diff --git a/core-java/pom.xml b/core-java/pom.xml
index 84a56c8bc7..ee0d6b4b4a 100644
--- a/core-java/pom.xml
+++ b/core-java/pom.xml
@@ -417,4 +417,4 @@
         <maven-surefire-plugin.version>2.19.1</maven-surefire-plugin.version>
 
     </properties>
-</project>
\ No newline at end of file
+</project>
diff --git a/core-java/src/test/java/com/baeldung/temporaladjusters/CustomTemporalAdjusterTest.java b/core-java/src/test/java/com/baeldung/temporaladjusters/CustomTemporalAdjusterTest.java
index 9e5ba5b158..aed40002be 100644
--- a/core-java/src/test/java/com/baeldung/temporaladjusters/CustomTemporalAdjusterTest.java
+++ b/core-java/src/test/java/com/baeldung/temporaladjusters/CustomTemporalAdjusterTest.java
@@ -1,11 +1,15 @@
 package com.baeldung.temporaladjusters;
 
+import java.text.SimpleDateFormat;
+
 import java.time.DayOfWeek;
 import java.time.LocalDate;
 import java.time.Period;
 import java.time.temporal.TemporalAdjuster;
 import java.time.temporal.TemporalAdjusters;
 
+import java.util.Calendar;
+
 import org.junit.Assert;
 import org.junit.Test;
 
diff --git a/core-java/src/test/java/com/baeldung/temporaladjusters/TemporalAdjustersTest.java b/core-java/src/test/java/com/baeldung/temporaladjusters/TemporalAdjustersTest.java
index b80668e09b..f2e18e7cd0 100644
--- a/core-java/src/test/java/com/baeldung/temporaladjusters/TemporalAdjustersTest.java
+++ b/core-java/src/test/java/com/baeldung/temporaladjusters/TemporalAdjustersTest.java
@@ -4,9 +4,15 @@ import java.time.DayOfWeek;
 import java.time.LocalDate;
 import java.time.temporal.TemporalAdjusters;
 
+import java.text.SimpleDateFormat;
+
+import java.util.Calendar;
+
 import org.junit.Assert;
 import org.junit.Test;
 
+import com.baeldung.temporaladjuster.TemporalAdjusterUtil;
+
 public class TemporalAdjustersTest {
 
     @Test
diff --git a/guava/src/main/java/org/baeldung/guava/CustomEvent.java b/guava/src/main/java/org/baeldung/guava/CustomEvent.java
index 2d5c3382d4..8534d7da1c 100644
--- a/guava/src/main/java/org/baeldung/guava/CustomEvent.java
+++ b/guava/src/main/java/org/baeldung/guava/CustomEvent.java
@@ -3,11 +3,11 @@ package org.baeldung.guava;
 public class CustomEvent {
     private String action;
 
-    public CustomEvent(String action) {
+    CustomEvent(String action) {
         this.action = action;
     }
 
-    public String getAction() {
+    String getAction() {
         return action;
     }
 
diff --git a/guava/src/main/java/org/baeldung/guava/EventListener.java b/guava/src/main/java/org/baeldung/guava/EventListener.java
index 438fcade63..60beebeea5 100644
--- a/guava/src/main/java/org/baeldung/guava/EventListener.java
+++ b/guava/src/main/java/org/baeldung/guava/EventListener.java
@@ -28,11 +28,11 @@ public class EventListener {
         eventsHandled++;
     }
 
-    public int getEventsHandled() {
+    int getEventsHandled() {
         return eventsHandled;
     }
 
