman = new Employee(1, "norman", "lewis", 28)
+        Employee normanCopy = new Employee(1, "norman", "lewis", 28)
+        assert norman.equals(normanCopy)
+        assert norman.hashCode() == normanCopy.hashCode()
+    }
+    
+    void testAutoCloneAnnotation() {
+        try {
+            Employee norman = new Employee(1, "norman", "lewis", 28)
+            def normanCopy = norman.clone()
+            assert norman == normanCopy
+        } catch(CloneNotSupportedException e) {
+            e.printStackTrace()
+        }
+    }
+
+    void testLoggingAnnotation() {
+        Employee employee = new Employee(1, "Norman", "Lewis", 28)
+        employee.logEmp()
+    }
+}

core-java-arrays/README.MD

@@ -1,3 +0,0 @@
-## Relevant Articles
-
-- [Extending an Array’s Length](https://www.baeldung.com/java-array-add-element-at-the-end)

core-java-modules/core-java-12/README.md

@@ -1,3 +1,4 @@
-## Relevant articles:
+## Relevant Articles:
+
 
 - [String API Updates in Java 12](https://www.baeldung.com/java12-string-api)

core-java-modules/core-java-8-2/README.md

@@ -7,3 +7,4 @@
 - [How to Delay Code Execution in Java](https://www.baeldung.com/java-delay-code-execution)
 - [Run JAR Application With Command Line Arguments](https://www.baeldung.com/java-run-jar-with-arguments)
 - [Java 8 Stream skip() vs limit()](https://www.baeldung.com/java-stream-skip-vs-limit)
+- [Guide to Java BiFunction Interface](https://www.baeldung.com/java-bifunction-interface)

core-java-modules/core-java-8/README.md

@@ -4,14 +4,10 @@
 
 ### Relevant Articles: 
 - [Guide to Java 8’s Collectors](http://www.baeldung.com/java-8-collectors)
-- [Functional Interfaces in Java 8](http://www.baeldung.com/java-8-functional-interfaces)
-- [Java 8 – Powerful Comparison with Lambdas](http://www.baeldung.com/java-8-sort-lambda)
 - [New Features in Java 8](http://www.baeldung.com/java-8-new-features)
-- [Lambda Expressions and Functional Interfaces: Tips and Best Practices](http://www.baeldung.com/java-8-lambda-expressions-tips)
 - [The Double Colon Operator in Java 8](http://www.baeldung.com/java-8-double-colon-operator)
 - [Guide to Java 8 groupingBy Collector](http://www.baeldung.com/java-groupingby-collector)
 - [Strategy Design Pattern in Java 8](http://www.baeldung.com/java-strategy-pattern)
-- [Exceptions in Java 8 Lambda Expressions](http://www.baeldung.com/java-lambda-exceptions)
 - [Guide to Java 8 Comparator.comparing()](http://www.baeldung.com/java-8-comparator-comparing)
 - [Guide To Java 8 Optional](http://www.baeldung.com/java-optional)
 - [Guide to the Java 8 forEach](http://www.baeldung.com/foreach-java)
@@ -38,6 +34,5 @@
 - [Java @SafeVarargs Annotation](https://www.baeldung.com/java-safevarargs)
 - [Java @Deprecated Annotation](https://www.baeldung.com/java-deprecated)
 - [Java 8 Predicate Chain](https://www.baeldung.com/java-predicate-chain)
-- [Method References in Java](https://www.baeldung.com/java-method-references)
 - [Creating a Custom Annotation in Java](https://www.baeldung.com/java-custom-annotation)
 - [The Difference Between Collection.stream().forEach() and Collection.forEach()](https://www.baeldung.com/java-collection-stream-foreach)

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

@@ -16,7 +16,6 @@
 *.ear
 
 # Files generated by integration tests
-*.txt
 backup-pom.xml
 /bin/
 /temp

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

@@ -0,0 +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-arrays-2/pom.xml

@@ -0,0 +1,50 @@
+<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>
+    <artifactId>core-java-arrays-2</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-arrays-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>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+            <version>${commons-lang3.version}</version>
+        </dependency>
+        <!-- test scoped -->
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj-core.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <finalName>core-java-arrays-2</finalName>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+    </build>
+
+    <properties>
+
+        <!-- util -->
+        <commons-lang3.version>3.9</commons-lang3.version>
+        <!-- testing -->
+        <assertj-core.version>3.10.0</assertj-core.version>
+    </properties>
+
+</project>

core-java-modules/core-java-arrays-2/src/test/java/com/baeldung/array/looping/LoopDiagonallyUnitTest.java

