How to Maintain Insertion Order of the Elements in Java HashMap?

https://www.geeksforgeeks.org/how-to-maintain-insertion-order-of-the-elements-in-java-hashmap/

 

When elements get from the HashMap due to hashing the order they inserted is not maintained while retrieval. We can achieve the given task using LinkedHashMap. The LinkedHashMap class implements a doubly-linked list so that it can traverse through all the elements.

Example:

 

Input : HashMapInput = {c=6, a=1, b=2}
Output: HashMapPrint = {c=6, a=1, b=2}

Input : HashMapInput = {"first"=1, "second"=3}
Output: HashMapPrint = {"first"=1, "second"=3}

Syntax:

public LinkedHashMap(Map m)

 

It creates an object of the LinkedHashMap class with the same mappings specified in the original Map object.

Order Not Maintain Here: HashMap Implementation:

• Java

 

// Java Program to maintain insertion order

// of the elements in HashMap

  

// Using HashMap (Order not maintain)

import java.io.*;

import java.util.*;

class GFG {

  

    public static void main(String args[])

    {

  

        // creating a hashmap

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

  

        // putting elements

        hm.put("01", "aaaaaaa");

        hm.put("03", "bbbbbbb");

        hm.put("04", "zzzzzzz");

        hm.put("02", "kkkkkkk");

  

        System.out.println("Iterate over original HashMap");

        

        // printing hashmap

        for (Map.Entry<String, String> entry :

             hm.entrySet()) {

            System.out.println(entry.getKey() + " => "

                               + ": " + entry.getValue());

        }

    }

}

Output

Iterate over original HashMap
01 => : aaaaaaa
02 => : kkkkkkk
03 => : bbbbbbb
04 => : zzzzzzz

Here, the original insertion order of HashMap is [01, 03, 04, 02], but the output is different [01, 02, 03, 04]. It did not maintain the original insertion order of the elements.

Order Maintain Here: LinkedHashMap implementation

• Java

 

// Java Program to maintain insertion order

// of the elements in HashMap

  

// LinkedHashMap

import java.io.*;

import java.util.*;

class GFG {

  

    public static void main(String args[])

    {

  

        // creating a hashmap

        HashMap<String, String> hm = new LinkedHashMap<>();

  

        // putting elements

        hm.put("01", "aaaaaaa");

        hm.put("03", "bbbbbbb");

        hm.put("04", "zzzzzzz");

        hm.put("02", "kkkkkkk");

  

        // printing LinkedHashMap

        System.out.println("Iterate over LinkedHashMap");

        for (Map.Entry<String, String> entry :

             hm.entrySet()) {

            System.out.println(entry.getKey() + " => "

                               + ": " + entry.getValue());

        }

    }

}

Output

Iterate over LinkedHashMap
01 => : aaaaaaa
03 => : bbbbbbb
04 => : zzzzzzz
02 => : kkkkkkk