Merge branch 'master' of https://github.com/JonCook/tutorials

core-java-modules/core-java-lang/src/main/java/com/baeldung/assertion/Assertion.java

@@ -0,0 +1,26 @@
+package com.baeldung.assertion;
+
+/**
+ * Simple demonstration of using Java assert keyword.
+ */
+public class Assertion {
+
+  public static void main(String[] args) {
+    Assertion assertion = new Assertion();
+    assertion.setup();
+  }
+
+  public void setup() {
+    Object conn = getConnection();
+    assert conn != null : "Connection is null";
+
+    // continue with other setup ...
+  }
+
+  // Simulate failure to get a connection; using Object
+  // to avoid dependencies on JDBC or some other heavy
+  // 3rd party library
+  public Object getConnection() {
+    return null;
+  }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/className/RetrievingClassName.java

@@ -0,0 +1,9 @@
+package com.baeldung.className;
+
+public class RetrievingClassName {
+
+    public class InnerClass {
+
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/comparable/Player.java

@@ -0,0 +1,51 @@
+package com.baeldung.comparable;
+
+public class Player implements Comparable<Player> {
+
+    private int ranking;
+
+    private String name;
+
+    private int age;
+
+    public Player(int ranking, String name, int age) {
+        this.ranking = ranking;
+        this.name = name;
+        this.age = age;
+    }
+
+    public int getRanking() {
+        return ranking;
+    }
+
+    public void setRanking(int ranking) {
+        this.ranking = ranking;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public int getAge() {
+        return age;
+    }
+
+    public void setAge(int age) {
+        this.age = age;
+    }
+
+    @Override
+    public String toString() {
+        return this.name;
+    }
+
+    @Override
+    public int compareTo(Player otherPlayer) {
+        return (this.getRanking() - otherPlayer.getRanking());
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/comparable/PlayerSorter.java

@@ -0,0 +1,25 @@
+package com.baeldung.comparable;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+public class PlayerSorter {
+
+    public static void main(String[] args) {
+
+        List<Player> footballTeam = new ArrayList<Player>();
+        Player player1 = new Player(59, "John", 20);
+        Player player2 = new Player(67, "Roger", 22);
+        Player player3 = new Player(45, "Steven", 24);
+        footballTeam.add(player1);
+        footballTeam.add(player2);
+        footballTeam.add(player3);
+
+        System.out.println("Before Sorting : " + footballTeam);
+        Collections.sort(footballTeam);
+        System.out.println("After Sorting : " + footballTeam);
+
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/comparator/Player.java

@@ -0,0 +1,46 @@
+package com.baeldung.comparator;
+
+public class Player {
+
+    private int ranking;
+
+    private String name;
+
+    private int age;
+
+    public Player(int ranking, String name, int age) {
+        this.ranking = ranking;
+        this.name = name;
+        this.age = age;
+    }
+
+    public int getRanking() {
+        return ranking;
+    }
+
+    public void setRanking(int ranking) {
+        this.ranking = ranking;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public int getAge() {
+        return age;
+    }
+
+    public void setAge(int age) {
+        this.age = age;
+    }
+
+    @Override
+    public String toString() {
+        return this.name;
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/comparator/PlayerAgeComparator.java

@@ -0,0 +1,12 @@
+package com.baeldung.comparator;
+
+import java.util.Comparator;
+
+public class PlayerAgeComparator implements Comparator<Player> {
+
+    @Override
+    public int compare(Player firstPlayer, Player secondPlayer) {
+       return (firstPlayer.getAge() - secondPlayer.getAge());
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/comparator/PlayerAgeSorter.java

@@ -0,0 +1,27 @@
+package com.baeldung.comparator;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+public class PlayerAgeSorter {
+
+    public static void main(String[] args) {
+        
+        List<Player> footballTeam = new ArrayList<Player>();
+        Player player1 = new Player(59, "John", 22);
+        Player player2 = new Player(67, "Roger", 20);
+        Player player3 = new Player(45, "Steven", 24);
+        footballTeam.add(player1);
+        footballTeam.add(player2);
+        footballTeam.add(player3);
+        
+        System.out.println("Before Sorting : " + footballTeam);
+        //Instance of PlayerAgeComparator
+        PlayerAgeComparator playerComparator = new PlayerAgeComparator();
+        Collections.sort(footballTeam, playerComparator);
+        System.out.println("After Sorting by age : " + footballTeam);
+        
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/comparator/PlayerRankingComparator.java

@@ -0,0 +1,12 @@
+package com.baeldung.comparator;
+
+import java.util.Comparator;
+
+public class PlayerRankingComparator implements Comparator<Player> {
+
+    @Override
+    public int compare(Player firstPlayer, Player secondPlayer) {
+       return (firstPlayer.getRanking() - secondPlayer.getRanking());
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/comparator/PlayerRankingSorter.java

@@ -0,0 +1,27 @@
+package com.baeldung.comparator;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+public class PlayerRankingSorter {
+
+    public static void main(String[] args) {
+        
+        List<Player> footballTeam = new ArrayList<Player>();
+        Player player1 = new Player(59, "John", 22);
+        Player player2 = new Player(67, "Roger", 20);
+        Player player3 = new Player(45, "Steven", 40);
+        footballTeam.add(player1);
+        footballTeam.add(player2);
+        footballTeam.add(player3);
+        
+        System.out.println("Before Sorting : " + footballTeam);
+        //Instance of PlayerRankingComparator
+        PlayerRankingComparator playerComparator = new PlayerRankingComparator();
+        Collections.sort(footballTeam, playerComparator);
+        System.out.println("After Sorting by ranking : " + footballTeam);
+        
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/enums/values/Element1.java

@@ -0,0 +1,17 @@
+package com.baeldung.enums.values;
+
+/**
+ * This is a simple enum of periodic table elements
+ */
+public enum Element1 {
+    H,
+    HE,
+    LI,
+    BE,
+    B,
+    C,
+    N,
+    O,
+    F,
+    NE
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/enums/values/Element2.java

@@ -0,0 +1,52 @@
+package com.baeldung.enums.values;
+
+/**
+ * The simple enum has been enhanced to add the name of the element.
+ */
+public enum Element2 {
+    H("Hydrogen"),
+    HE("Helium"),
+    LI("Lithium"),
+    BE("Beryllium"),
+    B("Boron"),
+    C("Carbon"),
+    N("Nitrogen"),
+    O("Oxygen"),
+    F("Flourine"),
+    NE("Neon");
+
+    /** a final variable to store the label, which can't be changed */
+    public final String label;
+
+    /**
+     * A private constructor that sets the label.
+     * @param label 
+     */
+    private Element2(String label) {
+        this.label = label;
+    }
+
+    /**
+     * Look up Element2 instances by the label field. This implementation iterates through 
+     * the values() list to find the label.
+     * @param label The label to look up
+     * @return The Element2 instance with the label, or null if not found.
+     */
+    public static Element2 valueOfLabel(String label) {
+        for (Element2 e2 : values()) {
+            if (e2.label.equals(label)) {
+                return e2;
+            }
+        }
+        return null;
+    }
+
+    /**
+     * Override the toString() method to return the label instead of the declared name.
+     * @return 
+     */
+    @Override
+    public String toString() {
+        return this.label;
+    }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/enums/values/Element3.java