@@ -4,7 +4,7 @@ import org.junit.Test;
 
 import static org.junit.Assert.assertEquals;
 
-public class LoopDiagonallyTest {
+public class LoopDiagonallyUnitTest {
 
     @Test
     public void twoArrayIsLoopedDiagonallyAsExpected() {

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

@@ -7,7 +7,7 @@
 - [Check if a Java Array Contains a Value](http://www.baeldung.com/java-array-contains-value)
 - [Initializing Arrays in Java](http://www.baeldung.com/java-initialize-array)
 - [Guide to the java.util.Arrays Class](http://www.baeldung.com/java-util-arrays)
-- [Jagged Arrays In Java](http://www.baeldung.com/java-jagged-arrays)
+- [Multi-Dimensional Arrays In Java](http://www.baeldung.com/java-jagged-arrays)
 - [Find Sum and Average in a Java Array](http://www.baeldung.com/java-array-sum-average)
 - [Arrays in Java: A Reference Guide](https://www.baeldung.com/java-arrays-guide)
 - [How to Invert an Array in Java](http://www.baeldung.com/java-invert-array)
@@ -16,3 +16,4 @@
 - [Sorting Arrays in Java](https://www.baeldung.com/java-sorting-arrays)
 - [Convert a Float to a Byte Array in Java](https://www.baeldung.com/java-convert-float-to-byte-array)
 - [Converting Between Stream and Array in Java](https://www.baeldung.com/java-stream-to-array)
+- [Removing an Element from an Array in Java](https://www.baeldung.com/java-array-remove-element)

core-java-modules/core-java-arrays/src/main/java/com/baeldung/array/SortedArrayChecker.java

@@ -2,7 +2,10 @@ package com.baeldung.array;
 
 import com.baeldung.arraycopy.model.Employee;
 
+import java.util.Comparator;
+
 public class SortedArrayChecker {
+
     boolean isSorted(int[] array, int length) {
         if (array == null || length < 2)
             return true;
@@ -22,7 +25,7 @@ public class SortedArrayChecker {
         return true;
     }
 
-    boolean isSorted(String[] array, int length) {
+    boolean isSorted(Comparable[] array, int length) {
         if (array == null || length < 2)
             return true;
 
@@ -32,40 +35,31 @@ public class SortedArrayChecker {
         return isSorted(array, length - 1);
     }
 
-boolean isSorted(String[] array) {
+    boolean isSorted(Comparable[] array) {
-    for (int i = 0; i < array.length - 1; ++i) {
-        if (array[i].compareTo(array[i + 1]) > 0)
-            return false;
-    }
-
-    return true;
-}
-
-    boolean isSortedByName(Employee[] array) {
         for (int i = 0; i < array.length - 1; ++i) {
-            if (array[i].getName().compareTo(array[i + 1].getName()) > 0)
+            if (array[i].compareTo(array[i + 1]) > 0)
                 return false;
         }
 
         return true;
     }
 
-boolean isSortedByAge(Employee[] array) {
+    boolean isSorted(Object[] array, Comparator comparator) {
-    for (int i = 0; i < array.length - 1; ++i) {
+        for (int i = 0; i < array.length - 1; ++i) {
-        if (array[i].getAge() > (array[i + 1].getAge()))
+            if (comparator.compare(array[i], (array[i + 1])) > 0)
-            return false;
+                return false;
+        }
+
+        return true;
     }
 
-    return true;
+    boolean isSorted(Object[] array, Comparator comparator, int length) {
-}
-
-    boolean isSortedByAge(Employee[] array, int length) {
         if (array == null || length < 2)
             return true;
 
-        if (array[length - 2].getAge() > array[length - 1].getAge())
+        if (comparator.compare(array[length - 2], array[length - 1]) > 0)
             return false;
 
-        return isSortedByAge(array, length - 1);
+        return isSorted(array, comparator, length - 1);
     }
 }

core-java-modules/core-java-arrays/src/test/java/com/baeldung/array/SortedArrayCheckerUnitTest.java

@@ -4,32 +4,33 @@ import com.baeldung.arraycopy.model.Employee;
 import org.junit.Before;
 import org.junit.Test;
 
+import java.util.Comparator;
+
 import static org.assertj.core.api.Assertions.assertThat;
 
