Java Sorted Set

gistfile1.java

/*
 * Just some X class
 */
public static class X implements Comparable<X> {

	public int getX() {
		return 42;
	}

	@Override
	public int compareTo(X o) {
		return 0;
	}
}

/*
 * Scenario 1: SortedSet with comparator
 */
SortedSet<X> s = new ConcurrentSkipListSet<X>(Comparator.comparingInt(X::getX));
SortedSet s2 = s;

s2.add("cake");
//ok

s2.add("cake");
s2.add("lie");
//java.lang.ClassCastException: java.lang.String cannot be cast to X

/*
 * Scenario 2: SortedSet without comparator
 */
SortedSet<X> s = new ConcurrentSkipListSet<X>();
SortedSet s2 = s;

s2.add("cake");
//ok

s2.add("cake");
s2.add("lie");
//ok

s2.add("cake");
s2.add("lie");
s2.add(new X());
//java.lang.ClassCastException: java.lang.String cannot be cast to X