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?

Try applying @Override on the methods which you think are overridden — denis.solonenko
– denis.solonenko, Commented Jan 6, 2012 at 8:24

Thilo · Accepted Answer · 2012-01-06 08:24:18Z

15

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.

answered Jan 6, 2012 at 8:24

Thilo

264k107 gold badges527 silver badges674 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Peter Lawrey Over a year ago

+1: Similar to using compareto instead of compareTo. There is a hashcode method in the Java 6 JDK ;)

Thilo Over a year ago

I think FindBugs would also have caught this. Something about implementing equals but not hashCode.

Collectives™ on Stack Overflow

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

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related