-class SortedArrayCheckerUnitTest {
+public class SortedArrayCheckerUnitTest {
-
     private static final int[] INTEGER_SORTED = {1, 3, 5, 7, 9};
     private static final int[] INTEGER_NOT_SORTED = {1, 3, 11, 7};
 
     private static final String[] STRING_SORTED = {"abc", "cde", "fgh"};
     private static final String[] STRING_NOT_SORTED = {"abc", "fgh", "cde", "ijk"};
 
-    private final Employee[] EMPLOYEES_SORTED_BY_NAME = {
+    private static final Employee[] EMPLOYEES_SORTED_BY_NAME = {
             new Employee(1, "Carlos", 26),
             new Employee(2, "Daniel", 31),
             new Employee(3, "Marta", 27)};
 
-    private final Employee[] EMPLOYEES_NOT_SORTED_BY_NAME = {
+    private static final Employee[] EMPLOYEES_NOT_SORTED_BY_NAME = {
             new Employee(1, "Daniel", 31),
             new Employee(2, "Carlos", 26),
             new Employee(3, "Marta", 27)};
 
-    private final Employee[] EMPLOYEES_SORTED_BY_AGE = {
+    private static final Employee[] EMPLOYEES_SORTED_BY_AGE = {
             new Employee(1, "Carlos", 26),
             new Employee(2, "Marta", 27),
             new Employee(3, "Daniel", 31)};
 
-    private final Employee[] EMPLOYEES_NOT_SORTED_BY_AGE = {
+    private static final Employee[] EMPLOYEES_NOT_SORTED_BY_AGE = {
             new Employee(1, "Marta", 27),
             new Employee(2, "Carlos", 26),
             new Employee(3, "Daniel", 31)};
@@ -61,13 +62,18 @@ class SortedArrayCheckerUnitTest {
 
     @Test
     public void givenEmployeeArray_thenReturnIfItIsSortedOrNot() {
-        assertThat(sortedArrayChecker.isSortedByName(EMPLOYEES_SORTED_BY_NAME)).isEqualTo(true);
+        assertThat(sortedArrayChecker.isSorted(EMPLOYEES_SORTED_BY_NAME, Comparator.comparing(Employee::getName))).isEqualTo(true);
-        assertThat(sortedArrayChecker.isSortedByName(EMPLOYEES_NOT_SORTED_BY_NAME)).isEqualTo(false);
+        assertThat(sortedArrayChecker.isSorted(EMPLOYEES_NOT_SORTED_BY_NAME, Comparator.comparing(Employee::getName))).isEqualTo(false);
 
-        assertThat(sortedArrayChecker.isSortedByAge(EMPLOYEES_SORTED_BY_AGE)).isEqualTo(true);
+        assertThat(sortedArrayChecker.isSorted(EMPLOYEES_SORTED_BY_AGE, Comparator.comparingInt(Employee::getAge))).isEqualTo(true);
-        assertThat(sortedArrayChecker.isSortedByAge(EMPLOYEES_NOT_SORTED_BY_AGE)).isEqualTo(false);
+        assertThat(sortedArrayChecker.isSorted(EMPLOYEES_NOT_SORTED_BY_AGE, Comparator.comparingInt(Employee::getAge))).isEqualTo(false);
 
-        assertThat(sortedArrayChecker.isSortedByAge(EMPLOYEES_SORTED_BY_AGE, EMPLOYEES_SORTED_BY_AGE.length)).isEqualTo(true);
+        assertThat(sortedArrayChecker
-        assertThat(sortedArrayChecker.isSortedByAge(EMPLOYEES_NOT_SORTED_BY_AGE, EMPLOYEES_NOT_SORTED_BY_AGE.length)).isEqualTo(false);
+                .isSorted(EMPLOYEES_SORTED_BY_AGE, Comparator.comparingInt(Employee::getAge), EMPLOYEES_SORTED_BY_AGE.length))
+                .isEqualTo(true);
+        assertThat(sortedArrayChecker
+                .isSorted(EMPLOYEES_NOT_SORTED_BY_AGE, Comparator.comparingInt(Employee::getAge), EMPLOYEES_NOT_SORTED_BY_AGE.length))
+                .isEqualTo(false);
     }
+
 }

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

@@ -0,0 +1,11 @@
+package com.baeldung.concurrent.mutex;
+
+public class SequenceGenerator {
+    private int currentValue = 0;
+
+    public int getNextSequence() throws InterruptedException {
+        currentValue = currentValue + 1;
+        Thread.sleep(500);
+        return currentValue;
+    }
+}