@@ -0,0 +1,63 @@
+package com.baeldung.enums.values;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * A Map has been added to cache labels for faster lookup.
+ */
+public enum Element3 {
+    H("Hydrogen"),
+    HE("Helium"),
+    LI("Lithium"),
+    BE("Beryllium"),
+    B("Boron"),
+    C("Carbon"),
+    N("Nitrogen"),
+    O("Oxygen"),
+    F("Flourine"),
+    NE("Neon");
+
+    /** 
+     * A map to cache labels and their associated Element3 instances.
+     * Note that this only works if the labels are all unique!
+     */
+    private static final Map<String, Element3> BY_LABEL = new HashMap<>();
+    
+    /** populate the BY_LABEL cache */
+    static {
+        for (Element3 e3 : values()) {
+            BY_LABEL.put(e3.label, e3);
+        }
+    }
+
+    /** a final variable to store the label, which can't be changed */
+    public final String label;
+
+    /**
+     * A private constructor that sets the label.
+     * @param label 
+     */
+    private Element3(String label) {
+        this.label = label;
+    }
+
+    /**
+     * Look up Element2 instances by the label field. This implementation finds the
+     * label in the BY_LABEL cache.
+     * @param label The label to look up
+     * @return The Element3 instance with the label, or null if not found.
+     */
+    public static Element3 valueOfLabel(String label) {
+        return BY_LABEL.get(label);
+    }
+
+    /**
+     * Override the toString() method to return the label instead of the declared name.
+     * @return 
+     */
+    @Override
+    public String toString() {
+        return this.label;
+    }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/enums/values/Element4.java

@@ -0,0 +1,95 @@
+package com.baeldung.enums.values;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Multiple fields have been added and the Labeled interface is implemented.
+ */
+public enum Element4 implements Labeled {
+    H("Hydrogen", 1, 1.008f),
+    HE("Helium", 2, 4.0026f),
+    LI("Lithium", 3, 6.94f),
+    BE("Beryllium", 4, 9.01722f),
+    B("Boron", 5, 10.81f),
+    C("Carbon", 6, 12.011f),
+    N("Nitrogen", 7, 14.007f),
+    O("Oxygen", 8, 15.999f),
+    F("Flourine", 9, 18.998f),
+    NE("Neon", 10, 20.180f);
+    /** 
+     * Maps cache labels and their associated Element3 instances.
+     * Note that this only works if the values are all unique!
+     */
+    private static final Map<String, Element4> BY_LABEL = new HashMap<>();
+    private static final Map<Integer, Element4> BY_ATOMIC_NUMBER = new HashMap<>();
+    private static final Map<Float, Element4> BY_ATOMIC_WEIGHT = new HashMap<>();
+    
+    /** populate the caches */
+    static {
+        for (Element4 e4 : values()) {
+            BY_LABEL.put(e4.label, e4);
+            BY_ATOMIC_NUMBER.put(e4.atomicNumber, e4);
+            BY_ATOMIC_WEIGHT.put(e4.atomicWeight, e4);
+        }
+    }
+
+    /** final variables to store the values, which can't be changed */
+    public final String label;
+    public final int atomicNumber;
+    public final float atomicWeight;
+
+    private Element4(String label, int atomicNumber, float atomicWeight) {
+        this.label = label;
+        this.atomicNumber = atomicNumber;
+        this.atomicWeight = atomicWeight;
+    }
+
+    /**
+     * Implement the Labeled interface.
+     * @return the label value
+     */
+    @Override
+    public String label() {
+        return label;
+    }
+
+    /**
+     * Look up Element2 instances by the label field. This implementation finds the
+     * label in the BY_LABEL cache.
+     * @param label The label to look up
+     * @return The Element4 instance with the label, or null if not found.
+     */
+    public static Element4 valueOfLabel(String label) {
+        return BY_LABEL.get(label);
+    }
+
+    /**
+     * Look up Element2 instances by the atomicNumber field. This implementation finds the
+     * atomicNUmber in the cache.
+     * @param number The atomicNumber to look up 
+     * @return The Element4 instance with the label, or null if not found.
+     */
+    public static Element4 valueOfAtomicNumber(int number) {
+        return BY_ATOMIC_NUMBER.get(number);
+    }
+
+    /**
+     * Look up Element2 instances by the atomicWeight field. This implementation finds the
+     * atomic weight in the cache.
+     * @param weight the atomic weight to look up
+     * @return The Element4 instance with the label, or null if not found.
+     */
+    public static Element4 valueOfAtomicWeight(float weight) {
+        return BY_ATOMIC_WEIGHT.get(weight);
+    }
+
+    /**
+     * Override the toString() method to return the label instead of the declared name.
+     * @return 
+     */
+    @Override
+    public String toString() {
+        return this.label;
+    }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/enums/values/Labeled.java

@@ -0,0 +1,5 @@
+package com.baeldung.enums.values;
+
+public interface Labeled {
+    String label();
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/finalize/CloseableResource.java

@@ -0,0 +1,30 @@
+package com.baeldung.finalize;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+
+public class CloseableResource implements AutoCloseable {
+    private BufferedReader reader;
+
+    public CloseableResource() {
+        InputStream input = this.getClass().getClassLoader().getResourceAsStream("file.txt");
+        reader = new BufferedReader(new InputStreamReader(input));
+    }
+
+    public String readFirstLine() throws IOException {
+        String firstLine = reader.readLine();
+        return firstLine;
+    }
+
+    @Override
+    public void close() {
+        try {
+            reader.close();
+            System.out.println("Closed BufferedReader in the close method");
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+    }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/finalize/Finalizable.java

@@ -0,0 +1,30 @@
+package com.baeldung.finalize;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+
+public class Finalizable {
+    private BufferedReader reader;
+
+    public Finalizable() {
+        InputStream input = this.getClass().getClassLoader().getResourceAsStream("file.txt");
+        reader = new BufferedReader(new InputStreamReader(input));
+    }
+
+    public String readFirstLine() throws IOException {
+        String firstLine = reader.readLine();
+        return firstLine;
+    }
+
+    @Override
+    public void finalize() {
+        try {
+            reader.close();
+            System.out.println("Closed BufferedReader in the finalizer");
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+    }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/recursion/BinaryNode.java

