Chapter 30: WeakHashMap

Concepts of weak Hashmap

Section 30.1: Concepts of WeakHashmap

Key Points:

• Implementation of Map.

• stores only weak references to its keys.

Weak References : The objects that are referenced only by weak references are garbage collected eagerly; the GC won’t wait until it needs memory in that case.

Difference between Hashmap and WeakHashMap:

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 in WeakHashMap.

Example:

public class WeakHashMapTest {

public  static  void  main(String[]  args)  {

Map  hashMap=  new  HashMap();

Map  weakHashMap  =  new  WeakHashMap();

String  keyHashMap  =  new  String(“keyHashMap”);

String  keyWeakHashMap  =  new  String(“keyWeakHashMap”);

hashMap.put(keyHashMap, “Ankita”);

weakHashMap.put(keyWeakHashMap, “Atul”);

System.gc();

System.out.println(“Before:  hash  map  value:”+hashMap.get(“keyHashMap”)+”  and  weak  hash  map value:”+weakHashMap.get(“keyWeakHashMap”));

keyHashMap = null;

keyWeakHashMap = null;

System.gc();

System.out.println(“After:  hash  map  value:”+hashMap.get(“keyHashMap”)+”  and  weak  hash  map value:”+weakHashMap.get(“keyWeakHashMap”));

}

Size differences (HashMap vs WeakHashMap):

Calling size() method on HashMap object will return the same number of key-value pairs. size will decrease only if remove() method is called explicitly on the HashMap object. Because the garbage collector may discard keys at anytime, a WeakHashMap may behave as though an unknown thread is silently removing entries. So it is possible for the size method to return smaller values over time.So, in WeakHashMap size decrease happens automatically