WeakHashMap class in Java

WeakHashMap is the Hash table based implementation of the Map interface, with weak keys. An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently from other Map implementations. Few important features of a weak hashmap are:

• Both null values and null keys are supported in WeakHashMap.
• It is not synchronized.
• This class is intended primarily for use with key objects whose equals methods test for object identity using the == operator.

Constructors in WeakHashMap:

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 load factor): This constructor is used to create an empty WeakHashMap with the given initial capacity and the given load factor.
4. WeakHashMap(Map m): This constructor is used to create a new WeakHashMap with the same mappings as the specified map.

Methods in WeakHashMap:

1. void clear(): The method removes all of the mappings from this map. The map will be empty after this call returns.

   Syntax: public void clear().
   Returns: NA.
   Exception: NA.
   

2. boolean contains the value(Object value): this method returns true if this map maps one or more keys to the specified value.

   Syntax: public boolean containsValue(Object value).
   Returns: true if this map maps one or more keys to the specified value.
   Exception: NA
   

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

   Syntax: public boolean containsKey(Object key).
   Returns: true if there is a mapping for key; false otherwise.
   Exception: NA
   

4. 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.

   Syntax: public put(K key, V value).
   Returns: the previous value associated with key, 
   or null if there was no mapping for key. 
   (A null return can also indicate that the 
   map previously associated null with key.)
   Exception: NA
   

5. boolean isEmpty(): Returns true if this map contains no key-value mappings. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced.

   Syntax: public boolean isEmpty()
   Returns: true if this map contains no key-value mappings.
   Exceptions: NA
   

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   // Java code illustrating close(), containsValue() // containsKey() and isEmpty() method    import java.util.Map; import java.util.WeakHashMap;    class WeakHashMapdemo {     public static void main(String[] arg)     {         Map<Number, String> weak = new WeakHashMap<Number, String>();         weak.put(1, "geeks");         weak.put(2, "for");         weak.put(3, "geeks");                    // Checking weak map         System.out.println("our weak map: " + weak);                    // Checking if "for" exist         if(weak.containsValue("for"))             System.out.println("for exist");                    // Checking if 1 exist as a key in map         if(weak.containsKey(1))             System.out.println("1 exist");                    // Removing all data         weak.clear();                    // Checking whether map is empty or not         if(weak.isEmpty())             System.out.println("empty map: " + weak);     } }

   Output:

   our weak map: {3=geeks, 2=for, 1=geeks}
   for exist
   1 exist
   empty map: {}
   

6. Set entrySet(): Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's remove operation, or the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

   Syntax: public Set entrySet()
   Returns: a set view of the mappings contained in this map.
   Exception: NA

7. Set keySet(): Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator. remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

   Syntax: public Set keySet().
   Returns: a set view of the keys contained in this map
   Exception: NA
   

8. Collection values(): Returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator's remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

   Syntax: public Collection values().
   Returns: a collection view of the values contained in this map
   Exception: NA
   

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   // Java code illustrating entrySet(), keySet() and Values()    import java.util.Collection; import java.util.Map; import java.util.Set; import java.util.WeakHashMap;    class WeakHashMapdemo {     public static void main(String[] arg)     {         Map<Number, String> weak = new WeakHashMap<Number, String>();         weak.put(1, "geeks");         weak.put(2, "for");         weak.put(3, "geeks");                    Set set1 = weak.entrySet();                    // Checking set         System.out.println(set1);                    // Creating set for key         Set keySet = weak.keySet();                    // Checking keySet         System.out.println("key set: " + keySet );                    Collection value = weak.values();                    // Checking values of map         System.out.println("values: " + value);     } }

   Output:

   [3=geeks, 2=for, 1=geeks]
   key set: [3, 2, 1]
   values: [geeks, for, geeks]
   

9. void 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.

   Syntax: public void putAll(Map m)
   Returns: NA
   Exception: 
   NullPointerException - if the specified map is null.
   

10. V get(Object key): Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

    Syntax: public V get(Object key)
    Returns: the value to which the specified 
    key is mapped, or null if this map contains no mapping for the key
    Exception: NA
    

11. V remove(Object key): Removes the mapping for a key from this weak hash map if it is present. More formally, if this map contains a mapping from key k to value v such that (key==null ? k==null: key.equals(k)), that mapping is removed.

    Syntax: public V remove(Object key)
    Returns: the previous value associated with key, or 
    null if there was no mapping for key
    Exception: NA.
    

12. int size(): Returns the number of key-value mappings in this map. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced.

    Syntax: public int size()
    Returns: the number of key-value mappings in this map.
    Exception: NA
    

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    // Java code remove(), putAll() // get() and size() method    import java.util.Collection; import java.util.Map; import java.util.Set; import java.util.WeakHashMap;    class WeakHashMapdemo {     public static void main(String[] arg)     {         Map<Number, String> weak = new WeakHashMap<Number, String>();         weak.put(1, "geeks");         weak.put(2, "for");         weak.put(3, "geeks");                    Map<Number, String> weak1 = new WeakHashMap<Number, String>();         weak1.putAll(weak);                    // getting value of key 2         System.out.println(weak1.get(2));                    // size of map         System.out.println("Size of map is: " + weak1.size());                    // removing 2nd element         weak1.remove(2);                    // size after removing key and value pair         System.out.println("Size after removing: " + weak1.size());     } }

    Output:

    for
    Size of map is: 3
    Size after removing: 2