@@ -0,0 +1,31 @@
+package com.baeldung.recursion;
+
+public class BinaryNode {
+    int value;
+    BinaryNode left;
+    BinaryNode right;
+    
+    public BinaryNode(int value){
+        this.value = value;
+    }
+    
+    public int getValue() {
+        return value;
+    }
+    public void setValue(int value) {
+        this.value = value;
+    }
+    public BinaryNode getLeft() {
+        return left;
+    }
+    public void setLeft(BinaryNode left) {
+        this.left = left;
+    }
+    public BinaryNode getRight() {
+        return right;
+    }
+    public void setRight(BinaryNode right) {
+        this.right = right;
+    }
+    
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/recursion/RecursionExample.java

@@ -0,0 +1,64 @@
+package com.baeldung.recursion;
+
+public class RecursionExample {
+    
+    public int sum(int n){
+        if (n >= 1){
+            return sum(n - 1) + n;
+        }
+        return n;
+    }
+    
+    public int tailSum(int currentSum, int n){
+        if (n <= 1) {
+            return currentSum + n;
+        }
+        return tailSum(currentSum + n, n - 1);
+    }
+    
+    public int iterativeSum(int n){
+        int sum = 0;
+        if(n < 0){
+            return -1;
+        }
+        for(int i=0; i<=n; i++){
+            sum += i;
+        }
+        return sum;
+    }
+    
+    public int powerOf10(int n){
+        if (n == 0){
+            return 1;
+        }
+        return powerOf10(n-1)*10;
+    }
+    
+    public int fibonacci(int n){
+        if (n <=1 ){
+            return n;
+        }
+        return fibonacci(n-1) + fibonacci(n-2);
+    }
+    
+    public String toBinary(int n){
+        if (n <= 1 ){
+            return String.valueOf(n);
+        }
+        return toBinary(n / 2) + String.valueOf(n % 2);
+    }
+    
+    public int calculateTreeHeight(BinaryNode root){
+        if (root!= null){
+            if (root.getLeft() != null || root.getRight() != null){
+                return 1 + max(calculateTreeHeight(root.left) , calculateTreeHeight(root.right));
+            }
+        }
+        return 0;
+    }
+    
+    public int max(int a,int b){
+        return a>b ? a:b;
+    }
+    
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/synthetic/BridgeMethodDemo.java

@@ -0,0 +1,23 @@
+package com.baeldung.synthetic;
+
+import java.util.Comparator;
+
+/**
+ * Class which contains a synthetic bridge method.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class BridgeMethodDemo implements Comparator<Integer> {
+
+    /*
+     * (non-Javadoc)
+     * 
+     * @see java.util.Comparator#compare(java.lang.Object, java.lang.Object)
+     */
+    @Override
+    public int compare(Integer o1, Integer o2) {
+        return 0;
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/synthetic/SyntheticConstructorDemo.java

@@ -0,0 +1,34 @@
+package com.baeldung.synthetic;
+
+/**
+ * Wrapper for a class which contains a synthetic constructor.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class SyntheticConstructorDemo {
+
+    /**
+     * We need to instantiate the {@link NestedClass} using a private
+     * constructor from the enclosing instance in order to generate a synthetic
+     * constructor.
+     */
+    private NestedClass nestedClass = new NestedClass();
+
+    /**
+     * Class which contains a synthetic constructor.
+     * 
+     * @author Donato Rimenti
+     *
+     */
+    class NestedClass {
+
+        /**
+         * In order to generate a synthetic constructor, this class must have a
+         * private constructor.
+         */
+        private NestedClass() {
+        }
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/synthetic/SyntheticFieldDemo.java

@@ -0,0 +1,22 @@
+package com.baeldung.synthetic;
+
+/**
+ * Wrapper for a class which contains a synthetic field reference to the outer
+ * class.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class SyntheticFieldDemo {
+
+    /**
+     * Class which contains a synthetic field reference to the outer class.
+     * 
+     * @author Donato Rimenti
+     *
+     */
+    class NestedClass {
+
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/synthetic/SyntheticMethodDemo.java

@@ -0,0 +1,48 @@
+package com.baeldung.synthetic;
+
+/**
+ * Wrapper for a class which contains two synthetic methods accessors to a
+ * private field.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class SyntheticMethodDemo {
+
+    /**
+     * Class which contains two synthetic methods accessors to a private field.
+     * 
+     * @author Donato Rimenti
+     *
+     */
+    class NestedClass {
+
+        /**
+         * Field for which will be generated synthetic methods accessors. It's
+         * important that this field is private for this purpose.
+         */
+        private String nestedField;
+    }
+
+    /**
+     * Gets the private nested field. We need to read the nested field in order
+     * to generate the synthetic getter.
+     * 
+     * @return the {@link NestedClass#nestedField}
+     */
+    public String getNestedField() {
+        return new NestedClass().nestedField;
+    }
+
+    /**
+     * Sets the private nested field. We need to write the nested field in order
+     * to generate the synthetic setter.
+     * 
+     * @param nestedField
+     *            the {@link NestedClass#nestedField}
+     */
+    public void setNestedField(String nestedField) {
+        new NestedClass().nestedField = nestedField;
+    }
+
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/system/ChatWindow.java

@@ -0,0 +1,17 @@
+package com.baeldung.system;
+
+import java.awt.event.WindowEvent;
+
+/**
+ * Note: This class is not meant for unit-testing since it uses system
+ * features at low level and that it uses 'System' gc() which suggests
+ * JVM for garbage collection. But the usage below demonstrates how the
+ * method can be used.
+ */
+public class ChatWindow {
+    public void windowStateChanged(WindowEvent event) {
+        if (event.getNewState() == WindowEvent.WINDOW_DEACTIVATED ) {
+            System.gc(); // if it ends up running, great!
+        }
+    }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/system/DateTimeService.java

@@ -0,0 +1,16 @@
+package com.baeldung.system;
+
+import java.util.Date;
+
+public class DateTimeService {
+
+    // One hour from now
+    public long nowPlusOneHour() {
+        return System.currentTimeMillis() + 3600 * 1000L;
+    }
+
+    // Human-readable format
+    public String nowPrettyPrinted() {
+        return new Date(System.currentTimeMillis()).toString();
+    }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/system/EnvironmentVariables.java

@@ -0,0 +1,7 @@
+package com.baeldung.system;
+
+public class EnvironmentVariables {
+    public String getPath() {
+        return System.getenv("PATH");
+    }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/system/SystemErrDemo.java

@@ -0,0 +1,18 @@
+package com.baeldung.system;
+
+/**
+ * Note: This class is not meant for unit-testing since it uses system
+ * features at low level and that it uses 'System' standard error stream
+ * methods to show output on screen. Also unit-tests in CI environments
+ * don't have console output for user to see messages. But the usage below
+ * demonstrates how the methods can be used.
+ */
+public class SystemErrDemo {
+    public static void main(String[] args) {
+        // Print without 'hitting' return
+        System.err.print("some inline error message");
+
+        // Print and then 'hit' return
+        System.err.println("an error message having new line at the end");
+    }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/system/SystemExitDemo.java

