WeakHashMap example

Question

I create a WeakHashMap as

WeakHashMap<Employee,String> map = new WeakHashMap<Employee,String>(); map.put(emp,"hello"); 

where emp is an Employee object. Now if I do emp = null or say emp object is no longer referenced, then will the entry be removed from the WeakHashMap i.e. will the size of Map be zero?
And will it be vice-versa in case of HashMap?
Is my understanding of WeakHashMap correct?

Answer 1

A very simple example, to enlighten what has already been said :

import java.util.WeakHashMap;  public class WeakHashMapDemo {      public static void main(String[] args) {         // -- Fill a weak hash map with one entry         WeakHashMap<Data, String> map = new WeakHashMap<Data, String>();         Data someDataObject = new Data("foo");         map.put(someDataObject, someDataObject.value);         System.out.println("map contains someDataObject ? " + map.containsKey(someDataObject));          // -- now make someDataObject elligible for garbage collection...         someDataObject = null;          for (int i = 0; i < 10000; i++) {             if (map.size() != 0) {                 System.out.println("At iteration " + i + " the map still holds the reference on someDataObject");             } else {                 System.out.println("somDataObject has finally been garbage collected at iteration " + i + ", hence the map is now empty");                 break;             }         }     }      static class Data {         String value;         Data(String value) {             this.value = value;         }     } } 

Output :

    map contains someDataObject ? true     ...     At iteration 6216 the map still holds the reference on someDataObject     At iteration 6217 the map still holds the reference on someDataObject     At iteration 6218 the map still holds the reference on someDataObject     somDataObject has finally been garbage collected at iteration 6219, hence the map is now empty 

Answer 2

I ran the sample code to understand the difference between HashMap and WeakHashMap

            Map hashMap= new HashMap();             Map weakHashMap = new WeakHashMap();              String keyHashMap = new String("keyHashMap");             String keyWeakHashMap = new String("keyWeakHashMap");              hashMap.put(keyHashMap, "helloHash");             weakHashMap.put(keyWeakHashMap, "helloWeakHash");             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")); 

The output will be:

Before: hash map value:helloHash and weak hash map value:helloWeakHash After: hash map value:helloHash and weak hash map value:null