HashMap vs LinkedHashMap: Difference and Comparison

Java is a class-oriented programming language for implementing orders with the least dependencies. Map Interface in Java represents the relation between a key and its value. Several groups of Map implementations maintain the Map Interface.

HashMap and LinkedHashMap are two popularly used Map implementations of Java. The similarity between the two is that they are not synchronized. They also have similarities in their performances. But what is the difference between the two?

Key Takeaways

1. HashMap is a collection that stores key-value pairs and allows null values, while LinkedHashMap maintains the insertion order.
2. HashMap provides better performance for frequent insertions and removals, while LinkedHashMap is better suited for applications where iteration order is important.
3. LinkedHashMap uses slightly more memory than HashMap due to its ordering mechanism.

HashMap vs LinkedHashMap

HashMap has elements that are stored using a hash table, which allows for constant time performance for operations such as insertion, deletion, and retrieval of elements. LinkedHashMap maintains the order of elements using a doubly linked list, having a slightly slower performance than a HashMap.

HashMap is the Map implementation of Java that uses AbstractMap class and extends it to help create a collection of inherits that belong to the same class. The elements entered do not have an order, and the order in which they are entered is also not stored.

LinkedHashMap is another Map implementation that is a subclass of HashMap. Since they have key insertion orders, their applications are in areas where the order of insertion and access matters. They also have more memory than HashMap.

Comparison Table

Parameters of Comparison	HashMap	LinkedHashMap
Meaning	It is a Java Map implementation that uses AbstractMap class and helps create a collection of inherits.	It is also a type of Java Map and is a subclass of HashMap that implements Hashtable and Linked List of Map.
Elements	The elements entered in a HashMap are not in order.	The elements are known to be in key insertion order.
Order	The order that the elements are entered is not preserved by the HashMap.	Since they are in key insertion order, the order of entering the elements is preserved.
Data structure	They are stored as a list of buckets and are implemented as a Hashtable in the Map interface.	LinkedHashMap has a doubly-linked list running through all the lists stored in it.
Retrieval	Retrieving or performing other such tasks on the lists stored in HashMap is easy.	Retrieving, removing or adding entries into the LinkedHashMap is harder than in HashMap.
Applications	It is used in places for easier retrieval and other general purposes.	It is applied or used in places where the insertion or access order matters, like LRU cache.

What is HashMap?

HashMap is a Map Implementation that implements and extends in AbstractMap to help create a collection of inherits belonging to the same class. This class allows one to store keys and their values to form a Map interface.

HashMap is not synchronized. It can also store null values, but only one null key should be defined. It would not work over multiple null keys. This is also because the keys stored have to be unique.

Even if a duplicate one is stored, it is replaced by the original one in the list. The feature that differentiates this from the other groups of map implementation is that they do not have the order to enter the key, nor do they store the order it is entered.

The data structure can be easily retrieved from a HashMap. The retrieved data can also be easily modified, added to, or removed. So their main applications are in places that require a fast retrieval process and other general purposes.

What is LinkedHashMap?

LinkedHashMap is also a map implementation group. They are a subgroup of HashMap but extend and implements HashMap itself, extending to AbstractMap and then to the map interface.

LinkedHashMap is also not synchronized or linked. Even though the values entered should be unique, they can have more than one null value in the list but only one null key.

In this map implementation, the keys are entered in order or what is known as key insertion order. The order of the keys entered is also preserved in the same manner.

This is overlooked and maintained by the doubly-linked lists running through all the lists since they maintain the order, and their application in the areas where the insertion and access order are important.

Difference Between HashMap and LinkedHashMap

1. HashMap is a popular Java Map implementation that uses AbstractMap class and can extend it to help create a collection of inherits of the AbstractMap. LinkedHashMap is a subclass of HashMap, and it extends HashMap and implements the Linked List of Map.
2. The elements entered into HashMap do not have a particular order, whereas LinkedHashMap follows key insertion order.
3. The main difference is the preservation of order in the two Maps. HashMap does not preserve the order in which the keys or elements are entered. Whereas, since LinkedHashMap follows key insertion order, they preserve the order in which the elements are entered.
4. The data structure of HashMap is stored as lists of buckets that have Hashtable implemented over them. If there are more than a particular number of entries, they are switched to a balanced form. On the other hand, LinkedHashMap has doubly-linked lists running through all the lists stored to manage their data structure.
5. Retrieving and performing functions like removing and adding data entries is easy in HashMap. But performing the same functions on LinkedHashMap is harder than in HashMap.
6. Since retrieving data entries is easier, HashMap is used for fast data retrieval and other general purposes. On the other hand, LinkedHashMap is used where the order of insertion or access is important, like in the LRU cache.

References

1. Efficient fuzzy search enabled hash map (ieee.org)
2. File uploading and Java Hash map data structure (researchgate.net)