Java by API/java.lang.ref/WeakReference

new WeakHashMap()

  
import java.lang.ref.WeakReference;
import java.util.Iterator;
import java.util.Map;
import java.util.WeakHashMap;
public class Main {
  public static void main(String[] argv) throws Exception {
    Object keyObject = "";
    Object valueObject = "";
    Map<Object, Object> weakMap = new WeakHashMap<Object, Object>();
    weakMap.put(keyObject, valueObject);
    WeakReference weakValue = new WeakReference<Object>(valueObject);
    weakMap.put(keyObject, weakValue);
    Iterator it = weakMap.keySet().iterator();
    while (it.hasNext()) {
      Object key = it.next();
      weakValue = (WeakReference) weakMap.get(key);
      if (weakValue == null) {
        System.out.println("Value has been garbage-collected");
      } else {
        System.out.println("Get value");
        valueObject = weakValue.get();
      }
    }
  }
}

new WeakReference(Object referent)

  
import java.lang.ref.WeakReference;
import java.util.Iterator;
import java.util.Map;
import java.util.WeakHashMap;
public class Main {
  public static void main(String[] argv) throws Exception {
    Object keyObject = "";
    Object valueObject = "";
    Map<Object, Object> weakMap = new WeakHashMap<Object, Object>();
    weakMap.put(keyObject, valueObject);
    WeakReference weakValue = new WeakReference<Object>(valueObject);
    weakMap.put(keyObject, weakValue);
    Iterator it = weakMap.keySet().iterator();
    while (it.hasNext()) {
      Object key = it.next();
      weakValue = (WeakReference) weakMap.get(key);
      if (weakValue == null) {
        System.out.println("Value has been garbage-collected");
      } else {
        System.out.println("Get value");
        valueObject = weakValue.get();
      }
    }
  }
}

new WeakReference(String referent, ReferenceQueue<? super String> q)

  
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
public class Main {
  public static void main(String[] argv) throws Exception {
    ReferenceQueue rq = new ReferenceQueue();
    WeakReference<String> wr = new WeakReference<String>("string", rq);
    while (true) {
      Reference r = rq.remove();
      if (r == wr) {
        System.out.println("no longer referenced");
      }
    }
  }
}

WeakReference: get()

  
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 WeakReference.");
    reference = new WeakReference(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());
  }
}

WeakReference: isEnqueued()

  

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 WeakReference.");
    reference = new WeakReference(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());
  }
}