@@ -0,0 +1,24 @@
+package com.baeldung.system;
+
+/**
+ * Note: This class is not meant for unit-testing since it uses system
+ * features at low level and that it uses 'System' exit() which will
+ * exit the JVM. Also unit-tests in CI environments are not meant to
+ * exit unit tests like that. But the usage below demonstrates how the
+ * method can be used.
+ */
+public class SystemExitDemo {
+    public static void main(String[] args) {
+        boolean error = false;
+
+        // do something and set error value
+
+        if (error) {
+            System.exit(1); // error case exit
+        } else {
+            System.exit(0); // normal case exit
+        }
+
+        // Will not do anything after exit()
+    }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/system/SystemOutDemo.java

@@ -0,0 +1,25 @@
+package com.baeldung.system;
+
+import java.io.FileNotFoundException;
+import java.io.PrintStream;
+
+/**
+ * Note: This class is not meant for unit-testing since it uses system
+ * features at low level and that it uses 'System' standard output stream
+ * methods to show output on screen. Also unit-tests in CI environments
+ * don't have console output for user to see messages. But the usage below
+ * demonstrates how the methods can be used.
+ */
+public class SystemOutDemo {
+
+    public static void main(String[] args) throws FileNotFoundException {
+        // Print without 'hitting' return
+        System.out.print("some inline message");
+
+        // Print and then 'hit' return
+        System.out.println("a message having new line at the end");
+
+        // Changes output stream to send messages to file.
+        System.setOut(new PrintStream("file.txt"));
+    }
+}

core-java-modules/core-java-lang/src/main/java/com/baeldung/system/UserCredentials.java

@@ -0,0 +1,35 @@
+package com.baeldung.system;
+
+import java.io.BufferedReader;
+import java.io.Console;
+import java.io.IOException;
+import java.io.InputStreamReader;
+
+/**
+ * Note: This class is not meant for unit-testing since it uses system
+ * features at low level and that it uses 'System' standard input stream
+ * methods to read text from user. Also unit-tests in CI environments
+ * don't have console input for user to type in text. But the usage below
+ * demonstrates how the methods can be used.
+ */
+public class UserCredentials {
+
+    public String readUsername(int length) throws IOException {
+        byte[] name = new byte[length];
+        System.in.read(name, 0, length); // by default, from the console
+        return new String(name);
+    }
+
+    public String readUsername() throws IOException {
+        BufferedReader reader =
+                new BufferedReader(new InputStreamReader(System.in));
+        return reader.readLine();
+    }
+
+    public String readUsernameWithPrompt() {
+        Console console = System.console();
+
+        return console == null ? null : // Console not available
+                console.readLine("%s", "Enter your name: ");
+    }
+}

core-java-modules/core-java-lang/src/main/resources/file.txt

@@ -0,0 +1 @@
+baeldung.com

core-java-modules/core-java-lang/src/test/java/com/baeldung/className/RetrievingClassNameUnitTest.java

