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.

Diffrence 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.