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