This pool is used to keep only one version of an object when they are equal

InternPool.java

import java.lang.ref.WeakReference;
import java.util.WeakHashMap;

/*
 * This pool is used to keep only one version of an object when they are equal
 * The object is garbage collected when it is no longer used. 
 * e.g.:
    InternPool<Integer> p = new InternPool<>();
    Integer a = pool.intern(new Integer(1));
    Integer b = pool.intern(new Integer(1));
    assert a == b;
 */

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;
    }
}