Merge pull request #3 from eugenp/master

sync with origin

apache-meecrowave/README.md

@@ -0,0 +1,3 @@
+### Relevant Articles: 
+================================
+- [Building a Microservice with Apache Meecrowave](http://www.baeldung.com/apache-meecrowave)

core-java-8/README.md

@@ -46,7 +46,6 @@
 - [Overview of Java Built-in Annotations](http://www.baeldung.com/java-default-annotations)
 - [Finding Min/Max in an Array with Java](http://www.baeldung.com/java-array-min-max)
 - [Internationalization and Localization in Java 8](http://www.baeldung.com/java-8-localization)
-- [Filtering Kotlin Collections](http://www.baeldung.com/kotlin-filter-collection)
 - [How to Find an Element in a List with Java](http://www.baeldung.com/find-list-element-java)
 - [Measure Elapsed Time in Java](http://www.baeldung.com/java-measure-elapsed-time)
 - [Java Optional – orElse() vs orElseGet()](http://www.baeldung.com/java-optional-or-else-vs-or-else-get)
@@ -55,3 +54,5 @@
 - [Java 8 Unsigned Arithmetic Support](http://www.baeldung.com/java-unsigned-arithmetic)
 - [How to Get the Start and the End of a Day using Java](http://www.baeldung.com/java-day-start-end)
 - [Generalized Target-Type Inference in Java](http://www.baeldung.com/java-generalized-target-type-inference)
+- [Image to Base64 String Conversion](http://www.baeldung.com/java-base64-image-string)
+- [Calculate Age in Java](http://www.baeldung.com/java-get-age)

core-java-8/pom.xml

@@ -99,6 +99,16 @@
             <artifactId>joda-time</artifactId>
             <version>${joda.version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.aspectj</groupId>
+            <artifactId>aspectjrt</artifactId>
+            <version>${asspectj.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.aspectj</groupId>
+            <artifactId>aspectjweaver</artifactId>
+            <version>${asspectj.version}</version>
+        </dependency>
     </dependencies>
 
     <build>
@@ -170,6 +180,7 @@
         <joda.version>2.10</joda.version>
         <!-- testing -->
         <assertj.version>3.6.1</assertj.version>
+        <asspectj.version>1.8.9</asspectj.version>
         <avaitility.version>1.7.0</avaitility.version>
         <jmh-core.version>1.19</jmh-core.version>
         <jmh-generator.version>1.19</jmh-generator.version>

core-java-8/src/main/java/com/baeldung/aspect/ChangeCallsToCurrentTimeInMillisMethod.aj

@@ -0,0 +1,9 @@
+package com.baeldung.aspect;
+
+public aspect ChangeCallsToCurrentTimeInMillisMethod {
+    long around():
+            call(public static native long java.lang.System.currentTimeMillis())
+                    && within(user.code.base.pckg.*) {
+        return 0;
+    }
+}

core-java-8/src/test/java/com/baeldung/list/AddElementsUnitTest.java

@@ -0,0 +1,87 @@
+package com.baeldung.list;
+
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.*;
+
+import static org.junit.Assert.*;
+
+public class AddElementsUnitTest {
+
+    List<Flower> flowers;
+
+    @Before
+    public void init() {
+        this.flowers = new ArrayList<>(Arrays.asList(
+                new Flower("Poppy", 12),
+                new Flower("Anemone", 8),
+                new Flower("Catmint", 12)));
+    }
+
+    @Test
+    public void givenAList_whenTargetListIsEmpty_thenReturnTargetListWithNewItems() {
+        List<Flower> anotherList = new ArrayList<>();
+        anotherList.addAll(flowers);
+
+        assertEquals(anotherList.size(), flowers.size());
+        Assert.assertTrue(anotherList.containsAll(flowers));
+    }
+
+    @Test
+    public void givenAList_whenTargetListIsEmpty_thenReturnTargetListWithOneModifiedElementByConstructor() {
+        List<Flower> anotherList = new ArrayList<>();
+        anotherList.addAll(flowers);
+
+        Flower flower = anotherList.get(0);
+        flower.setPetals(flowers.get(0).getPetals() * 3);
+
+        assertEquals(anotherList.size(), flowers.size());
+        Assert.assertTrue(anotherList.containsAll(flowers));
+    }
+
+    @Test
+    public void givenAListAndElements_whenUseCollectionsAddAll_thenAddElementsToTargetList() {
+        List<Flower> target = new ArrayList<>();
+
+        Collections.addAll(target, flowers.get(0), flowers.get(1), flowers.get(2), flowers.get(0));
+
+        assertEquals(target.size(), 4);
+    }
+
+    @Test
+    public void givenTwoList_whenSourceListDoesNotHaveNullElements_thenAddElementsToTargetListSkipFirstElementByStreamProcess() {
+        List<Flower> flowerVase = new ArrayList<>();
+
+        flowers.stream()
+                .skip(1)
+                .forEachOrdered(flowerVase::add);
+
+        assertEquals(flowerVase.size() + 1, flowers.size());
+        assertFalse(flowerVase.containsAll(flowers));
+    }
+
+    @Test
+    public void givenTwoList_whenSourceListDoesNotHaveNullElements_thenAddElementsToTargetListFilteringElementsByStreamProcess() {
+        List<Flower> flowerVase = new ArrayList<>();
+
+        flowers.stream()
+                .filter(f -> f.getPetals() > 10)
+                .forEachOrdered(flowerVase::add);
+
+        assertEquals(flowerVase.size() + 1, flowers.size());
+        assertFalse(flowerVase.containsAll(flowers));
+    }
+
+    @Test
+    public void givenAList_whenListIsNotNull_thenAddElementsToListByStreamProcessWihtOptional() {
+        List<Flower> target = new ArrayList<>();
+
+        Optional.ofNullable(flowers)
+                .ifPresent(target::addAll);
+
+        assertNotNull(target);
+        assertEquals(target.size(), 3);
+    }
+}

