Added tests for null-object pattern

null-object/src/test/java/com/iluwatar/nullobject/NullNodeTest.java

@@ -0,0 +1,43 @@
+package com.iluwatar.nullobject;
+
+import org.junit.Test;
+import org.mockito.Mockito;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+
+/**
+ * Date: 12/26/15 - 11:47 PM
+ *
+ * @author Jeroen Meulemeester
+ */
+public class NullNodeTest extends StdOutTest {
+
+  /**
+   * Verify if {@link NullNode#getInstance()} actually returns the same object instance
+   */
+  @Test
+  public void testGetInstance() {
+    final NullNode instance = NullNode.getInstance();
+    assertNotNull(instance);
+    assertSame(instance, NullNode.getInstance());
+  }
+
+  @Test
+  public void testFields() {
+    final NullNode node = NullNode.getInstance();
+    assertEquals(0, node.getTreeSize());
+    assertNull(node.getName());
+    assertNull(node.getLeft());
+    assertNull(node.getRight());
+  }
+
+  @Test
+  public void testWalk() throws Exception {
+    NullNode.getInstance().walk();
+    Mockito.verifyZeroInteractions(getStdOutMock());
+  }
+
+}