Java WeakHashMap Class

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Introduction

The Java WeakHashMap class is a hashtable-based Map implementation with weak keys. An entry in a WeakHashMap will automatically be removed by the garbage collector, when its key is no longer in use. Following are the important points about WeakHashMap −

  • Both null values and the null key are supported.

  • Like most collection classes, this class is also not synchronized.

  • This class is intended primarily for use with key objects whose equals methods test for object identity using the == operator.

  • Each key object in a WeakHashMap is stored indirectly as the referent of a weak reference.

  • This class is a member of the Java Collections Framework.

Class declaration

Following is the declaration for java.util.WeakHashMap class −

public class WeakHashMap<K,V> 
   extends AbstractMap<K,V>
   implements Map<K,V>

Here <K> is the type of keys maintained by this map and <V> is the type of mapped values.

Class constructors

Sr.No. Constructor & Description
1

WeakHashMap()

This constructor is used to create an empty WeakHashMap with the default initial capacity (16) and load factor (0.75).
2

WeakHashMap(int initialCapacity)

This constructor is used to create an empty WeakHashMap with the given initial capacity and the default load factor (0.75).
3

WeakHashMap(int initialCapacity, float loadFactor)

This constructor is used to create an empty WeakHashMap with the given initial capacity and the given load factor.
4

WeakHashMap(Map<? extends K,? extends V> m)

This constructor is used to create a new WeakHashMap with the same mappings as the specified map.

Class methods

Sr.No. Method & Description
1 void clear()

This method removes all of the mappings from this map.
2 boolean containsKey(Object key)

This method returns true if this map contains a mapping for the specified key.
3 boolean containsValue(Object value)

This method returns true if this map maps one or more keys to the specified value.
4 Set<Map.Entry>K,V>> entrySet()

This method returns a Set view of the mappings contained in this map.
5 v get(Object key)

This method returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
6 boolean isEmpty()

This method returns true if this map contains no key-value mappings.
7 Set<K> keySet()

This method returns a Set view of the keys contained in this map.
8 v put(K key, V value)

This method associates the specified value with the specified key in this map.
9 void putAll(Map<? extends K,? extends V> m)

This method copies all of the mappings from the specified map to this map.
10 v remove(Object key)

This method removes the mapping for a key from this weak hash map if it is present.
11 int size()

This method returns the number of key-value mappings in this map.
12 Collection<V> values()

This method returns a Collection view of the values contained in this map.

Methods inherited

This class inherits methods from the following classes −

  • java.util.AbstractMap
  • java.lang.Object
  • java.util.Map

Adding a Key-Value Pair to a WeakHashMap of Integer, Integer Pairs Example

The following example shows the usage of Java WeakHashMap put() method to put few values in a Map. We've created a Map object of Integer,Integer pairs. Then few entries are added using put() method and then map is printed.

package com.tutorialspoint;

import java.util.WeakHashMap;

public class WeakHashMapDemo {
   public static void main(String args[]) {
      
      // create hash map
      WeakHashMap<Integer,Integer> newmap = new WeakHashMap<>();

      // populate hash map
      newmap.put(1, 1);
      newmap.put(2, 2);
      newmap.put(3, 3); 

      System.out.println("Map elements: " + newmap);
   }
}

Output

Let us compile and run the above program, this will produce the following result.

Map elements: {3=3, 2=2, 1=1}
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