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copied!<p>In my humble opinion, you're thinking it in a more complicated way than it really is. In order to revert to previous state, clone of whole node is not required at all. Rather each <em>*</em>*Command class will have -</p> <ol> <li>reference to the node it is acting upon, </li> <li>memento object (having state variables just enough for the node to revert to)</li> <li>execute() method </li> <li>undo() method.</li> </ol> <p>Since command classes have reference to the node, we do not need ID mechanism to refer to objects in the diagram.</p> <p>In the example from your question, we want to move a node to a new position. For that, we have a NodePositionChangeCommand class.</p> <pre><code>public class NodePositionChangeCommand { // This command will act upon this node private Node node; // Old state is stored here private NodePositionMemento previousNodePosition; NodePositionChangeCommand(Node node) { this.node = node; } public void execute(NodePositionMemento newPosition) { // Save current state in memento object previousNodePosition // Act upon this.node } public void undo() { // Update this.node object with values from this.previousNodePosition } } </code></pre> <blockquote> <blockquote> <p>What about links? They should be movable too but I don't want to create a command just for links (and one just for nodes).</p> </blockquote> </blockquote> <p>I read in GoF book (in memento pattern discussion) that move of link with change in position of nodes are handled by some kind of constraint solver.</p>

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