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WeakHashMap Class in Java

Last Updated : 23 Apr, 2026

WeakHashMap is a part of the java.util package that implements the Map interface. It stores key–value pairs like a HashMap but allows keys to be garbage-collected when they are no longer in ordinary use. This means that entries can disappear automatically when the key is no longer referenced elsewhere.

Internally uses a hash table-based implementation with weak keys.
Keys eligible for garbage collection are removed automatically.
Both null keys and null values are supported.

Declaration

WeakHashMap<K, V> map = new WeakHashMap<>();

K -> Type of keys in the map
V –> Type of values in the map

Java

import java.util.Map;
import java.util.WeakHashMap;

public class WeakHashMapExample {
    public static void main(String[] args) {
        WeakHashMap<String, String> map = new WeakHashMap<>();

        // Add entries
        map.put("Java", "Language1");
        map.put("Python", "Language2");
        map.put("C++", "Language3");

        System.out.println("WeakHashMap elements: " + map);

        // Access an element
        System.out.println("Value for 'Python': " + map.get("Python"));

        // Remove an element
        map.remove("C++");
        System.out.println("After removing 'C++': " + map);

        // Iterate elements correctly using Map.Entry
        System.out.println("Iterating WeakHashMap:");
        for (Map.Entry<String, String> e : map.entrySet()) {
            System.out.println(e.getKey() + " -> " + e.getValue());
        }
    }
}

Output

WeakHashMap elements: {Java=Language1, C++=Language3, Python=Language2}
Value for 'Python': Language2
After removing 'C++': {Java=Language1, Python=Language2}
Iterating WeakHashMap:
Java -> Language1
...

Hierarchy

WeakHashMap extends AbstractMap<K, V> and implements Map<K, V>, Cloneable, Serializable in the java.util package.

Constructors

1. WeakHashMap()

Creates an empty WeakHashMap with default initial capacity 16 and load factor 0.75.

WeakHashMap<K, V> map = new WeakHashMap<>();

2. WeakHashMap(int initialCapacity)

Creates an empty WeakHashMap with the specified initial capacity and default load factor.

WeakHashMap<K, V> map = new WeakHashMap<>(50);

3. WeakHashMap(int initialCapacity, float loadFactor)

Creates an empty WeakHashMap with the specified capacity and load factor.

WeakHashMap<K, V> map = new WeakHashMap<>(50, 0.5f);

4. WeakHashMap(Map m)

Creates a new WeakHashMap with the same mappings as the specified map.

Map<K, V> m = new HashMap<>();
WeakHashMap<K, V> map = new WeakHashMap<>(m);

Java

import java.util.WeakHashMap;

public class WeakHashMapExample1 {
    public static void main(String[] args) {
        // Creating a WeakHashMap
        WeakHashMap<String, String> map = new WeakHashMap<>();

        // Adding elements using put()
        map.put("Java", "Language1");
        map.put("Python", "Language2");
        map.put("C++", "Language3");

        // Displaying the WeakHashMap
        System.out.println("WeakHashMap elements: " + map);

        // Removing an element
        map.remove("C++");
        System.out.println("After removing 'C++': " + map);
    }
}

Output

WeakHashMap elements: {Java=Language1, C++=Language3, Python=Language2}
After removing 'C++': {Java=Language1, Python=Language2}

Methods in WeakHashMap

Method	Action Performed
clear()	Removes all key-value mappings from the map.
containsValue(Object value)	Returns true if this map maps one or more keys to the specified value.
containsKey(Object key)	Returns true if this map contains a mapping for the specified key.
entrySet()	Returns a set of all key-value pairs in the map.
get(Object key)	Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
isEmpty()	Checks whether the map has no entries.
keySet()	Returns a set containing all keys in the map.
put(K key, V value)	Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.
putAll(Map m)	Copies all of the mappings from the specified map to this map. These mappings will replace any mappings that this map had for any of the keys currently in the specified map.
remove(Object key)	Deletes the mapping for the given key.
size()	Returns the total number of mappings.
values()	Returns a collection of all values in the map.

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