@@ -0,0 +1,156 @@
+package com.baeldung.className;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+
+public class RetrievingClassNameUnitTest {
+
+    // Retrieving Simple Name
+    @Test
+    public void givenRetrievingClassName_whenGetSimpleName_thenRetrievingClassName() {
+        assertEquals("RetrievingClassName", RetrievingClassName.class.getSimpleName());
+    }
+
+    @Test
+    public void givenPrimitiveInt_whenGetSimpleName_thenInt() {
+        assertEquals("int", int.class.getSimpleName());
+    }
+
+    @Test
+    public void givenRetrievingClassNameArray_whenGetSimpleName_thenRetrievingClassNameWithBrackets() {
+        assertEquals("RetrievingClassName[]", RetrievingClassName[].class.getSimpleName());
+        assertEquals("RetrievingClassName[][]", RetrievingClassName[][].class.getSimpleName());
+    }
+
+    @Test
+    public void givenAnonymousClass_whenGetSimpleName_thenEmptyString() {
+        assertEquals("", new RetrievingClassName() {}.getClass().getSimpleName());
+    }
+
+    // Retrieving Other Names
+    // - Primitive Types
+    @Test
+    public void givenPrimitiveInt_whenGetName_thenInt() {
+        assertEquals("int", int.class.getName());
+    }
+
+    @Test
+    public void givenPrimitiveInt_whenGetTypeName_thenInt() {
+        assertEquals("int", int.class.getTypeName());
+    }
+
+    @Test
+    public void givenPrimitiveInt_whenGetCanonicalName_thenInt() {
+        assertEquals("int", int.class.getCanonicalName());
+    }
+
+    // - Object Types
+    @Test
+    public void givenRetrievingClassName_whenGetName_thenCanonicalName() {
+        assertEquals("com.baeldung.className.RetrievingClassName", RetrievingClassName.class.getName());
+    }
+
+    @Test
+    public void givenRetrievingClassName_whenGetTypeName_thenCanonicalName() {
+        assertEquals("com.baeldung.className.RetrievingClassName", RetrievingClassName.class.getTypeName());
+    }
+
+    @Test
+    public void givenRetrievingClassName_whenGetCanonicalName_thenCanonicalName() {
+        assertEquals("com.baeldung.className.RetrievingClassName", RetrievingClassName.class.getCanonicalName());
+    }
+
+    // - Inner Classes
+    @Test
+    public void givenRetrievingClassNameInnerClass_whenGetName_thenCanonicalNameWithDollarSeparator() {
+        assertEquals("com.baeldung.className.RetrievingClassName$InnerClass", RetrievingClassName.InnerClass.class.getName());
+    }
+
+    @Test
+    public void givenRetrievingClassNameInnerClass_whenGetTypeName_thenCanonicalNameWithDollarSeparator() {
+        assertEquals("com.baeldung.className.RetrievingClassName$InnerClass", RetrievingClassName.InnerClass.class.getTypeName());
+    }
+
+    @Test
+    public void givenRetrievingClassNameInnerClass_whenGetCanonicalName_thenCanonicalName() {
+        assertEquals("com.baeldung.className.RetrievingClassName.InnerClass", RetrievingClassName.InnerClass.class.getCanonicalName());
+    }
+
+    // - Anonymous Classes
+    @Test
+    public void givenAnonymousClass_whenGetName_thenCallingClassCanonicalNameWithDollarSeparatorAndCountNumber() {
+        // These are the second and third appearences of an anonymous class in RetrievingClassNameUnitTest, hence $2 and $3 expectations
+        assertEquals("com.baeldung.className.RetrievingClassNameUnitTest$2", new RetrievingClassName() {}.getClass().getName());
+        assertEquals("com.baeldung.className.RetrievingClassNameUnitTest$3", new RetrievingClassName() {}.getClass().getName());
+    }
+
+    @Test
+    public void givenAnonymousClass_whenGetTypeName_thenCallingClassCanonicalNameWithDollarSeparatorAndCountNumber() {
+        // These are the fourth and fifth appearences of an anonymous class in RetrievingClassNameUnitTest, hence $4 and $5 expectations
+        assertEquals("com.baeldung.className.RetrievingClassNameUnitTest$4", new RetrievingClassName() {}.getClass().getTypeName());
+        assertEquals("com.baeldung.className.RetrievingClassNameUnitTest$5", new RetrievingClassName() {}.getClass().getTypeName());
+    }
+
+    @Test
+    public void givenAnonymousClass_whenGetCanonicalName_thenNull() {
+        assertNull(new RetrievingClassName() {}.getClass().getCanonicalName());
+    }
+
+    // - Arrays
+    @Test
+    public void givenPrimitiveIntArray_whenGetName_thenOpeningBracketsAndPrimitiveIntLetter() {
+        assertEquals("[I", int[].class.getName());
+        assertEquals("[[I", int[][].class.getName());
+    }
+
+    @Test
+    public void givenRetrievingClassNameArray_whenGetName_thenOpeningBracketsLetterLAndRetrievingClassNameGetName() {
+        assertEquals("[Lcom.baeldung.className.RetrievingClassName;", RetrievingClassName[].class.getName());
+        assertEquals("[[Lcom.baeldung.className.RetrievingClassName;", RetrievingClassName[][].class.getName());
+    }
+
+    @Test
+    public void givenRetrievingClassNameInnerClassArray_whenGetName_thenOpeningBracketsLetterLAndRetrievingClassNameInnerClassGetName() {
+        assertEquals("[Lcom.baeldung.className.RetrievingClassName$InnerClass;", RetrievingClassName.InnerClass[].class.getName());
+        assertEquals("[[Lcom.baeldung.className.RetrievingClassName$InnerClass;", RetrievingClassName.InnerClass[][].class.getName());
+    }
+
+    @Test
+    public void givenPrimitiveIntArray_whenGetTypeName_thenPrimitiveIntGetTypeNameWithBrackets() {
+        assertEquals("int[]", int[].class.getTypeName());
+        assertEquals("int[][]", int[][].class.getTypeName());
+    }
+
+    @Test
+    public void givenRetrievingClassNameArray_whenGetTypeName_thenRetrievingClassNameGetTypeNameWithBrackets() {
+        assertEquals("com.baeldung.className.RetrievingClassName[]", RetrievingClassName[].class.getTypeName());
+        assertEquals("com.baeldung.className.RetrievingClassName[][]", RetrievingClassName[][].class.getTypeName());
+    }
+
+    @Test
+    public void givenRetrievingClassNameInnerClassArray_whenGetTypeName_thenRetrievingClassNameInnerClassGetTypeNameWithBrackets() {
+        assertEquals("com.baeldung.className.RetrievingClassName$InnerClass[]", RetrievingClassName.InnerClass[].class.getTypeName());
+        assertEquals("com.baeldung.className.RetrievingClassName$InnerClass[][]", RetrievingClassName.InnerClass[][].class.getTypeName());
+    }
+
+    @Test
+    public void givenPrimitiveIntArray_whenGetCanonicalName_thenPrimitiveIntGetCanonicalNameWithBrackets() {
+        assertEquals("int[]", int[].class.getCanonicalName());
+        assertEquals("int[][]", int[][].class.getCanonicalName());
+    }
+
+    @Test
+    public void givenRetrievingClassNameArray_whenGetCanonicalName_thenRetrievingClassNameGetCanonicalNameWithBrackets() {
+        assertEquals("com.baeldung.className.RetrievingClassName[]", RetrievingClassName[].class.getCanonicalName());
+        assertEquals("com.baeldung.className.RetrievingClassName[][]", RetrievingClassName[][].class.getCanonicalName());
+    }
+
+    @Test
+    public void givenRetrievingClassNameInnerClassArray_whenGetCanonicalName_thenRetrievingClassNameInnerClassGetCanonicalNameWithBrackets() {
+        assertEquals("com.baeldung.className.RetrievingClassName.InnerClass[]", RetrievingClassName.InnerClass[].class.getCanonicalName());
+        assertEquals("com.baeldung.className.RetrievingClassName.InnerClass[][]", RetrievingClassName.InnerClass[][].class.getCanonicalName());
+    }
+
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/comparable/ComparableUnitTest.java

@@ -0,0 +1,29 @@
+package com.baeldung.comparable;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+import org.junit.Test;
+
+public class ComparableUnitTest {
+
+    @Test
+    public void whenUsingComparable_thenSortedList() {
+        List<Player> footballTeam = new ArrayList<Player>();
+        Player player1 = new Player(59, "John", 20);
+        Player player2 = new Player(67, "Roger", 22);
+        Player player3 = new Player(45, "Steven", 24);
+        footballTeam.add(player1);
+        footballTeam.add(player2);
+        footballTeam.add(player3);
+        Collections.sort(footballTeam);
+        assertEquals(footballTeam.get(0)
+            .getName(), "Steven");
+        assertEquals(footballTeam.get(2)
+            .getRanking(), 67);
+    }
+
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/comparator/ComparatorUnitTest.java

@@ -0,0 +1,47 @@
+package com.baeldung.comparator;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+import org.junit.Before;
+import org.junit.Test;
+
+public class ComparatorUnitTest {
+
+    List<Player> footballTeam;
+
+    @Before
+    public void setUp() {
+        footballTeam = new ArrayList<Player>();
+        Player player1 = new Player(59, "John", 20);
+        Player player2 = new Player(67, "Roger", 22);
+        Player player3 = new Player(45, "Steven", 24);
+        footballTeam.add(player1);
+        footballTeam.add(player2);
+        footballTeam.add(player3);
+    }
+
+    @Test
+    public void whenUsingRankingComparator_thenSortedList() {
+        PlayerRankingComparator playerComparator = new PlayerRankingComparator();
+        Collections.sort(footballTeam, playerComparator);
+        assertEquals(footballTeam.get(0)
+            .getName(), "Steven");
+        assertEquals(footballTeam.get(2)
+            .getRanking(), 67);
+    }
+
+    @Test
+    public void whenUsingAgeComparator_thenSortedList() {
+        PlayerAgeComparator playerComparator = new PlayerAgeComparator();
+        Collections.sort(footballTeam, playerComparator);
+        assertEquals(footballTeam.get(0)
+            .getName(), "John");
+        assertEquals(footballTeam.get(2)
+            .getRanking(), 45);
+    }
+
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/comparator/Java8ComparatorUnitTest.java

