Java - The WeakHashMap Class

WeakHashMap is an implementation of the Map interface that stores only weak references to its keys. Storing only weak references allows a key-value pair to be garbage-collected when its key is no longer referenced outside of the WeakHashMap.

This class provides the easiest way to harness the power of weak references. It is useful for implementing "registry-like" data structures, where the utility of an entry vanishes when its key is no longer reachable by any thread.

The WeakHashMap functions identically to the HashMap with one very important exception: if the Java memory manager no longer has a strong reference to the object specified as a key, then the entry in the map will be removed.

Weak Reference − If the only references to an object are weak references, the garbage collector can reclaim the object's memory at any time.it doesn't have to wait until the system runs out of memory. Usually, it will be freed the next time the garbage collector runs.

Following is the list of constructors supported by the WeakHashMap class.

Apart from the methods inherited from its parent classes, TreeMap defines the following methods −

Example

The following program illustrates several of the methods supported by this collection −

import java.util.*;
public class WeakHashMap_Demo {

   private static Map map;
   public static void main (String args[]) {
      map = new WeakHashMap();
      map.put(new String("Maine"), "Augusta");
      
      Runnable runner = new Runnable() {
         public void run() {
            while (map.containsKey("Maine")) {
               try {
                  Thread.sleep(500);
               } catch (InterruptedException ignored) {
               }
               System.out.println("Thread waiting");
               System.gc();
            }
         }
      };
      Thread t = new Thread(runner);
      t.start();
      System.out.println("Main waiting");
      try {
         t.join();
      } catch (InterruptedException ignored) {
      }
   }
}

This will produce the following result −

Output

Main waiting
Thread waiting

If you do not include the call to System.gc(), the system may never run the garbage collector as not much memory is used by the program. For a more active program, the call would be unnecessary.