### Question:
NullNode implements Node { @Override public void walk() { } private  NullNode(); static NullNode getInstance(); @Override int getTreeSize(); @Override Node getLeft(); @Override Node getRight(); @Override String getName(); @Override void walk();  }### Answer:
@Test public void testWalk() { NullNode.getInstance().walk(); }
@Test public void testWalk() throws Exception { NullNode.getInstance().walk(); }
@Test public void testWalk() throws Exception { NullNode.getInstance().walk(); Mockito.verifyZeroInteractions(getStdOutMock()); }