@@ -0,0 +1,68 @@
+package com.baeldung.comparator;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.List;
+
+import org.junit.Before;
+import org.junit.Test;
+
+public class Java8ComparatorUnitTest {
+
+    List<Player> footballTeam;
+
+    @Before
+    public void setUp() {
+        footballTeam = new ArrayList<Player>();
+        Player player1 = new Player(59, "John", 22);
+        Player player2 = new Player(67, "Roger", 20);
+        Player player3 = new Player(45, "Steven", 24);
+        footballTeam.add(player1);
+        footballTeam.add(player2);
+        footballTeam.add(player3);
+    }
+
+    @Test
+    public void whenComparing_UsingLambda_thenSorted() {
+        System.out.println("************** Java 8 Comaparator **************");
+        Comparator<Player> byRanking = (Player player1, Player player2) -> player1.getRanking() - player2.getRanking();
+
+        System.out.println("Before Sorting : " + footballTeam);
+        Collections.sort(footballTeam, byRanking);
+        System.out.println("After Sorting : " + footballTeam);
+        assertEquals(footballTeam.get(0)
+            .getName(), "Steven");
+        assertEquals(footballTeam.get(2)
+            .getRanking(), 67);
+    }
+
+    @Test
+    public void whenComparing_UsingComparatorComparing_thenSorted() {
+        System.out.println("********* Comaparator.comparing method *********");
+        System.out.println("********* byRanking *********");
+        Comparator<Player> byRanking = Comparator.comparing(Player::getRanking);
+
+        System.out.println("Before Sorting : " + footballTeam);
+        Collections.sort(footballTeam, byRanking);
+        System.out.println("After Sorting : " + footballTeam);
+        assertEquals(footballTeam.get(0)
+            .getName(), "Steven");
+        assertEquals(footballTeam.get(2)
+            .getRanking(), 67);
+        
+        System.out.println("********* byAge *********");
+        Comparator<Player> byAge = Comparator.comparing(Player::getAge);
+
+        System.out.println("Before Sorting : " + footballTeam);
+        Collections.sort(footballTeam, byAge);
+        System.out.println("After Sorting : " + footballTeam);
+        assertEquals(footballTeam.get(0)
+            .getName(), "Roger");
+        assertEquals(footballTeam.get(2)
+            .getRanking(), 45);
+    }
+
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/enums/iteration/DaysOfWeekEnum.java

@@ -0,0 +1,31 @@
+package com.baeldung.enums.iteration;
+
+import java.util.stream.Stream;
+
+public enum DaysOfWeekEnum {
+	SUNDAY("off"), 
+	MONDAY("working"), 
+	TUESDAY("working"), 
+	WEDNESDAY("working"), 
+	THURSDAY("working"), 
+	FRIDAY("working"), 
+	SATURDAY("off");
+
+	private String typeOfDay;
+
+	DaysOfWeekEnum(String typeOfDay) {
+		this.typeOfDay = typeOfDay;
+	}
+	
+	public String getTypeOfDay() {
+		return typeOfDay;
+	}
+
+	public void setTypeOfDay(String typeOfDay) {
+		this.typeOfDay = typeOfDay;
+	}
+
+	public static Stream<DaysOfWeekEnum> stream() {
+		return Stream.of(DaysOfWeekEnum.values());
+	}
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/enums/iteration/EnumIterationExamples.java

@@ -0,0 +1,43 @@
+package com.baeldung.enums.iteration;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.EnumSet;
+import java.util.List;
+
+
+public class EnumIterationExamples {
+	public static void main(String[] args) {
+		System.out.println("Enum iteration using EnumSet:");
+		EnumSet.allOf(DaysOfWeekEnum.class).forEach(day -> System.out.println(day));
+
+		System.out.println("Enum iteration using Stream:");
+		DaysOfWeekEnum.stream().filter(d -> d.getTypeOfDay().equals("off")).forEach(System.out::println);
+
+		System.out.println("Enum iteration using a for loop:");
+		for (DaysOfWeekEnum day : DaysOfWeekEnum.values()) {
+			System.out.println(day);
+		}
+		
+		System.out.println("Enum iteration using Arrays.asList():");
+	        Arrays.asList(DaysOfWeekEnum.values()).forEach(day -> System.out.println(day));
+	        
+	        System.out.println("Add Enum values to ArrayList:");
+	        List<DaysOfWeekEnum> days = new ArrayList<>();
+                days.add(DaysOfWeekEnum.FRIDAY);
+                days.add(DaysOfWeekEnum.SATURDAY);
+                days.add(DaysOfWeekEnum.SUNDAY);
+                for (DaysOfWeekEnum day : days) {
+                    System.out.println(day);
+                }
+                System.out.println("Remove SATURDAY from the list:");
+                days.remove(DaysOfWeekEnum.SATURDAY); 
+                if (!days.contains(DaysOfWeekEnum.SATURDAY)) { 
+                    System.out.println("Saturday is no longer in the list"); 
+                }
+                for (DaysOfWeekEnum day : days) {
+                    System.out.println(day);
+                }
+
+	}
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/enums/values/Element1UnitTest.java

@@ -0,0 +1,48 @@
+package com.baeldung.enums.values;
+
+import org.junit.After;
+import org.junit.AfterClass;
+import org.junit.Before;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+/**
+ *
+ * @author chris
+ */
+public class Element1UnitTest {
+    
+    public Element1UnitTest() {
+    }
+    
+    @BeforeClass
+    public static void setUpClass() {
+    }
+    
+    @AfterClass
+    public static void tearDownClass() {
+    }
+    
+    @Before
+    public void setUp() {
+    }
+    
+    @After
+    public void tearDown() {
+    }
+
+    @Test
+    public void whenAccessingToString_thenItShouldEqualName() {
+        for (Element1 e1 : Element1.values()) {
+            assertEquals(e1.name(), e1.toString());
+        }
+    }
+    
+    @Test
+    public void whenCallingValueOf_thenReturnTheCorrectEnum() {
+        for (Element1 e1 : Element1.values()) {
+            assertSame(e1, Element1.valueOf(e1.name()));
+        }
+    }
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/enums/values/Element2UnitTest.java

@@ -0,0 +1,59 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package com.baeldung.enums.values;
+
+import org.junit.After;
+import org.junit.AfterClass;
+import org.junit.Before;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import static org.junit.Assert.*;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ *
+ * @author chris
+ */
+public class Element2UnitTest {
+    private static final Logger LOGGER = LoggerFactory.getLogger(Element2UnitTest.class);
+    
+    public Element2UnitTest() {
+    }
+    
+    @BeforeClass
+    public static void setUpClass() {
+    }
+    
+    @AfterClass
+    public static void tearDownClass() {
+    }
+    
+    @Before
+    public void setUp() {
+    }
+    
+    @After
+    public void tearDown() {
+    }
+
+    @Test
+    public void whenLocatebyLabel_thenReturnCorrectValue() {
+        for (Element2 e2 : Element2.values()) {
+            assertSame(e2, Element2.valueOfLabel(e2.label));
+        }
+    }
+
+    /**
+     * Test of toString method, of class Element2.
+     */
+    @Test
+    public void whenCallingToString_thenReturnLabel() {
+        for (Element2 e2 : Element2.values()) {
+            assertEquals(e2.label, e2.toString());
+        }
+    }
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/enums/values/Element3UnitTest.java