core-java-8/src/test/java/com/baeldung/util/CurrentDateTimeUnitTest.java

@@ -1,6 +1,7 @@
 package com.baeldung.util;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
 
 import java.time.Clock;
 import java.time.Instant;
@@ -9,6 +10,8 @@ import java.time.LocalTime;
 import java.time.ZoneId;
 import java.time.temporal.ChronoField;
 
+import org.joda.time.DateTime;
+import org.joda.time.DateTimeUtils;
 import org.junit.Test;
 
 public class CurrentDateTimeUnitTest {
@@ -39,5 +42,4 @@ public class CurrentDateTimeUnitTest {
 
         assertEquals(clock.instant().getEpochSecond(), now.getEpochSecond());
     }
-
 }

core-java-9/pom.xml

@@ -19,11 +19,23 @@
             <version>${awaitility.version}</version>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj.version}</version>
+            <scope>test</scope>
+        </dependency>
         <dependency>
             <groupId>com.google.guava</groupId>
             <artifactId>guava</artifactId>
             <version>${guava.version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.junit.platform</groupId>
+            <artifactId>junit-platform-runner</artifactId>
+            <version>${junit.platform.version}</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <build>
@@ -50,6 +62,8 @@
 
     <properties>
         <!-- testing -->
+        <assertj.version>3.10.0</assertj.version>
+        <junit.platform.version>1.2.0</junit.platform.version>
         <awaitility.version>1.7.0</awaitility.version>
         <maven.compiler.source>1.9</maven.compiler.source>
         <maven.compiler.target>1.9</maven.compiler.target>

core-java-9/src/main/java/com/baeldung/java9/language/stream/StreamsGroupingCollectionFilter.java

@@ -0,0 +1,27 @@
+package com.baeldung.java9.language.stream;
+
+import static java.util.stream.Collectors.filtering;
+import static java.util.stream.Collectors.groupingBy;
+import static java.util.stream.Collectors.toList;
+
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+
+public class StreamsGroupingCollectionFilter {
+
+    static public Map<Integer, List<Integer>> findEvenNumbersAfterGroupingByQuantityOfDigits(Collection<Integer> baseCollection) {
+        Function<Integer, Integer> getQuantityOfDigits = item -> (int) Math.log10(item) + 1;
+
+        return baseCollection.stream()
+            .collect(groupingBy(getQuantityOfDigits, filtering(item -> item % 2 == 0, toList())));
+    }
+
+    static public Map<Integer, List<Integer>> findEvenNumbersBeforeGroupingByQuantityOfDigits(Collection<Integer> baseCollection) {
+
+        return baseCollection.stream()
+            .filter(item -> item % 2 == 0)
+            .collect(groupingBy(item -> (int) Math.log10(item) + 1, toList()));
+    }
+}

core-java-9/src/test/java/com/baeldung/java9/language/stream/CollectionFilterUnitTest.java

