Exception: javax.swing.KeyStroke cannot be cast to java.lang.Comparable? Need a reason for this error

Question

I'm working on an enterprise application. It was working fine earlier when the system had JDK1.6.+. I upgraded my JDK to 1.7.+. After that, I'm getting the error:

Caused by: java.lang.ClassCastException: javax.swing.KeyStroke cannot be cast to java.lang.Comparable
    at java.util.TreeMap.compare(Unknown Source)
    at java.util.TreeMap.put(Unknown Source)
    at java.util.TreeSet.add(Unknown Source)

Although I have solved my problem by switching between the JDK versions. I want to understand why the error is coming after upgrading to JDK 7. Obviously, there would be some changes in the implementation.But I want to understand that. I tried to check the Documentation of keystroke class. But I got nothing related to this.

Can someone explain this? Thanks in advance.

@Jarrod: This question doesn't have any answer. I asked the reason for why it's working when app is running with JDK 1.6 and not with JDK 1.7 — Raj Wadhwa, Apr 22 '15 at 09:58

score 0 · Answer 1 · edited May 23 '17 at 12:28

A TreeMap requires the key class to be orderable because it creates a tree of keys that are ordered. So you need to provide a way of ordering the keys, either via natural ordering (i.e., implementing the Comparable interface in the key class) or via a comparator class provided at TreeMap construction time.

KeyStroke does not implement Comparable and therefore does not provide natural ordering. This is the reason for the error. Simply provide a Comparator implementation that takes care of providing a way of ordering the keys, for example (ordering by keycode):

map = new TreeMap<KeyStroke, YourValueClass>(new Comparator<KeyStroke>() {
  public int compare(KeyStroke o1, KeyStroke o2) {
    return o1.getKeyCode() - o2.getKeyCode();
  }
});

More info about ordering requirements of TreeMap: How can I sort the keys of a Map in Java?

How Red-Black Trees (what TreeMap is) work, this should clarify why the key needs to be orderable (because it must be determined where to put it in the tree of keys): Explanation of Red-Black tree based implementation of TreeMap in JAVA

Thanks for the info. So, by that, do we mean that JDK 6, has the comparator available? — Raj Wadhwa, Apr 22 '15 at 10:00

score 0 · Answer 2 · edited May 23 '17 at 12:14

Why a TreeMap requires a class to be Comparable is nicely explained by other answers.

The other answers do not explain why you have the problem after changing from Java 6+ to Java 7+. The behavior of TreeMap is the same for Java 6 and 7 (I checked it in my IDE). Maybe your code uses another implementation in Java 7 due to Depency Injection. Just a guess. Could you check how your map is instantiated?

Exception: javax.swing.KeyStroke cannot be cast to java.lang.Comparable? Need a reason for this error

2 Answers2