Java by API/java.lang.ref/PhantomReference

new PhantomReference(T referent, ReferenceQueue<?> q)

  
import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
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());
  }
}

PhantomReference: get()

  
import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
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());
  }
}

PhantomReference: isEnqueued()

  
import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
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());
  }
}