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

@@ -0,0 +1,18 @@
+package com.baeldung.concurrent.mutex;
+
+import com.google.common.util.concurrent.Monitor;
+
+public class SequenceGeneratorUsingMonitor extends SequenceGenerator {
+
+    private Monitor monitor = new Monitor();
+
+    @Override
+    public int getNextSequence() throws InterruptedException {
+        monitor.enter();
+        try {
+            return super.getNextSequence();
+        } finally {
+            monitor.leave();
+        }
+    }
+}

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

@@ -0,0 +1,18 @@
+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() throws InterruptedException {
+        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 +1,18 @@
+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() throws InterruptedException {
+        try {
+            mutex.acquire();
+            return super.getNextSequence();
+        } finally {
+            mutex.release();
+        }
+    }
+}

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

@@ -0,0 +1,12 @@
+package com.baeldung.concurrent.mutex;
+
+public class SequenceGeneratorUsingSynchronizedBlock extends SequenceGenerator {
+
+    @Override
+    public int getNextSequence() throws InterruptedException {
+        synchronized (this) {
+            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() throws InterruptedException {
+        return super.getNextSequence();
+    }
+
+}

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

@@ -0,0 +1,81 @@
+package com.baeldung.concurrent.mutex;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+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;
+
+import com.baeldung.concurrent.mutex.SequenceGenerator;
+import com.baeldung.concurrent.mutex.SequenceGeneratorUsingMonitor;
+import com.baeldung.concurrent.mutex.SequenceGeneratorUsingReentrantLock;
+import com.baeldung.concurrent.mutex.SequenceGeneratorUsingSemaphore;
+import com.baeldung.concurrent.mutex.SequenceGeneratorUsingSynchronizedBlock;
+import com.baeldung.concurrent.mutex.SequenceGeneratorUsingSynchronizedMethod;
+
+public class MutexUnitTest {
+
+    private final int RANGE = 30;
+
+    @Test
+    public void givenUnsafeSequenceGenerator_whenRaceCondition_thenUnexpectedBehavior() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGenerator());
+        Assert.assertNotEquals(RANGE, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingSynchronizedMethod_whenRaceCondition_thenSuccess() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingSynchronizedMethod());
+        Assert.assertEquals(RANGE, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingSynchronizedBlock_whenRaceCondition_thenSuccess() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingSynchronizedBlock());
+        Assert.assertEquals(RANGE, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingReentrantLock_whenRaceCondition_thenSuccess() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingReentrantLock());
+        Assert.assertEquals(RANGE, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingSemaphore_whenRaceCondition_thenSuccess() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingSemaphore());
+        Assert.assertEquals(RANGE, uniqueSequences.size());
+    }
+
+    @Test
+    public void givenSequenceGeneratorUsingMonitor_whenRaceCondition_thenSuccess() throws Exception {
+        Set<Integer> uniqueSequences = getASetOFUniqueSequences(new SequenceGeneratorUsingMonitor());
+        Assert.assertEquals(RANGE, uniqueSequences.size());
+    }
+
+    private Set<Integer> getASetOFUniqueSequences(SequenceGenerator generator) throws Exception {
+        ExecutorService executor = Executors.newFixedThreadPool(3);
+        Set<Integer> uniqueSequences = new HashSet<>();
+        List<Future<Integer>> futures = new ArrayList<>();
+
+        for (int i = 0; i < RANGE; i++) {
+            futures.add(executor.submit(generator::getNextSequence));
+        }
+
+        for (Future<Integer> future : futures) {
+            uniqueSequences.add(future.get());
+        }
+
+        executor.awaitTermination(15, TimeUnit.SECONDS);
+        executor.shutdown();
+
+        return uniqueSequences;
+    }
+
+}

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

@@ -1,3 +1,3 @@
-## Relevant articles:
+## Relevant Articles:
 
 - [Will an Error Be Caught by Catch Block in Java?](https://www.baeldung.com/java-error-catch)

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

@@ -0,0 +1,65 @@
+<?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>
+
+    <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-jpms/README.md

@@ -0,0 +1,3 @@
+### Relevant Articles
+
+- [Design Strategies for Decoupling Java Modules](https://www.baeldung.com/java-modules-decoupling-design-strategies)