@@ -0,0 +1,51 @@
+package com.baeldung.java9.language.stream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+import org.junit.platform.runner.JUnitPlatform;
+import org.junit.runner.RunWith;
+
+@RunWith(JUnitPlatform.class)
+public class CollectionFilterUnitTest {
+
+    private static final Collection<Integer> BASE_INTEGER_COLLECTION = Arrays.asList(9, 12, 55, 56, 101, 115, 8002, 223, 2668, 19, 8);
+    private static final Map<Integer, List<Integer>> EXPECTED_EVEN_FILTERED_AFTER_GROUPING_MAP = createExpectedFilterAfterGroupingMap();
+    private static Map<Integer, List<Integer>> createExpectedFilterAfterGroupingMap() {
+        Map<Integer, List<Integer>> map = new HashMap<>();
+        map.put(1, Arrays.asList(8));
+        map.put(2, Arrays.asList(12, 56));
+        map.put(3, Collections.emptyList());
+        map.put(4, Arrays.asList(8002, 2668));
+        return map;
+
+    }
+
+    private static final Map<Integer, List<Integer>> EXPECTED_EVEN_FILTERED_BEFORE_GROUPING_MAP = createExpectedFilterBeforeGroupingMap();
+    private static Map<Integer, List<Integer>> createExpectedFilterBeforeGroupingMap() {
+        Map<Integer, List<Integer>> map = new HashMap<>();
+        map.put(1, Arrays.asList(8));
+        map.put(2, Arrays.asList(12, 56));
+        map.put(4, Arrays.asList(8002, 2668));
+        return map;
+
+    }
+
+    @Test
+    public void givenAStringCollection_whenFilteringFourLetterWords_thenObtainTheFilteredCollection() {
+        Map<Integer, List<Integer>> filteredAfterGroupingMap = StreamsGroupingCollectionFilter.findEvenNumbersAfterGroupingByQuantityOfDigits(BASE_INTEGER_COLLECTION);
+        Map<Integer, List<Integer>> filteredBeforeGroupingMap = StreamsGroupingCollectionFilter.findEvenNumbersBeforeGroupingByQuantityOfDigits(BASE_INTEGER_COLLECTION);
+
+        assertThat(filteredAfterGroupingMap).containsAllEntriesOf(EXPECTED_EVEN_FILTERED_AFTER_GROUPING_MAP);
+        assertThat(filteredBeforeGroupingMap).doesNotContainKey(3)
+            .containsAllEntriesOf(EXPECTED_EVEN_FILTERED_BEFORE_GROUPING_MAP);
+    }
+
+}

core-java-collections/README.md

@@ -30,3 +30,4 @@
 - [How to TDD a List Implementation in Java](http://www.baeldung.com/java-test-driven-list)
 - [How to Store Duplicate Keys in a Map in Java?](http://www.baeldung.com/java-map-duplicate-keys)
 - [Getting the Size of an Iterable in Java](http://www.baeldung.com/java-iterable-size)
+- [Iterating Backward Through a List](http://www.baeldung.com/java-list-iterate-backwards)

core-java-collections/pom.xml

@@ -36,19 +36,37 @@
             <artifactId>commons-lang3</artifactId>
             <version>${commons-lang3.version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.eclipse.collections</groupId>
+            <artifactId>eclipse-collections-api</artifactId>
+            <version>${eclipse.collections.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.eclipse.collections</groupId>
+            <artifactId>eclipse-collections</artifactId>
+            <version>${eclipse.collections.version}</version>
+        </dependency>
         <dependency>
             <groupId>org.assertj</groupId>
             <artifactId>assertj-core</artifactId>
             <version>${assertj.version}</version>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.junit.platform</groupId>
+            <artifactId>junit-platform-runner</artifactId>
+            <version>${junit.platform.version}</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <properties>
+        <junit.platform.version>1.2.0</junit.platform.version>
         <commons-lang3.version>3.5</commons-lang3.version>
         <commons-collections4.version>4.1</commons-collections4.version>
         <collections-generic.version>4.01</collections-generic.version>
         <avaitility.version>1.7.0</avaitility.version>
         <assertj.version>3.6.1</assertj.version>
+        <eclipse.collections.version>9.2.0</eclipse.collections.version>
     </properties>
 </project>

core-java-collections/src/main/java/com/baeldung/convertlisttomap/ConvertListToMapService.java

@@ -1,13 +1,13 @@
 package com.baeldung.convertlisttomap;
 
+import com.google.common.collect.Maps;
+import org.apache.commons.collections4.MapUtils;
+
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.stream.Collectors;
 
-import org.apache.commons.collections4.MapUtils;
-import com.google.common.collect.Maps;
-
 public class ConvertListToMapService {
 
     public Map<Integer, Animal> convertListBeforeJava8(List<Animal> list) {

core-java-collections/src/main/java/com/baeldung/java/filtering/CollectionUtilsCollectionFilter.java

