Java HashMap : Collection Class

1. HashMap class uses a hashtable to implement the Map interface
2. HashMap is a data structure which works on hashing techniques.
3. HashMap allows us to store the object as key-value pair.
4. HashMap is used to retrieve the object in constant time O(1)
5. HashMap implements the Map interface and extends AbstractMap class.
6. HashMap can only contain unique keys.
7. HashMap does not maintains any order.

Lets create an HashMap using following line of code -

Example #1 : Create HashMap

package com.c4learn.collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
public class HashMapExample {
  public static void main(String[] args) {
  HashMap<String,String> hMap = new HashMap<String,String>();
    hMap.put("A1", "Aman");
    hMap.put("A2", "Arun");
    hMap.put("A3", "Akash");
    hMap.put("A4", "Aniket");
    hMap.put("A5", "Anil");
    Set set = hMap.entrySet();
    Iterator itr = set.iterator();
    while (itr.hasNext()) {
      Map.Entry m = (Map.Entry) itr.next();
      System.out.println(m.getKey() + " " + m.getValue());
    }
  }
}

Output :

A2 Arun
A1 Aman
A4 Aniket
A3 Akash
A5 Anil

Explanation :

Following line of code is used to create the HashMap in Java.

HashMap<String,String> hMap = new HashMap<String,String>();

Now if we need to add a pair of name & value to the map then we can use put method for adding element to the Map.

hMap.put("A1", "Aman");
hMap.put("A2", "Arun");
hMap.put("A3", "Akash");
hMap.put("A4", "Aniket");
hMap.put("A5", "Anil");

We have added 5 elements to the Map.