problem with TreeMap - element removal fails.

Hello,
I can't solve some problem. I'm looking for mistake in my thinking or
any walkaround.

There is some sample code:

import java.util.TreeMap;
class A {
public A() {
}
public static final void main(String[] args) {
A a = new A();
TreeMap<A, Integer> map = new TreeMap<A, Integer>();
map.put(a, new Integer(1));
map.remove(a);
}
}

Executing last line of this code throws ClassCastException. Why element
can be putted into this TreeMap and can't be removed? I've been trying
some solutions but none has worked :( .

Thank You for any answer.

--
Pozdrawiam,
Tomasz Milewski

.