@@ -0,0 +1,16 @@
+package com.baeldung.java.filtering;
+
+import java.util.Collection;
+
+import org.apache.commons.collections4.CollectionUtils;
+import org.apache.commons.collections4.Predicate;
+
+public class CollectionUtilsCollectionFilter {
+
+    static public Collection<Integer> findEvenNumbers(Collection<Integer> baseCollection) {
+        Predicate<Integer> apacheEventNumberPredicate = item -> item % 2 == 0;
+
+        CollectionUtils.filter(baseCollection, apacheEventNumberPredicate);
+        return baseCollection;
+    }
+}

core-java-collections/src/main/java/com/baeldung/java/filtering/EclipseCollectionsCollectionFilter.java

@@ -0,0 +1,32 @@
+package com.baeldung.java.filtering;
+
+import java.util.Collection;
+
+import org.eclipse.collections.api.block.predicate.Predicate;
+import org.eclipse.collections.impl.factory.Lists;
+import org.eclipse.collections.impl.utility.Iterate;
+
+public class EclipseCollectionsCollectionFilter {
+
+    static public Collection<Integer> findEvenNumbers(Collection<Integer> baseCollection) {
+        Predicate<Integer> eclipsePredicate = item -> item % 2 == 0;
+        Collection<Integer> filteredList = Lists.mutable.ofAll(baseCollection)
+            .select(eclipsePredicate);
+
+        return filteredList;
+    }
+
+    static public Collection<Integer> findEvenNumbersUsingIterate(Collection<Integer> baseCollection) {
+        Predicate<Integer> eclipsePredicate = new Predicate<Integer>() {
+            private static final long serialVersionUID = 1L;
+
+            @Override
+            public boolean accept(Integer arg0) {
+                return arg0 % 2 == 0;
+            }
+        };
+        Collection<Integer> filteredList = Iterate.select(baseCollection, eclipsePredicate);
+
+        return filteredList;
+    }
+}

core-java-collections/src/main/java/com/baeldung/java/filtering/GuavaCollectionFilter.java

@@ -0,0 +1,17 @@
+package com.baeldung.java.filtering;
+
+import java.util.Collection;
+
+import com.google.common.base.Predicate;
+import com.google.common.collect.Collections2;
+
+public class GuavaCollectionFilter {
+
+    static public Collection<Integer> findEvenNumbers(Collection<Integer> baseCollection) {
+        Predicate<Integer> guavaPredicate = item -> item % 2 == 0;
+
+        Collection<Integer> filteredCollection = Collections2.filter(baseCollection, guavaPredicate);
+        return filteredCollection;
+    }
+
+}

core-java-collections/src/main/java/com/baeldung/java/filtering/StreamsCollectionFilter.java

@@ -0,0 +1,26 @@
+package com.baeldung.java.filtering;
+
+import java.util.Collection;
+import java.util.function.Predicate;
+import java.util.stream.Collectors;
+
+public class StreamsCollectionFilter {
+
+    public static <T> Collection<T> filterCollectionHelperMethod(Collection<T> baseCollection, Predicate<T> predicate) {
+        return baseCollection.stream()
+            .filter(predicate)
+            .collect(Collectors.toList());
+    }
+
+    static public Collection<Integer> findEvenNumbersUsingHelperMethod(Collection<Integer> baseCollection) {
+        return filterCollectionHelperMethod(baseCollection, item -> item % 2 == 0);
+    }
+
+    static public Collection<Integer> findEvenNumbers(Collection<Integer> baseCollection) {
+        Predicate<Integer> streamsPredicate = item -> item % 2 == 0;
+
+        return baseCollection.stream()
+            .filter(streamsPredicate)
+            .collect(Collectors.toList());
+    }
+}

core-java-collections/src/test/java/com/baeldung/convertlisttomap/ConvertListToMapServiceUnitTest.java

@@ -1,13 +1,14 @@
 package com.baeldung.convertlisttomap;
 
-import static org.hamcrest.MatcherAssert.assertThat;
-import static org.hamcrest.Matchers.*;
+import org.junit.Before;
+import org.junit.Test;
+
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 
-import org.junit.Before;
-import org.junit.Test;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.containsInAnyOrder;
 
 public class ConvertListToMapServiceUnitTest {
     List<Animal> list;

core-java-collections/src/test/java/com/baeldung/convertlisttomap/ConvertListWithDiplicatedIdToMapServiceUnitTest.java

@@ -1,15 +1,15 @@
 package com.baeldung.convertlisttomap;
 
-import static org.hamcrest.MatcherAssert.assertThat;
-import static org.hamcrest.Matchers.hasItem;
-import static org.hamcrest.Matchers.hasSize;
+import org.junit.Before;
+import org.junit.Test;
 
