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>();

?

Answer 1

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…