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HashMap in Java with Example

By Chaitanya Singh | Filed Under: Java Collections

HashMap is a Map based collection class that is used for storing Key & value pairs, it is denoted as HashMap<Key, Value> or HashMap<K, V>. This class makes no guarantees as to the order of the map. It is similar to the Hashtable class except that it is unsynchronized and permits nulls(null values and null key).

It is not an ordered collection which means it does not return the keys and values in the same order in which they have been inserted into the HashMap. It does not sort the stored keys and Values. You must need to import java.util.HashMap or its super class in order to use the HashMap class and methods.

HashMap Example in Java:

In this example we have demonstrated almost all the important methods of HashMap class.

import java.util.HashMap;
import java.util.Map;
import java.util.Iterator;
import java.util.Set;
public class Details {

   public static void main(String args[]) {

      /* This is how to declare HashMap */
      HashMap<Integer, String> hmap = new HashMap<Integer, String>();

      /*Adding elements to HashMap*/
      hmap.put(12, "Chaitanya");
      hmap.put(2, "Rahul");
      hmap.put(7, "Singh");
      hmap.put(49, "Ajeet");
      hmap.put(3, "Anuj");

      /* Display content using Iterator*/
      Set set = hmap.entrySet();
      Iterator iterator = set.iterator();
      while(iterator.hasNext()) {
         Map.Entry mentry = (Map.Entry)iterator.next();
         System.out.print("key is: "+ mentry.getKey() + " & Value is: ");
         System.out.println(mentry.getValue());
      }

      /* Get values based on key*/
      String var= hmap.get(2);
      System.out.println("Value at index 2 is: "+var);

      /* Remove values based on key*/
      hmap.remove(3);
      System.out.println("Map key and values after removal:");
      Set set2 = hmap.entrySet();
      Iterator iterator2 = set2.iterator();
      while(iterator2.hasNext()) {
          Map.Entry mentry2 = (Map.Entry)iterator2.next();
          System.out.print("Key is: "+mentry2.getKey() + " & Value is: ");
          System.out.println(mentry2.getValue());
       }

   }
}

Output:

key is: 49 & Value is: Ajeet
key is: 2 & Value is: Rahul
key is: 3 & Value is: Anuj
key is: 7 & Value is: Singh
key is: 12 & Value is: Chaitanya
Value at index 2 is: Rahul
Map key and values after removal:
Key is: 49 & Value is: Ajeet
Key is: 2 & Value is: Rahul
Key is: 7 & Value is: Singh
Key is: 12 & Value is: Chaitanya

HashMap Class Methods

Here is the list of methods available in HashMap class. I have also covered examples using these methods at the end of this post.

  1. void clear(): It removes all the key and value pairs from the specified Map.
  2. Object clone(): It returns a copy of all the mappings of a map and used for cloning them into another map.
  3. boolean containsKey(Object key): It is a boolean function which returns true or false based on whether the specified key is found in the map.
  4. boolean containsValue(Object Value): Similar to containsKey() method, however it looks for the specified value instead of key.
  5. Value get(Object key): It returns the value for the specified key.
  6. boolean isEmpty(): It checks whether the map is empty. If there are no key-value mapping present in the map then this function returns true else false.
  7. Set keySet(): It returns the Set of the keys fetched from the map.
  8. value put(Key k, Value v): Inserts key value mapping into the map. Used in the above example.
  9. int size(): Returns the size of the map – Number of key-value mappings.
  10. Collection values(): It returns a collection of values of map.
  11. Value remove(Object key): It removes the key-value pair for the specified key. Used in the above example.
  12. void putAll(Map m): Copies all the elements of a map to the another specified map.

HashMap Tutorials

Here is the list of tutorials published on HashMap class. Happy Learning:)

HashMap Basics

  1. How to iterate HashMap
  2. Sort HashMap by Keys and values
  3. Get Size of HashMap
  4. Remove Key-value mapping from HashMap
  5. Remove all mapping from HashMap
  6. How to check if HashMap is empty or not?

Get/Search

  1. Check if particular key exists in HashMap
  2. Check if particular value exists in HashMap

Serialize/Synchronize

  1. Serialize HashMap
  2. Synchronize HashMap

Differences

  1. HashMap vs ArrayList
  2. HashMap vs Hashtable
  3. HashSet vs HashMap

Other Tutorials

  1. HashMap Iterator example
  2. Copy one HashMap to another
  3. Get value from HashMap using Key
  4. Get Set view of keys from HashMap
  5. Clone a HashMap

Reference:

HashMap Documentation

Comments

  1. Raju says

    March 31, 2014 at 4:27 PM

    Mind Blowing……

    Do something so that your website can have more views………this is very much useful for beginners……………….

    Reply
    • Chaitanya Singh says

      April 6, 2014 at 3:33 PM

      Thanks for your kind words Raju :)

      Reply
      • piyush shukla says

        July 26, 2014 at 3:44 PM

        very good explanation…

        Reply
  2. May says

    July 17, 2014 at 2:59 PM

    Hi Chaitanya,
    This tutorial is really very understandable.
    I have been trying to learn Java from a long time but am not really able to get to undertsand the concepts well enough to use them wherever needed. Can you please suggest me how to learn so I can use the concepts in my day today job.

    Reply
  3. Farhan Ali says

    October 25, 2015 at 6:51 PM

    Hello Sir,
    I don’t understand why have you used set for iterating while it can be easily done with keySet();.

    Reply
    • sunny says

      June 14, 2016 at 6:12 AM

      keyset() also return SET.
      but if you use KeySet() – it will just return all keys in one go but if you want to traverse each key and do some operation you required something else and the same was explained by Chaitanya..

      Hope this help!

      Reply
  4. Line says

    May 18, 2016 at 3:24 PM

    Why do I get NullPointerException?

    Reply
    • Tamil selvan s says

      November 22, 2016 at 4:17 AM

      There may be no objects available at that area or space in which you are accessing.

      For example we can take an array

      int a[] = new int[5];
      here the available values are a[0],a[1],a[2],a[3],a[4] if u try to fix values for a[5] then you will get nullpointerexception!!!!!!!

      Reply
  5. Andrew says

    April 30, 2017 at 5:26 PM

    I want to know why the order of the output is not the same as we input them. Would you mind email me your reply? Thanks!

    Reply
    • Max Bradley says

      August 17, 2017 at 11:20 AM

      Hi Andrew,

      I was wondering the same, I think it is just a property of HashMaps that there is no ordering. When you add items to them you are not adding after any existing entries, you are just added to a pool of key-value pairs.

      Reply
      • Karthi says

        August 16, 2018 at 1:25 PM

        Read these lines again please

        It is not an ordered collection which means it does not return the keys and values in the same order in which they have been inserted into the Hash Map. It does not sort the stored keys and Values.

        Reply

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