FIFO tie breaker in Comparator?

For a homework, I need to compare Nodes based on a heuristic so that I can put them in a TreeSet. However, when 开发者_开发知识库the heuristic values for two Nodes are equal I need some way to break the tie.

I'm not allowed to modify the Node class provided and as far as I can tell there aren't any other values / properties of Node that would help me break the tie that I'm not already using. (We're dealing with puzzles, not that it really matters.) Is there some way I could break ties based on when I added them to my TreeSet? I just don't know how to go about it....

I saw an example in the documentation for a Priority Blocking Queue but I want to use the Comparator interface, and I can't get it to work.

Thanks in advance; any tips / hints are very much appreciated.

Are you allowed to create a wrapper for Nodes?

public class NodeWrapper implements Comparable<NodeWrapper> {
  private static AtomicLong serialNumGenerator = new AtomicLong(0L);

  private final Node node;
  private final long serialNum;

  public NodeWrapper(Node node) {
    this.node = node;
    this.serialNum = serialNumGenerator.getAndIncrement();
  }

  @Override
  public int compareTo(NodeWrapper other) {
    int compare = this.node.compareTo(other.node);
    if (compare == 0) {
      compare = (this.serialNum < other.serialNum)
          ? -1
          : ((this.serialNum > other.serialNum) ? 1 : 0);
    }
    return compare;
  }
  // implement other methods including equals() and hashCode()
}

You can try System.identityHashCode(Object) to get an int that you can use to sort the objects.