Java: Retrieving object from the set just by computing its hashcode

Question

I have created the Event class. As you can see, both hashCode and equals methods use only the id field of type long.

public class Event {
private long id;
private Map<String, Integer> terms2frequency;
private float vectorLength;

@Override
public long hashCode() {
    return this.id;
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    Event other = (Event) obj;
    if (id != other.id)
        return false;
    return true;
}

I will store the objects of this class in the HashSet Collection.

Set<Event> events = new HashSet<Event>();

Since for the hash computation only the field of the type long I'd like to retrieve the elements from the events hashset by computing the hash of the id. E.g.:

events.get(3);

Is it possible or should I use the hashMap for it:

Map<Long, Event> id2event = new HashMap<Long, Event>();

?

There are multiple weird things: getClass() != obj.getClass() but yet you cast to SimpleEvent and not Event. Float.floatToIntBits() on a long is also weird. The hashCode formula also doesn't make much sense (just adding 31?) — Thomas Mueller
– Thomas Mueller, Commented Jul 1, 2011 at 8:59
Isn't using if (Float.floatToIntBits(id) != Float.floatToIntBits(other.id)) the same as if (id != other.id)? — Qwerky
– Qwerky, Commented Jul 1, 2011 at 9:26
@Thomas Mueller Right remarks. It happened due to the previous name of the class and the types of the variables in it. I didn't notice it while changing the class name and the variable type. — Jakub
– Jakub, Commented Jul 1, 2011 at 10:39

Jon Skeet · Accepted Answer · 2011-07-01 08:40:47Z

4

You should absolutely not rely on hash code uniqueness. A long has 2⁶⁴ possible values; an int only has 2³². Therefore hash collisions are entirely possible. Don't use hash codes as your sole equality test. That's not what they're designed for.

Hash codes are designed to quickly get from a key to a set of potential matches, which are then checked more rigorously with normal equality.

(As an aside, I don't think it's a great idea to use floatToIntBits to compute the hash code to start with. Look at what Long.hashCode() does.)

EDIT: Of course, even if you did want to rely on that, HashSet<E> doesn't expose a method for getting an element by its hash code, precisely because it's a really bad idea in almost all cases... if you want a mapping, create a Map...

edited Jul 1, 2011 at 8:40

answered Jul 1, 2011 at 8:34

Jon Skeet

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9 Comments

bettersayhello Over a year ago

are there cases in which such approach would be acceptable, just curious, thanks

Jon Skeet Over a year ago

@ultrajohn: If you can guarantee absolutely that there will be no collisions, then it would at least be correct - but still not a good idea, IMO. (You can't do it with HashSet anyway, of course.)

JB Nizet Over a year ago

No. A Set stores objects, and doesn't let you get an object by a key. That's what Maps do.

bestsss Over a year ago

@Jon, all hashbased maps in java, besides (hashtable) do scramble the bits to 'improve' the hashing, so unique stuff doesn't really matter. floatToIntBits is ok (that's the impl. of java.lang.Float anyways, iirc). As floats are 32bits, you cant better general hash (they have less than 2^32 possible values too). HashMap's extra hashing looks like that h ^= (h >>> 20) ^ (h >>> 12); return h ^ (h >>> 7) ^ (h >>> 4); The extra hashing is needed due to pow2 sized bukets (otherwise too many collisions w/ standard hashes)

Jakub Over a year ago

@Jon Skeet Thank you for your comprehensive answer. Hereby I'd like to thank all other people who answered to my question. I will use the HashMap.

|

Foredoomed · Accepted Answer · 2011-07-01 10:00:45Z

0

You can generate a static class (eg:Utility), and create a method which can generate unique hash in it.But you must estimate that there are how many objects will be created in possible, and then create a algorithm to generate the unique to distinguish the objects. And also the hashcode method of the Event class should be overrided because you overided the equals method.

And use the map :

Map id2event = new HashMap();

answered Jul 1, 2011 at 10:00

Foredoomed

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