UID key in TreeMap causing ClassCastException

Question

Hello all,
I'm trying to use the UID class as a key in a TreeMap, but I'm running into a ClassCastException when I try to lookup using a UID as a key. From the docs, I see that a ClassCastException is thrown when the "key cannot be compared with the keys currently in the map." My guess is that the key needs to implement the comparable interface but I can't find mention of it anywhere in the docs. (other than the 'compared with keys' statement) ... So, two questions:
1) Can someone verify my suspicions about comparable interface?
2) Is there a better solution that implementing my own 'comparable subclass of UID? (I don't mind subclassing, but I'm loath to write/maintain/test/document a custom solution if I can use a built in class)
     protected void testTreemapUID() {
          TreeMap tree = new TreeMap();
          Double testDouble = new Double(23);
          tree.put( testDouble, new String("frank"));
          String result = (String)tree.get(testDouble);
          System.out.println("about to get value from Double " );
          System.out.println("Double result = " + result );
          tree = new TreeMap();
          UID uid = new UID();
          tree.put(uid, new String("bob"));
          System.out.println("about to get value from UID " );
          result = (String)tree.get( uid );
          System.out.println("UID result = " + result );
     }console output:
about to get value from Double
Double result = frank
about to get value from UID
Exception in thread "main" java.lang.ClassCastException: java.rmi.server.UID
     at java.util.TreeMap.compare(TreeMap.java:1093)
     at java.util.TreeMap.getEntry(TreeMap.java:347)
     at java.util.TreeMap.get(TreeMap.java:265)
     at SandBoxTester.testTreemapUID(SandBoxTester.java:70)
     at SandBoxTester.<init>(SandBoxTester.java:53)
     at SandBoxTester.main(SandBoxTester.java:100)
as always, many thanks!
Message was edited by:
oldmicah

Answer 1

Because you are using a TreeMap, which is a sorted map, the keys must be mutually comparable.
java.rmi.server.UID doesn't implement Comparable. That's why the ClassCastException is thrown.
If you need a sorted map, then you will have to provide a Comparator*.
If you don't care about sorting, you could use a HashMap instead.