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/test/java/com/baeldung/staticdemo/CarIntegrationTest.java

@@ -0,0 +1,14 @@
+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"));
+    }
+}