This is merely a short snippet to illustration how to use weak ref in Java.

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import java.lang.ref.*;

public class weak {
    public static void main(String[] args) throws InterruptedException {
      String obj = new String("Random");

      ReferenceQueue<String> queue = new ReferenceQueue<>();
      WeakReference<String> weak_ref = new WeakReference<>(obj, queue);

      System.out.println("weak ref: " + weak_ref);

      // drop strong ref to the object
      obj = null;

      while (!weak_ref.isEnqueued()) {
         // loop until weak ref is enqueued
         System.gc();
      }

      System.out.println("weak ref enqueued: " + queue.poll());
      System.out.println("referent nullified: " + weak_ref.get());

   }
}

Output is the following. So, when the referent becomes collectible, the enclosing weak ref is enqueued into the registered ref queue, which provides a way to inform the application of such reachability change.

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weak ref: java.lang.ref.WeakReference@50f8360d
weak ref enqueued: java.lang.ref.WeakReference@50f8360d
referent nullified: null