### Question:
NodeDiff implements GraphDiff, Comparable<NodeDiff> { public Object getNodeId() { return nodeId; } NodeDiff(Object nodeId);  NodeDiff(Object nodeId, int diffId); @Override boolean isNoop(); @Override abstract void apply(GraphChangeHandler tracker); @Override abstract void undo(GraphChangeHandler tracker); Object getNodeId(); int getDiffId(); void setDiffId(int diffId); @Override int compareTo(NodeDiff o);  }### Answer:
@Test public void testGetNodeId() { Object id = new Object(); NodeDiff diff = new ConcreteNodeDiff(id); assertSame(id, diff.getNodeId()); }