Java: After adding 2 identical objects to a Set, it contains the 2 elements

Question

After adding two identical objects to a Set, I would expect the set to contain only one element.

public void addIdenticalObjectsToSet(){
    Set<Foo> set = new HashSet<Foo>();
    set.add(new Foo("totoro"));
    set.add(new Foo("totoro"));
    Assert.assertEquals(1, set.size());            // PROBLEM: SIZE=2
}

private class Foo {
    private String id;
    public Foo(String id) {
        this.id = id;
    }
    public String getId() {
        return id;
    }
    public boolean equals(Object obj) {
        return obj!= null && obj instanceof Foo &&
            ((Foo)obj).getId().equals(this.getId());
    }
    public int hashcode() {
        return this.getId().hashCode();
    }
}

I consider two objects as identical if they have the same id (String).

Other strange thing: Neither Foo.equals nor Foo.hashcode are accessed, as far as I can tell using debug/breakpoints. What am I missing?

Answer 1

public int hashcode() {
        return this.getId().hashCode();
    }

should be

@Override
public int hashCode() {
        return this.getId().hashCode();
    }

The annotation would have told you about the spelling mistake.

There should also be a (missing) little triangle symbol in your IDE on the method to indicate if an interface is being implemented or a parent method overridden.