-    public void resetEventsHandled() {
+    void resetEventsHandled() {
         eventsHandled = 0;
     }
 }
diff --git a/guava/src/test/java/org/baeldung/guava/BloomFilterTest.java b/guava/src/test/java/org/baeldung/guava/BloomFilterTest.java
new file mode 100644
index 0000000000..d7c25b7c8d
--- /dev/null
+++ b/guava/src/test/java/org/baeldung/guava/BloomFilterTest.java
@@ -0,0 +1,54 @@
+package org.baeldung.guava;
+
+
+import com.google.common.hash.BloomFilter;
+import com.google.common.hash.Funnels;
+import org.junit.Test;
+
+import java.util.stream.IntStream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class BloomFilterTest {
+
+    @Test
+    public void givenBloomFilter_whenAddNStringsToIt_thenShouldNotReturnAnyFalsePositive() {
+        //when
+        BloomFilter<Integer> filter = BloomFilter.create(
+                Funnels.integerFunnel(),
+                500,
+                0.01);
+
+        //when
+        filter.put(1);
+        filter.put(2);
+        filter.put(3);
+
+        //then
+        // the probability that it returns true, but is actually false is 1%
+        assertThat(filter.mightContain(1)).isTrue();
+        assertThat(filter.mightContain(2)).isTrue();
+        assertThat(filter.mightContain(3)).isTrue();
+
+        assertThat(filter.mightContain(100)).isFalse();
+    }
+
+    @Test
+    public void givenBloomFilter_whenAddNStringsToItMoreThanDefinedExpectedInsertions_thenItWillReturnTrueForAlmostAllElements() {
+        //when
+        BloomFilter<Integer> filter = BloomFilter.create(
+                Funnels.integerFunnel(),
+                5,
+                0.01);
+
+        //when
+        IntStream.range(0, 100_000).forEach(filter::put);
+
+
+        //then
+        assertThat(filter.mightContain(1)).isTrue();
+        assertThat(filter.mightContain(2)).isTrue();
+        assertThat(filter.mightContain(3)).isTrue();
+        assertThat(filter.mightContain(1_000_000)).isTrue();
+    }
+}
diff --git a/guava/src/test/java/org/baeldung/guava/RateLimiterLongRunningUnitTest.java b/guava/src/test/java/org/baeldung/guava/RateLimiterLongRunningUnitTest.java
new file mode 100644
index 0000000000..914de01a6d
--- /dev/null
+++ b/guava/src/test/java/org/baeldung/guava/RateLimiterLongRunningUnitTest.java
@@ -0,0 +1,82 @@
+package org.baeldung.guava;
+
+
+import com.google.common.util.concurrent.RateLimiter;
+import org.junit.Test;
+
+import java.time.ZonedDateTime;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.IntStream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class RateLimiterLongRunningUnitTest {
+
+    @Test
+    public void givenLimitedResource_whenUseRateLimiter_thenShouldLimitPermits() {
+        //given
+        RateLimiter rateLimiter = RateLimiter.create(100);
+
+        //when
+        long startTime = ZonedDateTime.now().getSecond();
+        IntStream.range(0, 1000).forEach(i -> {
+            rateLimiter.acquire();
+            doSomeLimitedOperation();
+        });
+        long elapsedTimeSeconds = ZonedDateTime.now().getSecond() - startTime;
+
+        //then
+        assertThat(elapsedTimeSeconds >= 10);
+    }
+
+    @Test
+    public void givenLimitedResource_whenRequestTwice_thenShouldPermitWithoutBlocking() {
+        //given
+        RateLimiter rateLimiter = RateLimiter.create(2);
+
+        //when
+        long startTime = ZonedDateTime.now().getSecond();
+        rateLimiter.acquire(1);
+        doSomeLimitedOperation();
+        rateLimiter.acquire(1);
+        doSomeLimitedOperation();
+        long elapsedTimeSeconds = ZonedDateTime.now().getSecond() - startTime;
+
+        //then
+        assertThat(elapsedTimeSeconds <= 1);
+    }
+
+    @Test
+    public void givenLimitedResource_whenRequestOnce_thenShouldPermitWithoutBlocking() {
+        //given
+        RateLimiter rateLimiter = RateLimiter.create(100);
+
+        //when
+        long startTime = ZonedDateTime.now().getSecond();
+        rateLimiter.acquire(100);
+        doSomeLimitedOperation();
+        long elapsedTimeSeconds = ZonedDateTime.now().getSecond() - startTime;
+
+        //then
+        assertThat(elapsedTimeSeconds <= 1);
+    }
+
+    @Test
+    public void givenLimitedResource_whenTryAcquire_shouldNotBlockIndefinitely() {
+        //given
+        RateLimiter rateLimiter = RateLimiter.create(1);
+
+        //when
+        rateLimiter.acquire();
+        boolean result = rateLimiter.tryAcquire(2, 10, TimeUnit.MILLISECONDS);
+
+        //then
+        assertThat(result).isFalse();
+
+    }
+
+    private void doSomeLimitedOperation() {
+        //some computing
+    }
+
+}
diff --git a/libraries/pom.xml b/libraries/pom.xml
index a3b78f1695..0373d54429 100644
--- a/libraries/pom.xml
+++ b/libraries/pom.xml
@@ -349,6 +349,12 @@
             <artifactId>java-lsh</artifactId>
             <version>${java-lsh.version}</version>
         </dependency>
+        <!-- https://mvnrepository.com/artifact/com.google.guava/guava -->
+        <dependency>
+            <groupId>com.google.guava</groupId>
+            <artifactId>guava</artifactId>
+            <version>21.0</version>
+        </dependency>
         <dependency>
             <groupId>au.com.dius</groupId>
             <artifactId>pact-jvm-consumer-junit_2.11</artifactId>
diff --git a/libraries/src/main/java/com/baeldung/awaitility/AsyncService.java b/libraries/src/main/java/com/baeldung/awaitility/AsyncService.java
index 3bc82afaac..adfa3e1fb2 100644
--- a/libraries/src/main/java/com/baeldung/awaitility/AsyncService.java
+++ b/libraries/src/main/java/com/baeldung/awaitility/AsyncService.java
@@ -8,25 +8,23 @@ public class AsyncService {
     private final int DELAY = 1000;
     private final int INIT_DELAY = 2000;
 