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 
-import org.junit.Before;
-import org.junit.Test;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.hasItem;
+import static org.hamcrest.Matchers.hasSize;
 
 public class ConvertListWithDiplicatedIdToMapServiceUnitTest {
     List<Animal> duplicatedIdList;

core-java-collections/src/test/java/com/baeldung/java/filtering/CollectionFiltersUnitTest.java

@@ -0,0 +1,44 @@
+package com.baeldung.java.filtering;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+
+import org.junit.jupiter.api.Test;
+import org.junit.platform.runner.JUnitPlatform;
+import org.junit.runner.RunWith;
+
+@RunWith(JUnitPlatform.class)
+public class CollectionFiltersUnitTest {
+
+    private static final Collection<Integer> BASE_INTEGER_COLLECTION = Arrays.asList(9, 14, 2, 7, 1, 5, 8);
+    private static final Collection<Integer> EXPECTED_EVEN_FILTERED_COLLECTION = Arrays.asList(14, 2, 8);
+
+    @Test
+    public void givenAStringCollection_whenFilteringFourLetterWords_thenObtainTheFilteredCollection() {
+        final Collection<String> baseStrings = Arrays.asList("java", "baeldung", "type", "example", "other");
+
+        Collection<String> filtered = StreamsCollectionFilter.filterCollectionHelperMethod(baseStrings, item -> item.length() == 4);
+
+        assertThat(filtered).containsExactlyInAnyOrder("java", "type");
+    }
+
+    @Test
+    public void givenAnIntegerCollection_whenFilteringEvenValues_thenObtainTheFilteredCollectionForAllCases() {
+        Collection<Integer> filteredWithStreams1 = StreamsCollectionFilter.findEvenNumbers(BASE_INTEGER_COLLECTION);
+        Collection<Integer> filteredWithCollectionUtils = CollectionUtilsCollectionFilter.findEvenNumbers(new ArrayList<>(BASE_INTEGER_COLLECTION));
+        Collection<Integer> filteredWithEclipseCollections = EclipseCollectionsCollectionFilter.findEvenNumbers(BASE_INTEGER_COLLECTION);
+        Collection<Integer> filteredWithEclipseCollectionsUsingIterate = EclipseCollectionsCollectionFilter.findEvenNumbersUsingIterate(BASE_INTEGER_COLLECTION);
+        Collection<Integer> filteredWithGuava = GuavaCollectionFilter.findEvenNumbers(BASE_INTEGER_COLLECTION);
+
+        assertThat(filteredWithStreams1).hasSameElementsAs(filteredWithCollectionUtils)
+            .hasSameElementsAs(filteredWithEclipseCollections)
+            .hasSameElementsAs(filteredWithEclipseCollectionsUsingIterate)
+            .hasSameElementsAs(filteredWithEclipseCollectionsUsingIterate)
+            .hasSameElementsAs(filteredWithGuava)
+            .hasSameElementsAs(EXPECTED_EVEN_FILTERED_COLLECTION);
+    }
+
+}

core-java-collections/src/test/java/com/baeldung/list/removefirst/RemoveFirstElementUnitTest.java

@@ -0,0 +1,51 @@
+package com.baeldung.list.removefirst;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.*;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+
+import org.junit.Before;
+import org.junit.Test;
+
+public class RemoveFirstElementUnitTest {
+
+    private List<String> list = new ArrayList<>();
+    private LinkedList<String> linkedList = new LinkedList<>();
+
+    @Before
+    public void init() {
+
+        list.add("cat");
+        list.add("dog");
+        list.add("pig");
+        list.add("cow");
+        list.add("goat");
+
+        linkedList.add("cat");
+        linkedList.add("dog");
+        linkedList.add("pig");
+        linkedList.add("cow");
+        linkedList.add("goat");
+    }
+
+    @Test
+    public void givenList_whenRemoveFirst_thenRemoved() {
+        list.remove(0);
+
+        assertThat(list, hasSize(4));
+        assertThat(list, not(contains("cat")));
+    }
+
+    @Test
+    public void givenLinkedList_whenRemoveFirst_thenRemoved() {
+
+        linkedList.removeFirst();
+
+        assertThat(linkedList, hasSize(4));
+        assertThat(linkedList, not(contains("cat")));
+    }
+
+}

core-java-io/README.md

@@ -28,3 +28,5 @@
 - [Guide to Java NIO2 Asynchronous Channel APIs](http://www.baeldung.com/java-nio-2-async-channels)
 - [A Guide to NIO2 Asynchronous Socket Channel](http://www.baeldung.com/java-nio2-async-socket-channel)
 - [Download a File From an URL in Java](http://www.baeldung.com/java-download-file)
+- [Create a Symbolic Link with Java](http://www.baeldung.com/java-symlink)
+- [Quick Use of FilenameFilter](http://www.baeldung.com/java-filename-filter)

