Difference between HashMap and LinkedHashMap in Java

Difference between HashMap and LinkedHashMap in Java

Difference between LinkedHashMap and HashMap in Java

HashMap and LinkedHashMap are two most common used Map implementation in Java and main difference between HashMap and LinkedHashMap is that LinkedHashMap maintain insertion order of keys, Order in which keys are inserted in to LinkedHashMap. On the other hand HashMap doesn't maintain any order or keys or values. In terms of Performance there is not much difference betweenHashMap and LinkedHashMap but yes LinkedHashMap has more memory foot print than HashMap to maintain doubly LinkedList which it uses to keep track of insertion order of keys.

LinkedHashMap and HashMap in Java - Similarities
There are lot of similarity between LinkedHashMap and HashMap in Java, as they both implement Map interface.

let's have a look :

1) Both LinkedHashMap and HashMap are not synchronized and subject to race condition if shared between multiple threads without proper synchronization. Use Collections.synchronizedMap() for making them synchronized.

2) Iterator returned by HashMap and LinkedHashMap are fail-fast in nature.

3) Performance of HashMap and LinkedHashMap are similar also.

Difference between LinkedHashMap and HashMap in Java
Now let's see some differences between LinkedHashMap and HashMap in Java:

1) First and foremost difference between LinkedHashMap and HashMap is order, HashMap doesn't maintain any order whileLinkedHashMap maintains insertion order of elements in Java.

2) LinkedHashMap also requires more memory than HashMap because of this ordering feature. As I said before LinkedHashMapuses doubly LinkedList to keep order of elements.

3) LinkedHashMap actually extends HashMap and implements Map interface.

Given the insertion order guarantee of LinkedHashMap, Its a good compromise between HashMap and TreeMap in Java because with TreeMap you get increased cost of iteration due to sorting and performance drops on to log(n) level from constant time.