Testing PhantomReference : PhantomReference « Reflection « Java Tutorial

1. Java Tutorial
2. Reflection
3. PhantomReference

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    ReferenceQueue referenceQueue = new ReferenceQueue();
    Object object = new Object() {
      public String toString() {
        return "Referenced Object";
      }
    };

    Object data = new Object() {
      public String toString() {
        return "Data";
      }
    };

    HashMap map = new HashMap();
    Reference reference = null;
    System.out.println("Testing PhantomReference.");
    reference = new PhantomReference(object, referenceQueue);

    map.put(reference, data);

    System.out.println(reference.get());
    System.out.println(map.get(reference));
    System.out.println(reference.isEnqueued());

    System.gc();
    System.out.println(reference.get());
    System.out.println(map.get(reference));
    System.out.println(reference.isEnqueued());

    object = null;
    data = null;

    System.gc();
    System.out.println(reference.get());
    System.out.println(map.get(reference));
    System.out.println(reference.isEnqueued());
  }
}