Question

Hello I cannot find any info on what I need to do to make two keys seems equal. That is, i need to provide a custom compare method that will be used by map.put() Implementing comparable doesnt help.

For example, this code doesnt work as intended – for the purposes of my program, two keys n and n2 ARE the same.

private class N implements Comparable<N> {
        int value;

        int stuff;

        String z;

        @Override
        public int compareTo(N arg0) {
            if (arg0.z.equals(z))
                return 0;
            return 1;
        }

    }

    public void dostuff() {
        HashMap m = new HashMap();

        N n = new N();
        n.z = "1";

        N n2 = new N();
        n2.z = "1";

        m.put(n, "one");
        m.put(n2, "two");

            // will print refs to two instances! - wrong
        Iterator it = m.keySet().iterator();
        while (it.hasNext()) {
            System.err.println(it.next());
        }
    }

Answer 1

You need to override equals and hashCode – HashMap doesn’t use compareTo, which is meant for sorting.

Note that your compareTo implementation is already broken, as it’s really only testing for equality. In particular, x.compareTo(y) and y.compareTo(x) both returning 1 violates the contract of compareTo:

The implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y.