Using a simple class with overridden .equals() for TreeSet.contains() doesn't work

I have this class, which is defined as an inner class of another class
(so it's not the most beautiful class ever, but I needed it for a
really simple task):

#######################################################################

class SimpleClass {
            String attrib_one;
            String attrib_two;
            String attrib_three;

            SimpleClass(String attrib_one, String attrib_two, String
attrib_three) {
                this.attrib_one = attrib_one;
                this.attrib_two = attrib_two;
                attrib_three = attrib_three;
            }

            public boolean equals(Object obj) {
                if (obj instanceof SimpleClass) {
                    SimpleClass sc = (SimpleClass) obj;
                    return(attrib_two.equals(sc.attrib_two)
                            && (attrib_one.equals(sc.attrib_one))
                            && (attrib_three.equals(sc.attrib_three)));
                } else
                    return false;
            }

            public int hashCode() {
                return attrib_one.hashCode() + attrib_two.hashCode() +
attrib_three.hashCode();
            }

            public String toString() {
                return ("[attrib_one: " + attrib_one + "; attrib_two: " +
attrib_two + "; attrib_three: " + attrib_three + "]");
            }
        }

#######################################################################

As you can see, there are three attributes and three methods of the
Object class overwritten (equals(), hashCode() - i don't actually need
it -, and toString()).

Somewhere else in the code, i check if a SimpleClass object is already
in a specified TreeSet

#######################################################################

TreeSet ts = new TreeSet();
[...]
SimpleClass sc = new SimpleClass(string1, string2, string3);
if (ts.contains(sc)) {
   //blah blah
}

#######################################################################

and in the "if" branch I put the generated object in the TreeSet
itself.

#######################################################################

   //blah blah
   ts.add(sc);
   //blah blah

#######################################################################

Now, the first .contains() is executed normally (I suppose that the
TreeSet simply sees it's empty), while the second throws a
ClassCastException. I don't know why is this happening. I'm absolutely
sure that when i cast a (SimpleClass), the object IS a SimpleClass.

I've searched the archives but I didn't find any clue. I'm using java
1.4.2.

Thanks for your help.

StM