-    private AtomicLong value = new AtomicLong(0);
-    private Executor executor = Executors.newFixedThreadPool(4);
+    private final AtomicLong value = new AtomicLong(0);
+    private final Executor executor = Executors.newFixedThreadPool(4);
     private volatile boolean initialized = false;
 
-    public void initialize() {
+    void initialize() {
         executor.execute(() -> {
             sleep(INIT_DELAY);
             initialized = true;
         });
     }
 
-    public boolean isInitialized() {
+    boolean isInitialized() {
         return initialized;
     }
 
-    public void addValue(long val) {
-        if (!isInitialized()) {
-            throw new IllegalStateException("Service is not initialized");
-        }
+    void addValue(long val) {
+        throwIfNotInitialized();
         executor.execute(() -> {
             sleep(DELAY);
             value.addAndGet(val);
@@ -34,9 +32,7 @@ public class AsyncService {
     }
 
     public long getValue() {
-        if (!isInitialized()) {
-            throw new IllegalStateException("Service is not initialized");
-        }
+        throwIfNotInitialized();
         return value.longValue();
     }
 
@@ -47,4 +43,10 @@ public class AsyncService {
             e.printStackTrace();
         }
     }
+
+    private void throwIfNotInitialized() {
+        if (!initialized) {
+            throw new IllegalStateException("Service is not initialized");
+        }
+    }
 }
diff --git a/libraries/src/main/java/com/baeldung/zip/ZipCollectionExample.java b/libraries/src/main/java/com/baeldung/zip/ZipCollectionExample.java
new file mode 100644
index 0000000000..3df9baad69
--- /dev/null
+++ b/libraries/src/main/java/com/baeldung/zip/ZipCollectionExample.java
@@ -0,0 +1,40 @@
+package com.baeldung.zip;
+
+import com.google.common.collect.Streams;
+import org.jooq.lambda.Seq;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.IntStream;
+
+public class ZipCollectionExample {
+    static List<String> names = Arrays.asList("John", "Jane", "Jack", "Dennis");
+
+    static List<Integer> ages = Arrays.asList(24, 25, 27);
+
+    public static void main(String[] args) {
+        //      Using Streams API from Guava 21
+        Streams
+          .zip(names.stream(), ages.stream(), (name, age) -> name + ":" + age)
+          .forEach(System.out::println);
+
+        //        Using native Java 8 Int Stream
+        IntStream
+          .range(0, Math.min(names.size(), ages.size()))
+          .mapToObj(i -> names.get(i) + ":" + ages.get(i))
+          .forEach(System.out::println);
+
+        //      Using jOOL
+        Seq
+          .of("John", "Jane", "Dennis")
+          .zip(Seq.of(24, 25, 27));
+
+        Seq
+          .of("John", "Jane", "Dennis")
+          .zip(Seq.of(24, 25, 27), (x, y) -> x + ":" + y);
+
+        Seq
+          .of("a", "b", "c")
+          .zipWithIndex();
+    }
+}
diff --git a/libraries/src/test/java/com/baeldung/awaitility/AsyncServiceUnitTest.java b/libraries/src/test/java/com/baeldung/awaitility/AsyncServiceUnitTest.java
index 4f07d556b9..677d989fba 100644
--- a/libraries/src/test/java/com/baeldung/awaitility/AsyncServiceUnitTest.java
+++ b/libraries/src/test/java/com/baeldung/awaitility/AsyncServiceUnitTest.java
@@ -1,19 +1,20 @@
 package com.baeldung.awaitility;
 
