List to TreeSet conversion produces: "java.lang.ClassCastException: MyClass cannot be cast to java.lang.Comparable"

Question

List<MyClass> myclassList = (List<MyClass>) rs.get();

TreeSet<MyClass> myclassSet = new TreeSet<MyClass>(myclassList);

I don't understand why this code generates this:

java.lang.ClassCastException: MyClass cannot be cast to java.lang.Comparable

MyClass does not implement Comparable. I just want to use a Set to filter the unique elements of the List since my List contains unncessary duplicates.

Answer 1

Does MyClass implements Comparable<MyClass> or anything like that?

If not, then that's why.

For TreeSet, you either have to make the elements Comparable, or provide a Comparator. Otherwise TreeSet can't function since it wouldn't know how to order the elements.

Remember, TreeMap implements SortedSet, so it has to know how to order the elements one way or another.

You should familiarize yourself with how implementing Comparable defines natural ordering for objects of a given type.

The interface defines one method, compareTo, that must return a negative integer, zero, or a positive integer if this object is less than, equal to, or greater than the other object respectively.

The contract requires that:

• sgn(x.compareTo(y)) == -sgn(y.compareTo(x))
• it's transitive: x.compareTo(y)>0 && y.compareTo(z)>0 implies x.compareTo(z)>0
• x.compareTo(y)==0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)) for all z

Additionally, it recommends that:

• (x.compareTo(y)==0) == (x.equals(y)), i.e. "consistent with equals

This may seem like much to digest at first, but really it's quite natural with how one defines total ordering.

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If your objects can not be ordered one way or another, then a TreeSet wouldn't make sense. You may want to use a HashSet instead, which have its own contracts. You are likely to be required to @Override hashCode() and equals(Object) as appropriate for your type (see: Overriding equals and hashCode in Java)