Consistent Equals() results, but inconsistent TreeMap.containsKey() result

问题

I have the following object Node:

    private class Node implements Comparable<Node>(){
         private String guid();

         ...

         public boolean equals(Node o){
             return (this == o);
         }

         public int hashCode(){
              return guid.hashCode();
         }

         public int compareTo(Node o){
            return (this.hashCode() - o.hashCode());
         }

         ...

    }

And I use it in the following TreeMap:

TreeMap<Node, TreeSet<Edge>> nodes = new TreeMap<Node, TreeSet<Edge>>();

Now, the tree map is used in a class called Graph to store nodes currently in the graph, along with a set of their edges (from the class Edge). My problem is when I try to execute:

   public containsNode(n){
        for (Node x : nodes.keySet()) {
            System.out.println("HASH CODE: ");
            System.out.print(x.hashCode() == n.hashCode());
            System.out.println("EQUALS: ");
            System.out.print(x.equals(n));
            System.out.println("CONTAINS: ");
            System.out.print(nodes.containsKey(n));
            System.out.println("N: " + n);
            System.out.println("X: " + x);
            System.out.println("COMPARES: ");
            System.out.println(n.compareTo(x));
            }
        }

I sometimes get the following:

HASHCODE: true EQUALS: true CONTAINS: false N: foo X: foo COMPARES: 0

Anyone have an idea as to what I'm doing wrong? I'm still new to all this, so I apologize in advance if I'm overlooking something simple (I know hashCode() doesn't really matter for TreeMap, but I figured I'd include it).

edit1: added compareTo() method information.

回答1:

There are a couple of things wrong here.

You have not overriden Object.equals. Use @Override public boolean equals(Object obj).
There is a potential integer overflow error in compareTo. This is probably the cause of this particular error. It will upset the sorting, and hence the search may well not succeed.
The compareTo method claims two instances are equal if the hash code happens to match (could be a difficult error to catch, without code review).

For integer overflow issue, see question Why is my simple comparator broken?

回答2:

TreeSet does not use equals() to determine equality. It uses the Comparator (or Comparable) instead. In order to make it work right, you must follow the consistency with equals rule:

"The ordering imposed by a comparator c on a set of elements S is said to be consistent with equals if and only if c.compare(e1, e2)==0 has the same boolean value as e1.equals(e2) for every e1 and e2 in S".

I guess you are not following this rule (you didn't provide the implementation of the compareTo method). When the rule is not followed, the tree set will not have a normal behavior of a Set.

For details see http://eyalsch.wordpress.com/2009/11/23/comparators/.

--EDIT--

Now that you provided your compareTo implementation, is is clear that it has a flaw. It may return 0 for 2 nodes which are not equal (and have the same hashcode). As a result of this, you can't add 2 items with the same hash code into your TreeSet!