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

@@ -1,3 +1,8 @@
 ## Relevant articles:
 
 - [Why Do Local Variables Used in Lambdas Have to Be Final or Effectively Final?](https://www.baeldung.com/java-lambda-effectively-final-local-variables)
+- [Java 8 – Powerful Comparison with Lambdas](http://www.baeldung.com/java-8-sort-lambda)
+- [Functional Interfaces in Java 8](http://www.baeldung.com/java-8-functional-interfaces)
+- [Lambda Expressions and Functional Interfaces: Tips and Best Practices](http://www.baeldung.com/java-8-lambda-expressions-tips)
+- [Exceptions in Java 8 Lambda Expressions](http://www.baeldung.com/java-lambda-exceptions)
+- [Method References in Java](https://www.baeldung.com/java-method-references)

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

@@ -15,5 +15,11 @@
     <relativePath>../../parent-java</relativePath>
   </parent>
 
-
+  <dependencies>
+    <dependency>
+      <groupId>org.apache.commons</groupId>
+      <artifactId>commons-lang3</artifactId>
+      <version>${commons-lang3.version}</version>
+    </dependency>
+  </dependencies>
 </project>

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/relationships/aggregation/Car.java

@@ -0,0 +1,9 @@
+package com.baeldung.relationships.aggregation;
+
+import java.util.List;
+
+public class Car {
+
+    private List<Wheel> wheels;
+    
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/relationships/aggregation/CarWithStaticInnerWheel.java

@@ -0,0 +1,13 @@
+package com.baeldung.relationships.aggregation;
+
+import java.util.List;
+
+public class CarWithStaticInnerWheel {
+
+    private List<Wheel> wheels;
+
+    public static class Wheel {
+
+    }
+
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/relationships/aggregation/Wheel.java

@@ -0,0 +1,7 @@
+package com.baeldung.relationships.aggregation;
+
+public class Wheel {
+
+    private Car car;
+    
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/relationships/association/Child.java

@@ -0,0 +1,7 @@
+package com.baeldung.relationships.association;
+
+public class Child {
+
+    private Mother mother;
+
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/relationships/association/Mother.java

@@ -0,0 +1,9 @@
+package com.baeldung.relationships.association;
+
+import java.util.List;
+
+public class Mother {
+
+    private List<Child> children;
+
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/relationships/composition/Building.java

@@ -0,0 +1,18 @@
+package com.baeldung.relationships.composition;
+
+import java.util.List;
+
+public class Building {
+
+    private String address;
+    private List<Room> rooms;
+    
+    public class Room {
+        
+        public String getBuildingAddress() {
+            return Building.this.address;
+        }
+        
+    }
+    
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/relationships/composition/BuildingWithDefinitionRoomInMethod.java

@@ -0,0 +1,28 @@
+package com.baeldung.relationships.composition;
+
+public class BuildingWithDefinitionRoomInMethod {
+
+    public Room createAnonymousRoom() {
+        return new Room() {
+            @Override
+            public void doInRoom() {}
+        };
+    }
+
+    public Room createInlineRoom() {
+        class InlineRoom implements Room {
+            @Override
+            public void doInRoom() {}
+        }
+        return new InlineRoom();
+    }
+    
+    public Room createLambdaRoom() {
+        return () -> {};
+    }
+
+    public interface Room {
+        void doInRoom();
+    }
+
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/relationships/university/Department.java

@@ -0,0 +1,9 @@
+package com.baeldung.relationships.university;
+
+import java.util.List;
+
+public class Department {
+
+    private List<Professor> professors;
+
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/relationships/university/Professor.java

@@ -0,0 +1,10 @@
+package com.baeldung.relationships.university;
+
+import java.util.List;
+
+public class Professor {
+
+    private List<Department> department;
+    private List<Professor> friends;
+    
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/relationships/university/University.java

@@ -0,0 +1,9 @@
+package com.baeldung.relationships.university;
+
+import java.util.List;
+
+public class University {
+
+    private List<Department> department;
+    
+}

core-java-modules/core-java-lang-operators/pom.xml

@@ -0,0 +1,43 @@
+<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>
+    <artifactId>core-java-lang-operators</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-lang-operators</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>
+        <!-- test scoped -->
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj-core.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <finalName>core-java-lang-operators</finalName>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+    </build>
+
+    <properties>
+        <!-- testing -->
+        <assertj-core.version>3.10.0</assertj-core.version>
+    </properties>
+
+</project>