Generic InternPool in Java?

Question

How would I write a generic InternPool<T> in Java? Does it need a Internable interface?

String in Java has interning capabilities; I want to intern classes like BigDecimal and Account.

Answer 1

Something like this:

public class InternPool<T> {

    private WeakHashMap<T, WeakReference<T>> pool = 
        new WeakHashMap<T, WeakReference<T>>();

    public synchronized T intern(T object) {
        T res = null;
        // (The loop is needed to deal with race
        // conditions where the GC runs while we are
        // accessing the 'pool' map or the 'ref' object.)
        do {
            WeakReference<T> ref = pool.get(object);
            if (ref == null) {
                ref = new WeakReference<T>(object);
                pool.put(object, ref);
                res = object;
            } else {
                res = ref.get();
            }
        } while (res == null);
        return res;
    }
}

This depends on the pool element class implementing the equals and hashCode to provide "equality by value" and to obey to the API contracts for those methods. But BigDecimal certainly does.

---

UPDATE - for an explanation of why we need a WeakHashMap<T, WeakReference<T>> rather than a WeakHashMap<T, T>, see the javadocs. The short version is that the key weak-links in the latter won't be broken by the GC because the corresponding entry references are making the values strongly reachable.

Answer 2

For an example take a look at Interner from Guava. It does not require an Internable interface, it just relies on equals and hashCode.

Answer 3

I would separate the solution into two classes to have cleaner code and also this way getting rid of loop:

public class WeakPool<T> {
    private final WeakHashMap<T, WeakReference<T>> pool = new WeakHashMap<T, WeakReference<T>>();
    public T get(T object) {
        final T res;
        WeakReference<T> ref = pool.get(object);
        if (ref != null) {
            res = ref.get();
        } else {
            res = null;
        }
        return res;
    }
    public void put(T object) {
        pool.put(object, new WeakReference<T>(object));
    }
}

and the interning class using the weak pool is very simple:

public class InternPool<T> {

    private final WeakPool<T> pool = new WeakPool<T>();

    public synchronized T intern(T object) {
        T res = pool.get(object);
        if (res == null) {
            pool.put(object);
            res = object;
        }
        return res;
    }
}

Answer 4

This more sounds like that you're looking for flyweight pattern.

Flyweight is a software design pattern. A flyweight is an object that minimizes memory use by sharing as much data as possible with other similar objects

Click the link, it contains a Java example.

Answer 5

Just a quick caveat:

It has not been explicitly mentioned above, but it should be obvious that the objects being interned must be of an immutable type.

On a second note: You don't need to use another weak reference to the object as the value in the map, a reference to a static would suffice if you just rely on the map's keyset for the data. For example, declare:

WeakHashMap<T,Boolean>

And insert pairs as:

pool.put (object, Boolean.TRUE);

Which is a minor saving of a WeakReference instance (if you can't reuse the one used for the key).

...or create a WeakSet class, as @PeterVerhas has done with his WeakPool.

Answer 6

Shouldnt

"WeakReference ref = pool.get(object);"

instead be

WeakReference ref = pool.intern(object);

??