core-java/README.md

@@ -163,3 +163,8 @@
 - [Console I/O in Java](http://www.baeldung.com/java-console-input-output)
 - [Guide to the java.util.Arrays Class](http://www.baeldung.com/java-util-arrays)
 - [Create a Custom Exception in Java](http://www.baeldung.com/java-new-custom-exception)
+- [Guide to java.util.GregorianCalendar](http://www.baeldung.com/java-gregorian-calendar)
+- [Java Global Exception Handler](http://www.baeldung.com/java-global-exception-handler)
+- [Encrypting and Decrypting Files in Java](http://www.baeldung.com/java-cipher-input-output-stream)
+- [How to Get the Size of an Object in Java](http://www.baeldung.com/java-size-of-object)
+- [Exception Handling in Java](http://www.baeldung.com/java-exceptions)

core-java/pom.xml

@@ -173,6 +173,19 @@
             <artifactId>c3p0</artifactId>
             <version>${c3p0.version}</version>
         </dependency>
+        <!-- instrumentation -->
+        <dependency>
+            <groupId>org.javassist</groupId>
+            <artifactId>javassist</artifactId>
+            <version>${javaassist.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.sun</groupId>
+            <artifactId>tools</artifactId>
+            <version>1.8.0</version>
+            <scope>system</scope>
+            <systemPath>${java.home}/../lib/tools.jar</systemPath>
+        </dependency>
     </dependencies>
 
     <build>
@@ -400,6 +413,111 @@
                 </plugins>
             </build>
         </profile>
+
+        <!-- java instrumentation profiles to build jars -->
+        <profile>
+            <id>buildAgentLoader</id>
+            <build>
+                <plugins>
+                    <plugin>
+                        <groupId>org.apache.maven.plugins</groupId>
+                        <artifactId>maven-jar-plugin</artifactId>
+                        <executions>
+                            <execution>
+                                <phase>package</phase>
+                                <goals>
+                                    <goal>jar</goal>
+                                </goals>
+                                <configuration>
+                                    <classifier>agentLoader</classifier>
+                                    <classesDirectory>target/classes</classesDirectory>
+                                    <archive>
+                                        <manifest>
+                                            <addClasspath>true</addClasspath>
+                                        </manifest>
+                                        <manifestFile>${project.build.outputDirectory}/META-INF/MANIFEST.MF</manifestFile>
+                                    </archive>
+
+                                    <includes>
+                                        <include>com/baeldung/instrumentation/application/AgentLoader.class</include>
+                                        <include>com/baeldung/instrumentation/application/Launcher.class</include>
+                                    </includes>
+                                </configuration>
+                            </execution>
+                        </executions>
+                    </plugin>
+                </plugins>
+            </build>
+        </profile>
+        <profile>
+            <id>buildApplication</id>
+            <build>
+                <plugins>
+                    <plugin>
+                        <groupId>org.apache.maven.plugins</groupId>
+                        <artifactId>maven-jar-plugin</artifactId>
+                        <executions>
+                            <execution>
+                                <phase>package</phase>
+                                <goals>
+                                    <goal>jar</goal>
+                                </goals>
+                                <configuration>
+                                    <classifier>application</classifier>
+                                    <classesDirectory>target/classes</classesDirectory>
+                                    <archive>
+                                        <manifest>
+                                            <addClasspath>true</addClasspath>
+                                        </manifest>
+                                        <manifestFile>${project.build.outputDirectory}/META-INF/MANIFEST.MF</manifestFile>
+                                    </archive>
+
+                                    <includes>
+                                        <include>com/baeldung/instrumentation/application/MyAtm.class</include>
+                                        <include>com/baeldung/instrumentation/application/MyAtmApplication.class</include>
+                                        <include>com/baeldung/instrumentation/application/Launcher.class</include>
+                                    </includes>
+                                </configuration>
+                            </execution>
+                        </executions>
+                    </plugin>
+                </plugins>
+            </build>
+        </profile>
+        <profile>
+            <id>buildAgent</id>
+            <build>
+                <plugins>
+                    <plugin>
+                        <groupId>org.apache.maven.plugins</groupId>
+                        <artifactId>maven-jar-plugin</artifactId>
+                        <executions>
+                            <execution>
+                                <phase>package</phase>
+                                <goals>
+                                    <goal>jar</goal>
+                                </goals>
+                                <configuration>
+                                    <classifier>agent</classifier>
+                                    <classesDirectory>target/classes</classesDirectory>
+                                    <archive>
+                                        <manifest>
+                                            <addClasspath>true</addClasspath>
+                                        </manifest>
+                                        <manifestFile>${project.build.outputDirectory}/META-INF/MANIFEST.MF</manifestFile>
+                                    </archive>
+
+                                    <includes>
+                                        <include>com/baeldung/instrumentation/agent/AtmTransformer.class</include>
+                                        <include>com/baeldung/instrumentation/agent/MyInstrumentationAgent.class</include>
+                                    </includes>
+                                </configuration>
+                            </execution>
+                        </executions>
+                    </plugin>
+                </plugins>
+            </build>
+        </profile>
     </profiles>
 
