For a HashMap that maps from a custom class, how to make it so that two equivalent keys will map to say value?

Question

I have a custom class, for example's sake let's say it's tuples without order.

public class UnorderedTuple {  
    Integer i1 = null; Integer i2 = null;

    UnorderedTuple(Integer int1, Integer int2) { i1 = int1; i2 = int2; }

    boolean equals(UnorderedTuple t) { return t.i1 == i1 && t.i2 == t2 || t.i2 == i1 && t.i1 == i2; }
}

Like I said, a dumb example. Now, let's say i have

Map<UnorderedTuple, Integer> m = new HashMap<UnorderedTuple, Integer>();

Ideally, I'd like this functionality:

UnorderedTuple ut1 = new UnorderedTuple(1,2);
UnorderedTuple ut2 = new UnorderedTuple(2,1);

m.put(ut1,2);
m.put(ut2,3);
System.out.println(m.get(ut1)==3); //ideally returns true

Is there something I need to implement or extend such that I can have this functionality? In the same way that if you use 2 different, but equal strings, or Integers, or whatever as a key it will map it properly, but if I implement this as written, it treats ut1 and ut2 separately. Even if I construct ut1 and ut2 identically, it does the same thing.

Thanks for your help.

Answer 1

You need to override hashCode()... in this case you'd probably do it by XORing the hash codes of all the fields involved, to get the order-independence you want.

You also need to override equals(Object) rather than just implementing equals(UnorderedTuple).

I also note that you're using "==" to compare instances of Integer: don't do that. It will work in the test case you've given due to the rules around boxing, but if you try it with values over 127 you may well run into problems - it's only comparing references rather than values. Call equals to compare values properly.

Answer 2

You need to implement hashCode() in a way that is consistent with equals() (in other words, if a.equals(b) returns true, then a.hashCode() == b.hashCode() must be true as well).

Chapter 3 of Effective Java (free PDF!) has all the nitty-gritty details!

Edit: an you also need to make sure that you're actually implementing equals(Object). In your sample code you only implement equals(UnorderedTuple), which is not the same!