TreeSets to match lotto numbers?

Question

I'm pretty new to data structures, so I am not 100% sure I am going about this problem the right way. I am trying to compare two nested sets against each other, and store the matching numbers in the set titled 'Intersection'. However, the code below throws a ClassCastException.

public class Details {
    public static void main(String[] args) {
        Set<Integer> myNums = new TreeSet<Integer>();
        myNums.add(1);
        myNums.add(2);
        myNums.add(3);
        myNums.add(4);
        myNums.add(5);
        myNums.add(6);

        Set<Integer> myNums2 = new TreeSet<Integer>();
        myNums2.add(7);
        myNums2.add(8);
        myNums2.add(3);
        myNums2.add(10);
        myNums2.add(11);
        myNums2.add(12);

        Set<Set<Integer>> myTicket = new TreeSet<Set<Integer>>();
        myTicket.add(myNums);
        myTicket.add(myNums2);

        Set<Integer> lottoNums = new TreeSet<Integer>();
        lottoNums.add(1);
        lottoNums.add(2);
        lottoNums.add(3);
        lottoNums.add(4);
        lottoNums.add(5);
        lottoNums.add(6);

        Set<Set<Integer>> lottoNums1 = new TreeSet<Set<Integer>>();
        lottoNums1.add(lottoNums);

        // keep only the winning numbers from the user's ticket
        Set<Set<Integer>> intersection = new TreeSet<Set<Integer>>(myTicket);
        intersection.retainAll(lottoNums1);

        // print results
        System.out.println("Your ticket numbers are " + myTicket);
        System.out.println("The winning numbers are " + lottoNums1);
        System.out.println();
        System.out.println("You had " + intersection.size()
                + " matching numbers.");
        if (intersection.size() > 0) {
            double prize = 100 * Math.pow(2, intersection.size());
            System.out.println("The matched numbers are " + intersection);
            System.out.println("Your prize is $" + prize);
        }
    }
}

Answer 1

You should read the docs of the TreeSet() constructor you are using :

/**
 * Constructs a new, empty tree set, sorted according to the
 * natural ordering of its elements.  All elements inserted into
 * the set must implement the {@link Comparable} interface.
 * Furthermore, all such elements must be <i>mutually
 * comparable</i>: {@code e1.compareTo(e2)} must not throw a
 * {@code ClassCastException} for any elements {@code e1} and
 * {@code e2} in the set.  If the user attempts to add an element
 * to the set that violates this constraint (for example, the user
 * attempts to add a string element to a set whose elements are
 * integers), the {@code add} call will throw a
 * {@code ClassCastException}.
 */
public TreeSet() {
    this(new TreeMap<E,Object>());
}

You are adding a Set<Integer> to your Set<Set<Integer>>, but Set<Integer> doesn't implement Comparable.

The solution is to use the constructor that receives a Comparator as a parameter.

TreeSet(Comparator<? super E> comparator)

The Comparator (in your case Comparator<Set<Integer>>) would hold the logic of how to compare two Set<Integer> instances.