Set.contains returns false even when the set contains the element

Question

As per java documentation, set.contains(o) returns true if and only if this set contains an element e such that (o==null ? e==null : o.equals(e)). In my case it doesn't (I have tested with Java 1.50_09 and Java 1.6.0). Look at the small toy example below. The last assert statement fails. But as per the documentation, it should not, since an element sc exists in the set such that sc.equals(sc2).
Any help will be greatly appreciated.
thanks
Nilesh
import java.util.HashSet;
import java.util.Set;
class StringCapsule {
    String s;
    public StringCapsule(String s) {
        this.s = s;
    public boolean equals(StringCapsule s2) {
        return s.equals(s2.s);
public class TestSets {
    public static void main(String[] args) {
        Set<StringCapsule> set = new HashSet<StringCapsule>();
        String s = "test";
        StringCapsule sc = new StringCapsule(s);
        StringCapsule sc2 = new StringCapsule(s);
        set.add(sc);
        assert (set.contains(sc));
        assert (sc.equals(sc2));
        assert (set.contains(sc2)); // THIS FAILS
        System.out.println("Finished with set size " + set.size());
}Copy above code to TestSets.java, and compile and run to reproduce
javac TestSets.java
java -ea -cp . TestSets

Answer 1

If you have generics available, you should have annotations as well. One way to avoid this kind of problem in the future is to use
@Override
public boolean equals (Object o) when you know you want a method to override it's parent.