Soft, weak & phantom reference

Question

Hi all :)
A short while ago, I began to study "java.lang.ref" package. It's not so easy to me. Also I've gotten many relative questions & answers from this forum, here is a problem confused me for so long time :( Please help me out, thanks!
import java.lang.ref.*;
import java.util.Hashtable;
public class TestMe {
    private Hashtable ht = new Hashtable();
    private ReferenceQueue rq = new ReferenceQueue();
    public TestMe() { }
    private void clearQueue() {
        SoftReference r;
        while ((r = (SoftReference)rq.poll()) != null) {
            Object obj = r.get();
            if (obj != null) {
                //* Here is the problem: why this always doesn't work?
                // using weak or phantom reference is same like soft reference here :(
                System.out.println("=> " + obj);
            ht.remove(r);
    public void put(Object key, Object obj) {
        clearQueue();
        ht.put(new SoftReference(key, rq), obj);
    public static void main(String[] args) {
        TestMe tm = new TestMe();
        int i=0;
        while(true) {
            tm.put(Integer.toString(i++), new String("wow"));

Answer 1

The reference object is put into the reference queue after all SoftReferences to the reference object have been cleared. Since you're trying to get the reference pointed to by the SoftReference after it's been queued, it is null.
If you want to store some information to use after the reference object has been queued, you need to subclass the SoftReference class and store the information in a field.