5 Difference between HashSet and ArrayList in Java with Example

HashSet and ArrayList both are one of the most important classes of Java Collection framework. This is one of the frequently asked java collection interview question for java developers if you have experience around 1 to 3 years.Apart from the differences between HashSet and ArrayList we will see the similarities and examples as well.

Read Also :  Java Collection Interview Questions and Answers

Difference between HashSet and ArrayList in Java

1. Implementation : HashSet implements Set interface in Java while ArrayList  implements List interface.

2. Internal implementation: HashSet is backed by an HashMap while ArrayList is backed by an Array. Find out how HashSet works internally in java for more details.

3. Ordering : HashSet is an unordered collection and doesn't maintain any order while ArrayList maintains the order of the object in which they are inserted.


4. Duplicates : HashSet doesn't allow duplicates though ArrayList allows duplicate values.

5. Index based  :  The fifth difference between HashSet and ArrayList is that ArrayList is index based you can retrieve object by calling get(index) or remove objects by calling remove(index) while HashSet is completely object based.HashSet also doesn't provide get() method.

Example of HashSet and ArrayList in Java

import java.util.Hashtable;


public class HashSetArrayListExample {
    
    public static void main(String[] args) { 

        ArrayList<String> arrlistobj = new ArrayList<String>();

        arrlistobj.add("1. Alive is awesome");
        arrlistobj.add("2. Love yourself");
        System.out.println("ArrayList object output :"+ arrlistobj);

        HashSet hashsetobj = new HashSet();
        hashsetobj.add("Alive is awesome");  
        hashsetobj.add("Love yourself"); 
        System.out.println("HashSet object output :"+hashsetobj);   

 }
}

Output

ArrayList object output :[1. Alive is awesome, 2. Love yourself]
HashSet object output :[Alive is awesome, Love yourself]

Similarities between HashSet and ArrayList 

1. Allows null : Both HashSet and ArrayList allows null.

2. Synchronized : Both HashSet and ArrayList are non synchronized collection class. One needs to avoid using them in multithreading environment.

3. Traversal : Both HashSet and ArrayList can be traversed through iterator in java.

4. Iterator : Both HashSet and ArrayList classes iterator are fail-fast i.e they will throw ConcurrentModificationException as soon as they detect any structural change in ArrayList or HashSet.


Recap : Difference between HashSet and ArrayList in Java 

	HashSet	ArrayList
Implementation	Set	List
Internal implementation	backed by HashMap	backed by Array
Duplicates	Unique values	permit duplicates
Ordering	Unordered	Maintains order
Index based	No	Yes

In case you have any other query please mention in the comments .

That's all for the difference between HashSet and ArrayList in java with example. You can decide when to use ArrayList or HashSet based on the performance and properties (duplicates,ordering) .Please mention in the comments if you know any other differences between HashSet and ArrayList.