BAEL-1210 Guide to the Static Keyword in Java (#2767)

* Adding Source Files

* Guide to the Static Keyword in Java - Test files

core-java/src/main/java/com/baeldung/staticdemo/Car.java

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+package com.baeldung.staticdemo;
+
+/**
+ * This class demonstrates the use of static fields and static methods
+ * the instance variables engine and displacement are distinct for 
+ * each and every object whereas static/class variable numberOfCars
+ * is unique and is shared across all objects of this class.
+ * 
+ * @author baeldung
+ *
+ */
+public class Car {
+    private String name;
+    private String engine;
+    
+    public static int numberOfCars;
+    
+    public Car(String name, String engine) {
+        this.name = name;
+        this.engine = engine;
+        numberOfCars++;
+    }
+
+    //getters and setters
+    public static int getNumberOfCars() {
+        return numberOfCars;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public String getEngine() {
+        return engine;
+    }
+
+    public void setEngine(String engine) {
+        this.engine = engine;
+    }
+
+    public static void setNumberOfCars(int numberOfCars) {
+        Car.numberOfCars = numberOfCars;
+    }
+}

core-java/src/main/java/com/baeldung/staticdemo/Singleton.java

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+package com.baeldung.staticdemo;
+
+public class Singleton  {    
+    private Singleton() {}
+    
+    private static class SingletonHolder {    
+        public static final Singleton instance = new Singleton();
+    }    
+
+    public static Singleton getInstance() {    
+        return SingletonHolder.instance;    
+    }    
+}

core-java/src/main/java/com/baeldung/staticdemo/StaticBlock.java

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+package com.baeldung.staticdemo;
+
+import java.util.LinkedList;
+import java.util.List;
+
+public class StaticBlock {
+    private static List<String> ranks = new LinkedList<>();
+
+    static {
+        ranks.add("Lieutenant");
+        ranks.add("Captain");
+        ranks.add("Major");
+    }
+    
+    static {
+        ranks.add("Colonel");
+        ranks.add("General");
+    }
+
+    //getters and setters
+    public static List<String> getRanks() {
+        return ranks;
+    }
+
+    public static void setRanks(List<String> ranks) {
+        StaticBlock.ranks = ranks;
+    }
+}

core-java/src/test/java/com/baeldung/staticdemo/CarIntegrationTest.java

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+package com.baeldung.staticdemo;
+
+import static org.junit.Assert.*;
+
+import org.junit.Test;
+
+public class CarIntegrationTest {
+    @Test
+    public void whenNumberOfCarObjectsInitialized_thenStaticCounterIncreases() {
+        new Car("Jaguar", "V8");
+        new Car("Bugatti", "W16");
+        assertEquals(2, Car.numberOfCars);
+    }
+}

core-java/src/test/java/com/baeldung/staticdemo/SingletonIntegrationTest.java

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+package com.baeldung.staticdemo;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class SingletonIntegrationTest {
+
+    @Test
+    public void givenStaticInnerClass_whenMultipleTimesInstanceCalled_thenOnlyOneTimeInitialized() {
+        Singleton object1 = Singleton.getInstance();
+        Singleton object2 = Singleton.getInstance();
+        
+        Assert.assertSame(object1, object2);
+    }
+}

core-java/src/test/java/com/baeldung/staticdemo/StaticBlockIntegrationTest.java

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+package com.baeldung.staticdemo;
+
+import static org.hamcrest.collection.IsIterableContainingInOrder.contains;
+import static org.junit.Assert.assertThat;
+
+import java.util.List;
+
+import org.junit.Test;
+
+public class StaticBlockIntegrationTest {
+
+    @Test
+    public void whenAddedListElementsThroughStaticBlock_thenEnsureCorrectOrder() {
+        List<String> actualList = StaticBlock.getRanks();
+        assertThat(actualList, contains("Lieutenant", "Captain", "Major", "Colonel", "General"));
+    }
+}