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 −
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Both null values and the null key are supported.
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Like most collection classes, this class is also not synchronized.
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This class is intended primarily for use with key objects whose equals methods test for object identity using the == operator.
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Each key object in a WeakHashMap is stored indirectly as the referent of a weak reference.
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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 |
This method removes all of the mappings from this map. |
| 2 |
This method returns true if this map contains a mapping for the specified key. |
| 3 |
This method returns true if this map maps one or more keys to the specified value. |
| 4 |
This method returns a Set view of the mappings contained in this map. |
| 5 |
This method returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. |
| 6 |
This method returns true if this map contains no key-value mappings. |
| 7 |
This method returns a Set view of the keys contained in this map. |
| 8 |
This method associates the specified value with the specified key in this map. |
| 9 |
This method copies all of the mappings from the specified map to this map. |
| 10 |
This method removes the mapping for a key from this weak hash map if it is present. |
| 11 |
This method returns the number of key-value mappings in this map. |
| 12 |
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}