     <properties>
@@ -453,6 +571,8 @@
         <!-- Mime Type Libraries -->
         <tika.version>1.18</tika.version>
         <jmime-magic.version>0.1.5</jmime-magic.version>
+        <!-- instrumentation -->
+        <javaassist.version>3.21.0-GA</javaassist.version>
     </properties>
 
 </project>

core-java/src/main/java/com/baeldung/connectionpool/connectionpools/BasicConnectionPool.java

@@ -33,7 +33,7 @@ public class BasicConnectionPool implements ConnectionPool {
     
     @Override
     public Connection getConnection() throws SQLException {
-        if (connectionPool.size() == 0) {
+        if (connectionPool.isEmpty()) {
             if (usedConnections.size() < MAX_POOL_SIZE) {
                 connectionPool.add(createConnection(url, user, password));
             } else {
@@ -76,9 +76,7 @@ public class BasicConnectionPool implements ConnectionPool {
     }
 
     public void shutdown() throws SQLException {
-        for (Connection c : usedConnections) {
-            this.releaseConnection(c);
-        }
+        usedConnections.forEach(this::releaseConnection);
         for (Connection c : connectionPool) {
             c.close();
         }

core-java/src/main/java/com/baeldung/instrumentation/agent/AtmTransformer.java

@@ -0,0 +1,70 @@
+package com.baeldung.instrumentation.agent;
+
+import javassist.CannotCompileException;
+import javassist.ClassPool;
+import javassist.CtClass;
+import javassist.CtMethod;
+import javassist.NotFoundException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.lang.instrument.ClassFileTransformer;
+import java.lang.instrument.IllegalClassFormatException;
+import java.security.ProtectionDomain;
+
+public class AtmTransformer implements ClassFileTransformer {
+
+    private static Logger LOGGER = LoggerFactory.getLogger(AtmTransformer.class);
+
+    private static final String WITHDRAW_MONEY_METHOD = "withdrawMoney";
+
+    /** The internal form class name of the class to transform */
+    private String targetClassName;
+    /** The class loader of the class we want to transform */
+    private ClassLoader targetClassLoader;
+
+    public AtmTransformer(String targetClassName, ClassLoader targetClassLoader) {
+        this.targetClassName = targetClassName;
+        this.targetClassLoader = targetClassLoader;
+    }
+
+    @Override
+    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
+            ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
+        byte[] byteCode = classfileBuffer;
+
+        String finalTargetClassName = this.targetClassName.replaceAll("\\.", "/"); //replace . with /
+        if (!className.equals(finalTargetClassName)) {
+            return byteCode;
+        }
+
+        if (className.equals(finalTargetClassName) && loader.equals(targetClassLoader)) {
+            LOGGER.info("[Agent] Transforming class MyAtm");
+            try {
+                ClassPool cp = ClassPool.getDefault();
+                CtClass cc = cp.get(targetClassName);
+                CtMethod m = cc.getDeclaredMethod(WITHDRAW_MONEY_METHOD);
+                m.addLocalVariable("startTime", CtClass.longType);
+                m.insertBefore("startTime = System.currentTimeMillis();");
+
+                StringBuilder endBlock = new StringBuilder();
+
+                m.addLocalVariable("endTime", CtClass.longType);
+                m.addLocalVariable("opTime", CtClass.longType);
+                endBlock.append("endTime = System.currentTimeMillis();");
+                endBlock.append("opTime = (endTime-startTime)/1000;");
+
+                endBlock.append("LOGGER.info(\"[Application] Withdrawal operation completed in:\" + opTime + \" seconds!\");");
+
+                m.insertAfter(endBlock.toString());
+
+                byteCode = cc.toBytecode();
+                cc.detach();
+            } catch (NotFoundException | CannotCompileException | IOException e) {
+                LOGGER.error("Exception", e);
+            }
+        }
+        return byteCode;
+    }
+}

core-java/src/main/java/com/baeldung/instrumentation/agent/MyInstrumentationAgent.java