+import org.awaitility.Awaitility;
+import org.awaitility.Duration;
+import org.junit.Before;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.TimeUnit;
+
 import static org.awaitility.Awaitility.await;
 import static org.awaitility.Awaitility.fieldIn;
 import static org.awaitility.Awaitility.given;
 import static org.awaitility.proxy.AwaitilityClassProxy.to;
 import static org.hamcrest.Matchers.equalTo;
 
-import java.util.concurrent.Callable;
-import java.util.concurrent.TimeUnit;
-
-import org.awaitility.Awaitility;
-import org.awaitility.Duration;
-import org.junit.Before;
-import org.junit.Test;
-
 public class AsyncServiceUnitTest {
     private AsyncService asyncService;
 
@@ -25,18 +26,18 @@ public class AsyncServiceUnitTest {
     @Test
     public void givenAsyncService_whenInitialize_thenInitOccurs1() {
         asyncService.initialize();
-        Callable<Boolean> isInitialized = () -> asyncService.isInitialized();
+        Callable<Boolean> isInitialized = asyncService::isInitialized;
         await().until(isInitialized);
     }
 
     @Test
     public void givenAsyncService_whenInitialize_thenInitOccurs2() {
         asyncService.initialize();
-        Callable<Boolean> isInitialized = () -> asyncService.isInitialized();
+        Callable<Boolean> isInitialized = asyncService::isInitialized;
         await().atLeast(Duration.ONE_HUNDRED_MILLISECONDS)
-                .atMost(Duration.FIVE_SECONDS)
-                .with().pollInterval(Duration.ONE_HUNDRED_MILLISECONDS)
-                .until(isInitialized);
+          .atMost(Duration.FIVE_SECONDS)
+          .with().pollInterval(Duration.ONE_HUNDRED_MILLISECONDS)
+          .until(isInitialized);
     }
 
     @Test
@@ -46,7 +47,7 @@ public class AsyncServiceUnitTest {
         Awaitility.setDefaultTimeout(Duration.ONE_MINUTE);
 
         asyncService.initialize();
-        await().until(() -> asyncService.isInitialized());
+        await().until(asyncService::isInitialized);
     }
 
     @Test
@@ -59,14 +60,14 @@ public class AsyncServiceUnitTest {
     public void givenAsyncService_whenInitialize_thenInitOccurs3() {
         asyncService.initialize();
         await().until(fieldIn(asyncService)
-                .ofType(boolean.class)
-                .andWithName("initialized"), equalTo(true));
+          .ofType(boolean.class)
+          .andWithName("initialized"), equalTo(true));
     }
 
     @Test
     public void givenValue_whenAddValue_thenValueAdded() {
         asyncService.initialize();
-        await().until(() -> asyncService.isInitialized());
+        await().until(asyncService::isInitialized);
         long value = 5;
         asyncService.addValue(value);
         await().until(asyncService::getValue, equalTo(value));
@@ -76,9 +77,9 @@ public class AsyncServiceUnitTest {
     public void givenAsyncService_whenGetValue_thenExceptionIgnored() {
         asyncService.initialize();
         given().ignoreException(IllegalStateException.class)
-                .await()
-                .atMost(Duration.FIVE_SECONDS)
-                .atLeast(Duration.FIVE_HUNDRED_MILLISECONDS)
-                .until(asyncService::getValue, equalTo(0L));
+          .await()
+          .atMost(Duration.FIVE_SECONDS)
+          .atLeast(Duration.FIVE_HUNDRED_MILLISECONDS)
+          .until(asyncService::getValue, equalTo(0L));
     }
 }
diff --git a/libraries/src/test/java/com/baeldung/commons/collections/CollectionUtilsGuideTest.java b/libraries/src/test/java/com/baeldung/commons/collections/CollectionUtilsGuideTest.java
index 9166865484..90e30b01dc 100644
--- a/libraries/src/test/java/com/baeldung/commons/collections/CollectionUtilsGuideTest.java
+++ b/libraries/src/test/java/com/baeldung/commons/collections/CollectionUtilsGuideTest.java
@@ -54,8 +54,10 @@ public class CollectionUtilsGuideTest {
     
     @Test
     public void givenListOfCustomers_whenTransformed_thenListOfAddress() {
-        Collection<Address> addressCol = CollectionUtils.collect(list1, customer -> {
-            return new Address(customer.getLocality(), customer.getCity(), customer.getZip());
+        Collection<Address> addressCol = CollectionUtils.collect(list1, new Transformer<Customer, Address>() {
+            public Address transform(Customer customer) {
+                return new Address(customer.getLocality(), customer.getCity(), customer.getZip());
+            }
         });
         
