Re: hashCode

Daniel Pitts <>
Wed, 29 Aug 2012 11:06:23 -0700
On 8/28/12 5:02 PM, markspace wrote:

On 8/28/2012 4:33 PM, Daniel Pitts wrote:

interface Hasher<Type> {
    int hash(Type t);

Not really seeing how this is a good idea. How would you implement this?

So, that would change HashMap to take a Hasher<? super K> instance in
its constructor.

This is the problem; Map (and HashMap) were desired to be spec'd as
taking Object, not a subclass.

Actually, they are Generic, so they are not spec'd to take Object, but
to take a specific subtype defined at compile time. At least, now that
they have the addition of Generics. Pre-generics, they still had
Comparators which had the same behavior that I'm describing, but instead
of defining buckets, they define an ordering. See below.

A default Hasher<Object> could be implemented to use
System.identityHashCode and == for the common use-case.

Again not seeing how you'd actually use that to put an object in a Map.

Example usage:
// MyKeyHasher implements Hasher<MyKey>
Map<MyKey, MyValue> map=new HashMap<MyKey,MyValue>(new MyKeyHasher());

map.put(myFirstKey, myFirstValue);
map.put(mySecondKey, mySecondValue);

This is directly analogous to how TreeMap and Comparator currently work

Map<MyKey, MyValue> map =
         new TreeMap<MyKey, MyValue(new MyKeyComparator());

map.put(myFirstKey, myFirstValue);
map.put(mySecondKey, mySecondValue);

Now, it would be my opinion that the standard library should provide a
IdentityHasher implementation as such:
public final class IdentityHasher<T> implements Hasher<T> {
    public int hash(T object) {
       return System.identityHashCode(object);
    public boolean isEqual(T l, T r) {
       return l == r;

There could of course be a singleton instance of this, similar to how
they have a singleton for Collections.emptySet();

Does that make more sense?

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