ArrayList vs HashMap

ArrayList  and  HashMap  are two collection classes in Java. Even though they have the same role of collecting and organizing data, the way they back up and process data is different. In this article we will see the differences and similarities between the two.

Difference between ArrayList and HashMap in Java

One of the important differences between the HashMap and ArrayList is that the first is the hash table implementation and the second is a dynamic array that can resize. HashMap  and  ArrayList  are the two most popular classes of the Java Collection framework. Both are used in the storage of information and their implementation, use is completely different.
The main difference between  ArrayList  and  HashMap  is that  ArrayList  is based on the structure of the array index while HashMap  is a structure in a map that uses hashing to extract values.

Some crucial differences between  ArrayList  and  HashMap:

• The First Difference Between ArrayList  and  HashMap  is that  ArrayList  implements the interface List while HashMap  Implements the Map in Java.
• ArrayList  Backs up one object at a time while  HashMap  Saves the key and values< key,value>.
• ArrayList  Maintains the order of the added objects. HashMap  does not guarantee this functionality.
• ArrayList  Allows insertion of duplicate objects but  HashMap  does not allow inserting the same key twice, but it does allow the same key values.
• The complexity of the get(index) of  ArrayList  is O(1) but  HashMap  get(key) can be O(1) in the best case and O(n) in the worst case.

Similarities between ArrayList and HashMap in Java

Here are some common points between  ArrayList  and  HashMap  in Java:

• ArrayList  and  HashMap  are not synchronized. You should not use them in a multi-threaded program without external synchronization.
• The iteration of  ArrayList  and  HashMap  can trigger a  ConcurrentModificationException if a structure change just happened once Iterator is created.
• ArrayList  allowsnull.  HashMap  allowsnull for keys and values.
• ArrayList  Allows duplicate items and  HashMap  allows duplicate values.
• In terms of performance  ArrayList  has a constant time for reading if you know the index with the method get() similar to  HashMap  which also has a constant read time.
• The implementation of ArrayList and HashMap is based on an array.
• Both collections ArrayList and HashMap can be browsed with Iterator.