HashMap Of Java Collection With Easy Example

java.util.HashMap<K,V>   is used to implement Map data structure.It extends AbstractMap class , and implements Map interface.Internally HashMap use hash table to store objects. 

Characteristics of HashMap  

1. HashMap store objects as Key value Pair.

2. We can get value by its unique key.

3. HashMap can store only one null value as a key. 

4. HashMap is not thread safe, Multiple thread can access one HashMap simultaneously
5. HashMap does not maintain the order. 

Example HashMap<K,V> 

package com.javasuitor.demo;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class HashMapDemo {

 public static void main(String[] args) {
  
  // Map contains data in  Key, Value Pair, keys will be unique
  Map<Integer,String> studentMap = new HashMap<Integer,String>();
  
  studentMap.put(101, "Ashok Singh");
  studentMap.put(102, "Arvind");  
  studentMap.put(103, "Monika");
  studentMap.put(104, "Aakanksha");
  studentMap.put(105, "Sourabh");  
  
  // We can pass null as a key only once in map.
  studentMap.put(null,null);
  
  // If you pass duplicate key then value will be overwritten. 
  
  studentMap.put(null,"No name");
  
  Set<Integer> keys = studentMap.keySet();
  
  // View all values
  for(Integer i : keys){
   System.out.println("Key :" + i +" Value : "+ studentMap.get(i));
  }
    
  
 }
}

Output :

Key :null Value : No name
Key :102 Value : Arvind
Key :103 Value : Monika
Key :101 Value : Ashok Singh
Key :104 Value : Aakanksha
Key :105 Value : Sourabh

Difference Between HashTable and HashMap

HashTable is very similar to HashMap. Difference between HashTable and HashMap are :

1. HashTable can not store null value.
2. HashTable is thread safe, multiple thread cannot access HashTable simultaneously.
3. HashMap is faster than HashTable.


Reference :

http://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html