### Question:
NullNode implements Node { public static NullNode getInstance() { return instance; } private  NullNode(); static NullNode getInstance(); @Override int getTreeSize(); @Override Node getLeft(); @Override Node getRight(); @Override String getName(); @Override void walk();  }### Answer:
@Test void testGetInstance() { final var instance = NullNode.getInstance(); assertNotNull(instance); assertSame(instance, NullNode.getInstance()); }
@Test public void testGetInstance() { final NullNode instance = NullNode.getInstance(); assertNotNull(instance); assertSame(instance, NullNode.getInstance()); }