how do I get a unique ID per object in Java? [duplicate]

Answer 1

No, that's not how hashCode() works. The returned value does not have to be unique. The exact contract is spelled out in the documentation.

Also,

supposedly the Object.hashCode() method is a unique id per object

is not true. To quote the documentation:

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects.

Answer 2

The simplest solution is to add a field to the object. This is the fastest and most efficient solution and avoid any issues of objects failing to be cleaned up.

abstract Ided {
    static final AtomicLong NEXT_ID = new AtomicLong(0);
    final long id = NEXT_ID.getAndIncrement();

    public long getId() {
         return id;
    }
}

---

If you can't modify the class, you can use an IdentityHashMap like @glowcoder's deleted solution.

private static final Map<Object, Long> registry = new IdentityHashMap<Object, Long>();
private static long nextId = 0;

public static long idFor(Object o) {
    Long l = registry.get(o);
    if (l == null)
        registry.put(o, l = nextId++);
    return l;
}

public static void remove(Object o) {
    registry.remove(o);
}

Answer 3

java.lang.System.identityHashCode(obj); will do this for you, if you really need it and understand the repercussions. It gets the identity hashcode, even if the method to provide the hashcode has been overridden.

Answer 4

Trying to outperform the java GC sounds like premature optimization to me.

The GC is already tuned to handle small short-lived objects. If you have performance issues with GC, you ought to help the GC, not re-implement it (IMNSHO)