Java HashMap class

• A HashMap contains values based on the key. It implements the Map interface and extends AbstractMap class.
• It contains only unique elements.
• It may have one null key and multiple null values.
• It maintains no order.

Hierarchy of HashMap class:

Example of HashMap class:

1. import java.util.*;  
2. class TestCollection13{  
3.  public static void main(String args[]){  
4.    
5.   HashMap<Integer,String> hm=new HashMap<Integer,String>();  
6.   
7.   hm.put(100,"Amit");  
8.   hm.put(101,"Vijay");  
9.   hm.put(102,"Rahul");  
10.   
11.   for(Map.Entry m:hm.entrySet()){  
12.    System.out.println(m.getKey()+" "+m.getValue());  
13.   }  
14.  }  
15. }  

Output:102 Rahul
       100 Amit
       101 Vijay

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What is difference between HashSet and HashMap?

HashSet contains only values whereas HashMap contains entry(key and value).