Java WeakHashMap

In this tutorial, we will learn about Java WeakHashMap and its operations with the help of examples. We will also learn about the differences between WeakHashMap and HashMap

The WeakHashMap class of the Java collections framework provides the feature of the hash table data structure..

WeakHashMap is an implementation of the Map interface. WeakHashMap is almost same as HashMap except in case of WeakHashMap, if object is specified as key doesn’t contain any references- it is eligible for garbage collection even though it is associated with WeakHashMap. i.e Garbage Collector dominates over WeakHashMap.

Example:

// Java program to illustrate 
// WeakHashmap 
import java.util.*; 
class WeakHashMapDemo 
{ 
	public static void main(String args[])throws Exception 
	{ 
		WeakHashMap m = new WeakHashMap(); 
		Demo d = new Demo(); 
		
		// puts an entry into WeakHashMap 
		m.put(d," Hi "); 
		System.out.println(m); 
		
		d = null; 
		
		// garbage collector is called 
		System.gc(); 
		
		// thread sleeps for 4 sec 
		Thread.sleep(4000); . 
		
		System.out.println(m); 
	} 
} 

class Demo 
{ 
	public String toString() 
	{ 
		return "demo"; 
	} 
	
	// finalize method 
	public void finalize() 
	{ 
		System.out.println("finalize method is called"); 
	} 
} 

       Output:

{demo = Hi}
finalize method is called
{ }

Some more important differences between Hashmap and WeakHashmap:

1. Strong vs Weak References: Weak Reference Objects are not the default type/class of Reference Object and they should be explicitly specified while using them. This type of reference is used in WeakHashMap to reference the entry objects. Strong References: This is the default type/class of Reference Object. Any object which has an active strong reference are not eligible for garbage collection. In HashMap, key objects have strong reference.
2. Role of Garbage Collector: Garbage Collected : In HashMap , entry object(entry object stores key-value pairs) is not eligible for garbage collection i.e Hashmap is dominant over Garbage Collector. In WeakHashmap, wWhen a key is discarded then its entry is automatically removed from the map , in other words, garbage collected.
3. Clone method Implementation: HashMap implements Cloneable interface . WeakHashMap does not implement Cloneable interface , it only implements Map interface. Hence , there is no clone() method in the WeakHashMap class.

Method & Description in weakhash map