@@ -0,0 +1,59 @@
+package com.baeldung.instrumentation.agent;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.lang.instrument.Instrumentation;
+
+public class MyInstrumentationAgent {
+    private static Logger LOGGER = LoggerFactory.getLogger(MyInstrumentationAgent.class);
+
+    public static void premain(String agentArgs, Instrumentation inst) {
+        LOGGER.info("[Agent] In premain method");
+
+        String className = "com.baeldung.instrumentation.application.MyAtm";
+        transformClass(className,inst);
+    }
+
+    public static void agentmain(String agentArgs, Instrumentation inst) {
+        LOGGER.info("[Agent] In agentmain method");
+
+        String className = "com.baeldung.instrumentation.application.MyAtm";
+        transformClass(className,inst);
+    }
+
+    private static void transformClass(String className, Instrumentation instrumentation) {
+        Class<?> targetCls = null;
+        ClassLoader targetClassLoader = null;
+        // see if we can get the class using forName
+        try {
+            targetCls = Class.forName(className);
+            targetClassLoader = targetCls.getClassLoader();
+            transform(targetCls, targetClassLoader, instrumentation);
+            return;
+        } catch (Exception ex) {
+            LOGGER.error("Class [{}] not found with Class.forName");
+        }
+        // otherwise iterate all loaded classes and find what we want
+        for(Class<?> clazz: instrumentation.getAllLoadedClasses()) {
+            if(clazz.getName().equals(className)) {
+                targetCls = clazz;
+                targetClassLoader = targetCls.getClassLoader();
+                transform(targetCls, targetClassLoader, instrumentation);
+                return;
+            }
+        }
+        throw new RuntimeException("Failed to find class [" + className + "]");
+    }
+
+    private static void transform(Class<?> clazz, ClassLoader classLoader, Instrumentation instrumentation) {
+        AtmTransformer dt = new AtmTransformer(clazz.getName(), classLoader);
+        instrumentation.addTransformer(dt, true);
+        try {
+            instrumentation.retransformClasses(clazz);
+        } catch (Exception ex) {
+            throw new RuntimeException("Transform failed for class: [" + clazz.getName() + "]", ex);
+        }
+    }
+
+}

core-java/src/main/java/com/baeldung/instrumentation/application/AgentLoader.java

@@ -0,0 +1,46 @@
+package com.baeldung.instrumentation.application;
+
+import com.sun.tools.attach.VirtualMachine;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.File;
+import java.util.Optional;
+
+/**
+ * Created by adi on 6/10/18.
+ */
+public class AgentLoader {
+    private static Logger LOGGER = LoggerFactory.getLogger(AgentLoader.class);
+
+    public static void run(String[] args) {
+        String agentFilePath = "/home/adi/Desktop/agent-1.0.0-jar-with-dependencies.jar";
+        String applicationName = "MyAtmApplication";
+
+        //iterate all jvms and get the first one that matches our application name
+        Optional<String> jvmProcessOpt = Optional.ofNullable(VirtualMachine.list()
+          .stream()
+          .filter(jvm -> {
+              LOGGER.info("jvm:{}", jvm.displayName());
+              return jvm.displayName().contains(applicationName);
+          })
+          .findFirst().get().id());
+
+        if(!jvmProcessOpt.isPresent()) {
+            LOGGER.error("Target Application not found");
+            return;
+        }
+        File agentFile = new File(agentFilePath);
+        try {
+            String jvmPid = jvmProcessOpt.get();
+            LOGGER.info("Attaching to target JVM with PID: " + jvmPid);
+            VirtualMachine jvm = VirtualMachine.attach(jvmPid);
+            jvm.loadAgent(agentFile.getAbsolutePath());
+            jvm.detach();
+            LOGGER.info("Attached to target JVM and loaded Java agent successfully");
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+}

core-java/src/main/java/com/baeldung/instrumentation/application/Launcher.java

@@ -0,0 +1,14 @@
+package com.baeldung.instrumentation.application;
+
+/**
+ * Created by adi on 6/14/18.
+ */
+public class Launcher {
+    public static void main(String[] args) throws Exception {
+        if(args[0].equals("StartMyAtmApplication")) {
+            new MyAtmApplication().run(args);
+        } else if(args[0].equals("LoadAgent")) {
+            new AgentLoader().run(args);
+        }
+    }
+}

core-java/src/main/java/com/baeldung/instrumentation/application/MyAtm.java

@@ -0,0 +1,19 @@
+package com.baeldung.instrumentation.application;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Created by adi on 6/11/18.
+ */
+public class MyAtm {
+    private static Logger LOGGER = LoggerFactory.getLogger(MyAtm.class);
+
+    private static final int account = 10;
+
+    public static void withdrawMoney(int amount) throws InterruptedException {
+        Thread.sleep(2000l); //processing going on here
+        LOGGER.info("[Application] Successful Withdrawal of [{}] units!", amount);
+
+    }
+}