Added tests for null-object pattern

null-object/pom.xml

@@ -14,5 +14,10 @@
       <artifactId>junit</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.mockito</groupId>
+      <artifactId>mockito-core</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 </project>

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());
+  }
+
+}

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

@@ -0,0 +1,54 @@
+package com.iluwatar.nullobject;
+
+import org.junit.After;
+import org.junit.Before;
+
+import java.io.PrintStream;
+
+import static org.mockito.Mockito.mock;
+
+/**
+ * Date: 12/10/15 - 8:37 PM
+ *
+ * @author Jeroen Meulemeester
+ */
+public abstract class StdOutTest {
+
+  /**
+   * The mocked standard out {@link PrintStream}, required since walking through the tree has no
+   * influence on any other accessible object, except for writing to std-out using {@link
+   * System#out}
+   */
+  private final PrintStream stdOutMock = mock(PrintStream.class);
+
+  /**
+   * Keep the original std-out so it can be restored after the test
+   */
+  private final PrintStream stdOutOrig = System.out;
+
+  /**
+   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
+   */
+  @Before
+  public void setUp() {
+    System.setOut(this.stdOutMock);
+  }
+
+  /**
+   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
+   */
+  @After
+  public void tearDown() {
+    System.setOut(this.stdOutOrig);
+  }
+
+  /**
+   * Get the mocked stdOut {@link PrintStream}
+   *
+   * @return The stdOut print stream mock, renewed before each test
+   */
+  final PrintStream getStdOutMock() {
+    return this.stdOutMock;
+  }
+
+}

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

@@ -0,0 +1,101 @@
+package com.iluwatar.nullobject;
+
+import org.junit.Test;
+import org.mockito.InOrder;
+import org.mockito.Mockito;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertSame;
+
+/**
+ * Date: 12/26/15 - 11:44 PM
+ *
+ * @author Jeroen Meulemeester
+ */
+public class TreeTest extends StdOutTest {
+
+  /**
+   * During the tests, the same tree structure will be used, shown below. End points will be
+   * terminated with the {@link NullNode} instance.
+   *
+   * <pre>
+   * root
+   * ├── level1_a
+   * │   ├── level2_a
+   * │   │   ├── level3_a
+   * │   │   └── level3_b
+   * │   └── level2_b
+   * └── level1_b
+   * </pre>
+   */
+  private static final Node TREE_ROOT;
+
+  static {
+    final NodeImpl level1_b = new NodeImpl("level1_b", NullNode.getInstance(), NullNode.getInstance());
+    final NodeImpl level2_b = new NodeImpl("level2_b", NullNode.getInstance(), NullNode.getInstance());
+    final NodeImpl level3_a = new NodeImpl("level3_a", NullNode.getInstance(), NullNode.getInstance());
+    final NodeImpl level3_b = new NodeImpl("level3_b", NullNode.getInstance(), NullNode.getInstance());
+    final NodeImpl level2_a = new NodeImpl("level2_a", level3_a, level3_b);
+    final NodeImpl level1_a = new NodeImpl("level1_a", level2_a, level2_b);
+    TREE_ROOT = new NodeImpl("root", level1_a, level1_b);
+  }
+
+  /**
+   * Verify the number of items in the tree. The root has 6 children so we expect a {@link
+   * Node#getTreeSize()} of 7 {@link Node}s in total.
+   */
+  @Test
+  public void testTreeSize() {
+    assertEquals(7, TREE_ROOT.getTreeSize());
+  }
+
+  /**
+   * Walk through the tree and verify if every item is handled
+   */
+  @Test
+  public void testWalk() {
+    TREE_ROOT.walk();
+
+    final InOrder inOrder = Mockito.inOrder(getStdOutMock());
+    inOrder.verify(getStdOutMock()).println("root");
+    inOrder.verify(getStdOutMock()).println("level1_a");
+    inOrder.verify(getStdOutMock()).println("level2_a");
+    inOrder.verify(getStdOutMock()).println("level3_a");
+    inOrder.verify(getStdOutMock()).println("level3_b");
+    inOrder.verify(getStdOutMock()).println("level2_b");
+    inOrder.verify(getStdOutMock()).println("level1_b");
+    inOrder.verifyNoMoreInteractions();
+  }
+
+  @Test
+  public void testGetLeft() throws Exception {
+    final Node level1 = TREE_ROOT.getLeft();
+    assertNotNull(level1);
+    assertEquals("level1_a", level1.getName());
+    assertEquals(5, level1.getTreeSize());
+
+    final Node level2 = level1.getLeft();
+    assertNotNull(level2);
+    assertEquals("level2_a", level2.getName());
+    assertEquals(3, level2.getTreeSize());
+
+    final Node level3 = level2.getLeft();
+    assertNotNull(level3);
+    assertEquals("level3_a", level3.getName());
+    assertEquals(1, level3.getTreeSize());
+    assertSame(NullNode.getInstance(), level3.getRight());
+    assertSame(NullNode.getInstance(), level3.getLeft());
+  }
+
+  @Test
+  public void testGetRight() throws Exception {
+    final Node level1 = TREE_ROOT.getRight();
+    assertNotNull(level1);
+    assertEquals("level1_b", level1.getName());
+    assertEquals(1, level1.getTreeSize());
+    assertSame(NullNode.getInstance(), level1.getRight());
+    assertSame(NullNode.getInstance(), level1.getLeft());
+  }
+
+}