@@ -0,0 +1,57 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package com.baeldung.enums.values;
+
+import org.junit.After;
+import org.junit.AfterClass;
+import org.junit.Before;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+/**
+ *
+ * @author chris
+ */
+public class Element3UnitTest {
+    
+    public Element3UnitTest() {
+    }
+    
+    @BeforeClass
+    public static void setUpClass() {
+    }
+    
+    @AfterClass
+    public static void tearDownClass() {
+    }
+    
+    @Before
+    public void setUp() {
+    }
+    
+    @After
+    public void tearDown() {
+    }
+
+    @Test
+    public void whenLocatebyLabel_thenReturnCorrectValue() {
+        for (Element3 e3 : Element3.values()) {
+            assertSame(e3, Element3.valueOfLabel(e3.label));
+        }
+    }
+
+    /**
+     * Test of toString method, of class Element3.
+     */
+    @Test
+    public void whenCallingToString_thenReturnLabel() {
+        for (Element3 e3 : Element3.values()) {
+            assertEquals(e3.label, e3.toString());
+        }
+    }
+    
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/enums/values/Element4UnitTest.java

@@ -0,0 +1,71 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package com.baeldung.enums.values;
+
+import org.junit.After;
+import org.junit.AfterClass;
+import org.junit.Before;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+/**
+ *
+ * @author chris
+ */
+public class Element4UnitTest {
+    
+    public Element4UnitTest() {
+    }
+    
+    @BeforeClass
+    public static void setUpClass() {
+    }
+    
+    @AfterClass
+    public static void tearDownClass() {
+    }
+    
+    @Before
+    public void setUp() {
+    }
+    
+    @After
+    public void tearDown() {
+    }
+
+    @Test
+    public void whenLocatebyLabel_thenReturnCorrectValue() {
+        for (Element4 e4 : Element4.values()) {
+            assertSame(e4, Element4.valueOfLabel(e4.label));
+        }
+    }
+
+    @Test
+    public void whenLocatebyAtmNum_thenReturnCorrectValue() {
+        for (Element4 e4 : Element4.values()) {
+            assertSame(e4, Element4.valueOfAtomicNumber(e4.atomicNumber));
+        }
+    }
+
+    @Test
+    public void whenLocatebyAtmWt_thenReturnCorrectValue() {
+        for (Element4 e4 : Element4.values()) {
+            assertSame(e4, Element4.valueOfAtomicWeight(e4.atomicWeight));
+        }
+    }
+
+    /**
+     * Test of toString method, of class Element4.
+     */
+    @Test
+    public void whenCallingToString_thenReturnLabel() {
+        for (Element4 e4 : Element4.values()) {
+            assertEquals(e4.label, e4.toString());
+        }
+    }
+    
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/finalize/FinalizeUnitTest.java

@@ -0,0 +1,23 @@
+package com.baeldung.finalize;
+
+import java.io.IOException;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class FinalizeUnitTest {
+    @Test
+    public void whenGC_thenFinalizerExecuted() throws IOException {
+        String firstLine = new Finalizable().readFirstLine();
+        Assert.assertEquals("baeldung.com", firstLine);
+        System.gc();
+    }
+
+    @Test
+    public void whenTryWResourcesExits_thenResourceClosed() throws IOException {
+        try (CloseableResource resource = new CloseableResource()) {
+            String firstLine = resource.readFirstLine();
+            Assert.assertEquals("baeldung.com", firstLine);
+        }
+    }
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/recursion/RecursionExampleUnitTest.java

@@ -0,0 +1,61 @@
+package com.baeldung.recursion;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class RecursionExampleUnitTest {
+    
+    RecursionExample recursion = new RecursionExample();
+
+    @Test
+    public void testPowerOf10() {
+        int p0 = recursion.powerOf10(0);
+        int p1 = recursion.powerOf10(1);
+        int p4 = recursion.powerOf10(4);
+        
+        Assert.assertEquals(1, p0);
+        Assert.assertEquals(10, p1);
+        Assert.assertEquals(10000, p4);
+    }
+    
+    @Test
+    public void testFibonacci() {
+        int n0 = recursion.fibonacci(0);
+        int n1 = recursion.fibonacci(1);
+        int n7 = recursion.fibonacci(7);
+        
+        Assert.assertEquals(0, n0);
+        Assert.assertEquals(1, n1);
+        Assert.assertEquals(13, n7);
+    }
+    
+    @Test
+    public void testToBinary() {
+        String b0 = recursion.toBinary(0);
+        String b1 = recursion.toBinary(1);
+        String b10 = recursion.toBinary(10);
+        
+        Assert.assertEquals("0", b0);
+        Assert.assertEquals("1", b1);
+        Assert.assertEquals("1010", b10);
+    }
+    
+    @Test
+    public void testCalculateTreeHeight() {
+        BinaryNode root = new BinaryNode(1);
+        root.setLeft(new BinaryNode(1));
+        root.setRight(new BinaryNode(1));
+        
+        root.getLeft().setLeft(new BinaryNode(1));
+        root.getLeft().getLeft().setRight(new BinaryNode(1));
+        root.getLeft().getLeft().getRight().setLeft(new BinaryNode(1));
+        
+        root.getRight().setLeft(new BinaryNode(1));
+        root.getRight().getLeft().setRight(new BinaryNode(1));
+        
+        int height = recursion.calculateTreeHeight(root);
+        
+        Assert.assertEquals(4, height);
+    }
+
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/synthetic/SyntheticUnitTest.java

