Question

public class Empty {

    public static void main( String[] args ) {

        TreeSet<Class> classes = new TreeSet<Class>();
        classes.add( String.class );

        String test = new String();

        try{ 
            if( classes.contains(test.getClass()) ){
                System.out.println( "contains" );
            }
        }catch(ClassCastException cce){

            System.out.println( "Expected:  "  + classes );
            System.out.println( "But it was: " + test.getClass() );
        }
    }
}

Why does this throw a ClassCastException?

Answer 1

When instantiating TreeSet without an explicit comparator, it expects inserted elements to implement Comparable, but Class does not implement this interface.

To fix, create a comparator for Class:

Comparator<Class> classComp = new Comparator<Class>()
{
    @Override
    public int compare(Class o1, Class o2)
    {
        return o1.getName().compareTo(o2.getName());
    }
};
TreeSet<Class> classes = new TreeSet<Class>(classComp);