ClassCastException in TreeSet

[pHew]

Hello Java guys,

I have a problem. I keep getting a ClassCastException I don't understand. Let me tell you what happens:

Code:

Set<A> mySet = new HashSet<A>();

(filling mySet with objects of type A)

Now I want to order the Set:

Code:

Set<A> sortedSet = new TreeSet<A>(mySet);

BANG! ClassCastException in TreeMap.compare

I don't understand it. All the objects in mySet are of type A. Why could he be wanting to cast them anyway?

BTW, the toString() method of type A has been rewritten to return the member String name. If that has something to do with it.

Thanks for reading this,
pHew

[pHew]

Yay, I have it! I need to implement the "Comparable" interface. Thanks a lot guys!

[gibsonrocker800]

Yeah, good job! Keep in mind that TreeSets sort out your data, but how else would it know how to sort it unless you have defined compareTo() in the class, right? For example, say i had a BankAccount class, which should be sorted by balance. For all the TreeSet knows, you could want it to be sorted by the account number. That's why you must define compareTo() and make it implement Comparable.

"This class guarantees that the sorted set will be in ascending element order, sorted according to the natural order of the elements ([see Comparable), or by the comparator provided at set creation time, depending on which constructor is used."