@@ -0,0 +1,99 @@
+package com.baeldung.synthetic;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+/**
+ * Unit test for {@link SyntheticFieldDemo}, {@link SyntheticMethodDemo},
+ * {@link SyntheticConstructorDemo} and {@link BridgeMethodDemo} classes.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class SyntheticUnitTest {
+
+    /**
+     * Tests that the {@link SyntheticMethodDemo.NestedClass} contains two synthetic
+     * methods.
+     */
+    @Test
+    public void givenSyntheticMethod_whenIsSinthetic_thenTrue() {
+        // Checks that the nested class contains exactly two synthetic methods.
+        Method[] methods = SyntheticMethodDemo.NestedClass.class.getDeclaredMethods();
+        Assert.assertEquals("This class should contain only two methods", 2, methods.length);
+
+        for (Method m : methods) {
+            System.out.println("Method: " + m.getName() + ", isSynthetic: " + m.isSynthetic());
+            Assert.assertTrue("All the methods of this class should be synthetic", m.isSynthetic());
+        }
+    }
+
+    /**
+     * Tests that {@link SyntheticConstructorDemo.NestedClass} contains a synthetic
+     * constructor.
+     */
+    @Test
+    public void givenSyntheticConstructor_whenIsSinthetic_thenTrue() {
+        // Checks that the nested class contains exactly a synthetic
+        // constructor.
+        int syntheticConstructors = 0;
+        Constructor<?>[] constructors = SyntheticConstructorDemo.NestedClass.class.getDeclaredConstructors();
+        Assert.assertEquals("This class should contain only two constructors", 2, constructors.length);
+
+        for (Constructor<?> c : constructors) {
+            System.out.println("Constructor: " + c.getName() + ", isSynthetic: " + c.isSynthetic());
+
+            // Counts the synthetic constructors.
+            if (c.isSynthetic()) {
+                syntheticConstructors++;
+            }
+        }
+
+        // Checks that there's exactly one synthetic constructor.
+        Assert.assertEquals(1, syntheticConstructors);
+    }
+
+    /**
+     * Tests that {@link SyntheticFieldDemo.NestedClass} contains a synthetic field.
+     */
+    @Test
+    public void givenSyntheticField_whenIsSinthetic_thenTrue() {
+        // This class should contain exactly one synthetic field.
+        Field[] fields = SyntheticFieldDemo.NestedClass.class.getDeclaredFields();
+        Assert.assertEquals("This class should contain only one field", 1, fields.length);
+
+        for (Field f : fields) {
+            System.out.println("Field: " + f.getName() + ", isSynthetic: " + f.isSynthetic());
+            Assert.assertTrue("All the fields of this class should be synthetic", f.isSynthetic());
+        }
+    }
+
+    /**
+     * Tests that {@link BridgeMethodDemo} contains a synthetic bridge method.
+     */
+    @Test
+    public void givenBridgeMethod_whenIsBridge_thenTrue() {
+        // This class should contain exactly one synthetic bridge method.
+        int syntheticMethods = 0;
+        Method[] methods = BridgeMethodDemo.class.getDeclaredMethods();
+        for (Method m : methods) {
+            System.out.println(
+                    "Method: " + m.getName() + ", isSynthetic: " + m.isSynthetic() + ", isBridge: " + m.isBridge());
+
+            // Counts the synthetic methods and checks that they are also bridge
+            // methods.
+            if (m.isSynthetic()) {
+                syntheticMethods++;
+                Assert.assertTrue("The synthetic method in this class should also be a bridge method", m.isBridge());
+            }
+        }
+        
+        // Checks that there's exactly one synthetic bridge method.
+        Assert.assertEquals("There should be exactly 1 synthetic bridge method in this class", 1, syntheticMethods);
+    }
+
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/system/DateTimeServiceUnitTest.java

@@ -0,0 +1,15 @@
+package com.baeldung.system;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class DateTimeServiceUnitTest {
+
+    @Test
+    public void givenClass_whenCalledMethods_thenNotNullInResult() {
+        DateTimeService dateTimeService = new DateTimeService();
+
+        Assert.assertNotNull(dateTimeService.nowPlusOneHour());
+        Assert.assertNotNull(dateTimeService.nowPrettyPrinted());
+    }
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/system/EnvironmentVariablesUnitTest.java

@@ -0,0 +1,14 @@
+package com.baeldung.system;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class EnvironmentVariablesUnitTest {
+
+    @Test
+    public void givenEnvVars_whenReadPath_thenGetValueinResult() {
+        EnvironmentVariables environmentVariables = new EnvironmentVariables();
+
+        Assert.assertNotNull(environmentVariables.getPath());
+    }
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/system/SystemArrayCopyUnitTest.java

@@ -0,0 +1,27 @@
+package com.baeldung.system;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class SystemArrayCopyUnitTest {
+
+    @Test
+    public void givenTwoArraysAB_whenUseArrayCopy_thenArrayCopiedFromAToBInResult() {
+        int[] a = {34, 22, 44, 2, 55, 3};
+        int[] b = new int[a.length];
+
+        // copy all elements from a to b
+        System.arraycopy(a, 0, b, 0, a.length);
+        Assert.assertArrayEquals(a, b);
+    }
+
+    @Test
+    public void givenTwoArraysAB_whenUseArrayCopyPosition_thenArrayCopiedFromAToBInResult() {
+        int[] a = {34, 22, 44, 2, 55, 3};
+        int[] b = new int[a.length];
+
+        // copy 2 elements from a, starting at a[1] to b, starting at b[3]
+        System.arraycopy(a, 1, b, 3, 2);
+        Assert.assertArrayEquals(new int[] {0, 0, 0, 22, 44, 0}, b);
+    }
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/system/SystemNanoUnitTest.java

@@ -0,0 +1,16 @@
+package com.baeldung.system;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class SystemNanoUnitTest {
+
+    @Test
+    public void givenSystem_whenCalledNanoTime_thenGivesTimeinResult() {
+        long startTime = System.nanoTime();
+        // do something that takes time
+        long endTime = System.nanoTime();
+
+        Assert.assertTrue(endTime - startTime < 10000);
+    }
+}

core-java-modules/core-java-lang/src/test/java/com/baeldung/system/SystemPropertiesUnitTest.java

@@ -0,0 +1,56 @@
+package com.baeldung.system;
+
+import org.junit.Assert;
+import org.junit.Ignore;
+import org.junit.Test;
+
+import java.util.Properties;
+
+public class SystemPropertiesUnitTest {
+
+    @Test
+    public void givenSystem_whenCalledGetProperty_thenReturnPropertyinResult() {
+        Assert.assertNotNull(System.getProperty("java.vm.vendor"));
+    }
+
+    @Test
+    public void givenSystem_whenCalledSetProperty_thenSetPropertyasResult() {
+
+        // set a particular property
+        System.setProperty("abckey", "abcvaluefoo");
+        Assert.assertEquals("abcvaluefoo", System.getProperty("abckey"));
+    }
+
+    @Test
+    public void givenSystem_whenCalledClearProperty_thenDeletePropertyasResult() {
+
+        // Delete a property
+        System.clearProperty("abckey");
+        Assert.assertNull(System.getProperty("abckey"));
+    }
+
+    @Test
+    public void givenSystem_whenCalledGetPropertyDefaultValue_thenReturnPropertyinResult() {
+
+        System.clearProperty("dbHost");
+        String myKey = System.getProperty("dbHost", "db.host.com");
+        Assert.assertEquals("db.host.com", myKey);
+    }
+
+    @Test
+    public void givenSystem_whenCalledGetProperties_thenReturnPropertiesinResult() {
+        Properties properties = System.getProperties();
+
+        Assert.assertNotNull(properties);
+    }
+
+    @Test
+    @Ignore
+    public void givenSystem_whenCalledClearProperties_thenDeleteAllPropertiesasResult() {
+
+        // Clears all system properties. Use with care!
+        System.getProperties().clear();
+
+        Assert.assertTrue(System.getProperties().isEmpty());
+    }
+}