HashMap in java

HashMap in java

Java HashMap is a part of collection since jdk 1.2 versions. HashMap implements the Map interface and extends AbstractMap class. HashMap stores data in key & value pairs.

HashMap uses Hashing technique for storing key and value pair in java.

The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.

HashMap - Here k means Key which is any data type

 V means Value of any data types

HashMap in Java - techfloaters

HashMap class Declaration

public class HashMap extends AbstractMap

    implements Map, Cloneable, Serializable {

Features of HashMap class

1] HashMap stores unique key only. Duplicate key not allowed if we try to enter then recent one get override by previous one.

2] HashMap uses hashing mechanism to store unique element.

3] HashMap does not maintain insertion order because element are stored on the basis of hashing technique.

4] HashMap allows multiple null values but only one null key.

5] HashMap class is not thread safe, it means it is non synchronized.

Constructor and Description

HashMap<K, V> hs = new HashMap()

This is default constructor which constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).

HashMap<K, V> hs = new HashMap(int initialCapacity)

Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).

HashMap<K, V> hs = new HashMap(int initialCapacity, float loadFactor)

Constructs an empty HashMap with the specified initial capacity and load factor.

HashMap<K, V> hs = new HashMap(Map m)

Constructs a new HashMap with the same mappings as the specified Map.

Constructs a new set containing the elements in the specified collection.

Java HashMap Example

package com.vr.techfloaters;

import java.util.HashMap;

import java.util.Iterator;

import java.util.Map.Entry;

import java.util.Set;

public class HashMapDemo {

     public static void main(String[] args) {

          

           // This is how to declare HashMap

           HashMap hm = new HashMap<>();

          

           //Adding elements to HashMap

           hm.put(50, "java");

           hm.put(26, ".Net");

           hm.put(20, "php");

           hm.put(null, null);

           hm.put(25, ".Net");

          

           //Get values based on key

           System.out.println(hm.get(20));

          

           //Remove key and values based on key

           System.out.println(hm.remove(25));

          

           //Iterating the element in HashMap

           Set< Entry > set = hm.entrySet();

           

           Iterator> itr = set.iterator();

           

          while(itr.hasNext())

          {

             Entry entry = itr.next();

             System.out.println(entry.getKey() +" : "+entry.getValue());

          }

          

     }

}

Output

php

.Net

null : null

50 : java

20 : php

26 : .Net

From above output we can see duplicate key are not allowed and also insertion order is not followed.