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Table of contents
1.
Introduction
2.
What is HashMap in Java?
2.1.
Syntax of HashMap in Java
2.2.
Example 
3.
What is LinkedHashMap in Java?
3.1.
Syntax of LinkedHashMap in Java
3.2.
Example
4.
Difference between HashMap and LinkedHashMap
5.
LinkedHashMap and HashMap in Java - Similarities
6.
Frequently Asked Questions
6.1.
What is the major difference between hashMap and linkedHashMap?
6.2.
Is TreeMap quicker than LinkedHashMap?
6.3.
What is the difference between TreeMap HashMap and LinkedHashMap?
6.4.
What is the difference between a linked list and a linked HashMap?
6.5.
When to use LinkedHashMap over HashMap?
7.
Conclusion
Last Updated: Mar 27, 2024
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Difference between HashMap and LinkedHashMap in Java

Author Shiva
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Introduction

The Java Collections framework's classes include Java HashMap and LinkedHashMap. A collection, often known as a container, is just an object that unifies several items into one. Aggregate data is stored, retrieved, handled, and communicated via collections. Two of the most popular and all-purpose Map implementations in the Java platform are HashMap and LinkedHashMap. They are essentially hash-based classes used to create Maps and are pretty similar. The Map interface, which describes the actions supported by a set of key-to-value associations in which the keys are unique, is the final of the primary Collections Framework interfaces. These Map implementations use a hashing method as their foundation.

Difference Between Hashmap and Linkedhashmap

The class LinkedHashMap implements ordered maps in contrast to the class HashMap's unordered implementation. Since the LinkedHashMap implementation belongs to the HashMap class as a subclass, it has access to all of its capabilities. Performance-wise, there isn't much of a difference between the two. Let's look at the linkedhashmap vs hashmap.

What is HashMap in Java?

Before discussing the difference between hashMap and linkedHashMap. Let’s look at HashMap first; HashMap is a data structure that stores items using "key-value" pairs. They are great at mapping, as the name would imply. We can map one object to another using hash maps. It could be a multi-fielded object, a String, or an integer. It is a crucial data structure, and Java offers its implementation. 

Key and value pairs are found in maps. HashMap is a hashtable-based implementation of the Map interface, which is available in Java. Java's HashMap class is virtually identical to Hashtable, except that it is unsynchronized and supports nulls. Basic operations like insertion and deletion are performed in constant time by HashMap.

Syntax of HashMap in Java

HashMap<K, V> h = new HashMap<K, V>();

Example 

import java.util.*;
class HelloWorld {
	public static void main(String[] args) {
		HashMap<Integer,String> mp=new HashMap<Integer,String>();  
		mp.put(1,"Practice");  
		mp.put(2,"Coding");      
		mp.put(3,"Everyday.");     


		for(Map.Entry itr : mp.entrySet()){      
			System.out.println(itr.getKey()+" "+itr.getValue());      
		}    
	}
}


Output: 

1 Practice
2 Coding
3 Everyday.
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What is LinkedHashMap in Java?

As one of the four general-purpose implementations of the Map interface, LinkedHashMap inherits the functionality of the HashMap class since it is a subclass. 

Although it performs fairly similarly to HashMap in terms of speed, it differs from HashMap in that it preserves the order in which keys are placed into the Map or accessed from it. Ensuring the order in which its iterators return its elements improves the contract of its parent class. However, because it keeps a doubly-linked list in Java, it uses more memory than a HashMap.

Syntax of LinkedHashMap in Java

LinkedHashMap<K, V> hp = new LinkedHashMap<K, V>();

Example

import java.util.*;
class HelloWorld {
	public static void main(String args[]){    
		LinkedHashMap<Integer,String> lhmp=new LinkedHashMap<Integer,String>();   
		lhmp.put(1,"Solve");  
		lhmp.put(2,"Question Series");    
		lhmp.put(3,"On Coding Ninjas Studio.");  
		for(Map.Entry mp:lhmp.entrySet()){    
			System.out.println(mp.getKey()+" "+mp.getValue());    
		}    
	}    
}

 

Output:

1 Solve
2 Question Series
3 On Coding Ninjas Studio.

You can also read about the Longest Consecutive Sequence.

You can also check out Internal Working of HashMap

Difference between HashMap and LinkedHashMap

Here is a thorough difference between HashMap and LinkedHashMap below: 

Based on HashMap LinkedHashMap
Class Extension HashMap extends abstract map class LinkedHashMap extends HashMap class.
Performance HashMap is much faster than its counterpart. LinkedHashMap also supports a similar speed as HashMap.
Iteration order HashMap does not guarantee any order. LinkedHashMap is based on Insertion Order.
Memory  HashMap takes less memory. LinkedHashMap consumes relatively more memory.
Implementation ( Interface ) HashMap implements a map interface. LinkedHashMap implements a hashMap interface.
Synchronization Hash-Map is not synchronized. LinkedHashMap is also not synchronized.
Null values and keys HashMap supports multiple values but only one null key. LinkedHashMap supports multiple values but only one null key.
Implementation HashMap uses buckets for its implementation. LinkedHashMap uses double-linked buckets.
Thread Safety HashMap is not thread-safe. LinkedHashMap is not thread-safe.

Adding, removing, and finding entries in a HashMap is relatively faster than doing the same with a LinkedHashMap, even though the performance of both HashMap and HashMap classes is almost identical. However, HashMap uses less memory than LinkedHashMap because it does not guarantee the order in which iterations occur. A LinkedHashMap's elements are in key insertion order, which refers to the order in which the keys are entered into the map, whereas a HashMap's components are not in order by default. The sequence in which the entries are accessed is likewise maintained by LinkedHashMap for its elements. It performs worse than HashMap and requires the maintenance of a doubly-linked list, just as LinkedHashMap.

LinkedHashMap and HashMap in Java - Similarities

LinkedHashMap and HashMap are both classes in Java that implement the Map interface, and they share many similarities:

  • Both allow null values and null keys.
  • Both are used to store key-value pairs.
  • Both are not synchronized by default. However, they can be made thread-safe using Collections.synchronizedMap().
  • Both have put(), get(), and remove() methods to manipulate the entries in the map.
  • Both allow iteration over the entries in the map using entrySet(), keySet(), and values() methods.
  • Both have constant-time performance for get() and put() operations, and linear time for containsKey() and containsValue() operations.

Frequently Asked Questions

What is the major difference between hashMap and linkedHashMap?

The major difference between hashMap and linkedHashMap is hashmap implements the map interface, whereas linkedhashMap implements the hashmap interface.

Is TreeMap quicker than LinkedHashMap?

While slower than HashMap, LinkedHashMap is faster than TreeMap.

What is the difference between TreeMap HashMap and LinkedHashMap?

TreeMap orders entries based on keys' natural ordering, LinkedHashMap maintains the insertion order, and HashMap does not maintain any order. TreeMap has O(log n) complexity, while LinkedHashMap and HashMap have O(1) complexity for most operations.

What is the difference between a linked list and a linked HashMap?

A linked list is a data structure that allows fast insertion and deletion of elements at any position. A linked HashMap is a variant of HashMap that maintains the order of insertion, whereas a linked list does not store key-value pairs.

When to use LinkedHashMap over HashMap?

LinkedHashMap is useful when you need to maintain the order of insertion of entries, while HashMap is useful when order is not important. If you need the entries to be sorted based on the keys, then TreeMap can be used.

Conclusion

This article covers everything you need to know about the difference between hashMap and linkedHashMap. We hope this difference between hashMap and linkedHashMap helps you in your journey.

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