         List<Address> addressList = new ArrayList<>(addressCol);
@@ -65,9 +67,13 @@ public class CollectionUtilsGuideTest {
     
     @Test
     public void givenCustomerList_whenFiltered_thenCorrectSize() {
-        
-        boolean isModified = CollectionUtils.filter(linkedList1, customer -> Arrays.asList("Daniel","Kyle").contains(customer.getName()));
-        
+
+        boolean isModified = CollectionUtils.filter(linkedList1, new Predicate<Customer>() {
+            public boolean evaluate(Customer customer) {
+                return Arrays.asList("Daniel","Kyle").contains(customer.getName());
+            }
+        });
+
         //filterInverse does the opposite. It removes the element from the list if the Predicate returns true
         //select and selectRejected work the same way except that they do not remove elements from the given collection and return a new collection 
         
diff --git a/libraries/src/test/java/com/baeldung/zip/ZipCollectionTest.java b/libraries/src/test/java/com/baeldung/zip/ZipCollectionTest.java
new file mode 100644
index 0000000000..6f6a7b765f
--- /dev/null
+++ b/libraries/src/test/java/com/baeldung/zip/ZipCollectionTest.java
@@ -0,0 +1,85 @@
+package com.baeldung.zip;
+
+import com.google.common.collect.Streams;
+import org.jooq.lambda.Seq;
+import org.jooq.lambda.tuple.Tuple2;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+import static org.junit.Assert.assertEquals;
+
+public class ZipCollectionTest {
+
+    List<String> names;
+    List<Integer> ages;
+    List<String> expectedOutput;
+
+    @Before
+    public void setUp() throws Exception {
+        names = Arrays.asList("John", "Jane", "Jack", "Dennis");
+        ages = Arrays.asList(24, 25, 27);
+        expectedOutput = Arrays.asList("John:24", "Jane:25", "Jack:27");
+    }
+
+    @Test
+    public void zipCollectionUsingGuava21() {
+        List<String> output = Streams
+          .zip(names.stream(), ages.stream(), (name, age) -> name + ":" + age)
+          .collect(Collectors.toList());
+
+        assertEquals(output, expectedOutput);
+    }
+
+    @Test
+    public void zipCollectionUsingIntStream() {
+        List<String> output = IntStream
+          .range(0, Math.min(names.size(), ages.size()))
+          .mapToObj(i -> names.get(i) + ":" + ages.get(i))
+          .collect(Collectors.toList());
+
+        assertEquals(output, expectedOutput);
+    }
+
+    @Test
+    public void zipCollectionUsingJool() {
+        Seq<String> output = Seq
+          .of("John", "Jane", "Jack")
+          .zip(Seq.of(24, 25, 27), (x, y) -> x + ":" + y);
+
+        assertEquals(output.toList(), expectedOutput);
+    }
+
+    @Test
+    public void zipCollectionUsingJoolTuple() {
+        Seq<Tuple2<String, Integer>> output = Seq
+          .of("John", "Jane", "Dennis")
+          .zip(Seq.of(24, 25, 27));
+
+        Tuple2<String, Integer> element1 = new Tuple2<String, Integer>("John", 24);
+        Tuple2<String, Integer> element2 = new Tuple2<String, Integer>("Jane", 25);
+        Tuple2<String, Integer> element3 = new Tuple2<String, Integer>("Dennis", 27);
+
+        List<Tuple2> expectedOutput = Arrays.asList(element1, element2, element3);
+        assertEquals(output.collect(Collectors.toList()), expectedOutput);
+    }
+
+    @Test
+    public void zipCollectionUsingJoolWithIndex() {
+        Seq<Tuple2<String, Long>> output = Seq
+          .of("John", "Jane", "Dennis")
+          .zipWithIndex();
+
+        Tuple2<String, Long> element1 = new Tuple2<>("John", 0L);
+        Tuple2<String, Long> element2 = new Tuple2<>("Jane", 1L);
+        Tuple2<String, Long> element3 = new Tuple2<>("Dennis", 2L);
+
+        List<Tuple2> expectedOutput = Arrays.asList(element1, element2, element3);
+        assertEquals(output.collect(Collectors.toList()), expectedOutput);
+    }
+
+}
\ No newline at end of file