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 // LinkedHashMapimport 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
