Difference between HashSet and CopyOnWriteArraySet with Example

I have already shared how HashSet works internally in java . Both HashSet and CopyOnWriteArraySet does not allow duplicates . In this post we will learn about the differences , similarities between HashSet and CopyOnWriteArraySet.

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Difference between HashSet and CopyOnWriteArraySet in Java

1. Synchronization :  HashSet is not synchronized . CopyOnWriteArraySet is synchronized. Synchronization means at a time only one thread can access the object.

2. Performance :  HashSet is faster as it is not synchronized. That means many threads can execute the same piece of code simultaneously. In comparison , CopyOnWriteArraySet is slower.

3. Fail-fast vs Fail-safe : Iterators returned by HahSet's iterator and listiterator  methods are fail-fast. CopyOnWriteArraySet uses fail-safe iterator . We have already shared the fail-fast vs fail-safe iterators in java with example

4. ConcurrentModificationException : HashSet can throw ConcurrentModificationException  while CopyOnWriteArraySet can not .

5. Added in java version : HashSet class was added in java version 1.2 , while CopyOnWriteArraySet class was added in java version 1.5 (or java 5) .

6. Package :  HashSet class is present in java.util package , while CopyOnWriteArraySet class is present in java.util.concurrent package.

Example of ArraySet and CopyOnWriteArraySet in Java

import java.util.HashSet;
import java.util.concurrent.CopyOnWriteArraySet;
public class HashSetCopyOnWriteArraySetExample { public static void main(String[] args) { 
        HashSet<String> hsobj = new HashSet<String>();
        hsobj.add("Alive is awesome");
        hsobj.add("Love yourself");
        System.out.println("HashSet object output :"+ hsobj);

        CopyOnWriteArraySet<String> coponwrtobj = 
                                           new CopyOnWriteArraySet<String>();
        coponwrtobj.add("Alive is awesome");  
        coponwrtobj.add("Love yourself"); 
        System.out.println("CopyOnWriteArraySet object output :"+ coponwrtobj);   

Output :  HashSet object output :[Alive is awesome, Love yourself]
                 CopyOnWriteArraySet object output :[Alive is awesome, Love yourself]

Similarities between HashSet and CopyOnWriteArraySet

1. Permit null values : Both HashSet and CopyOnWriteArraySet permits null.

2. Java Collections Framework : Both classes are part of the Java Collections Framework.

3. Duplicate elements : Both classes does not allow to store duplicate elements.

When to prefer CopyOnWriteArraySet over HashSet 

difference between hashset and copyonwritearrayset in java with example
CopyOnWriteArraySet is a thread-safe variant of HashSet in which all mutative operations (add , set and so on) are implemented by making a fresh copy of the underlying array.
So, if your code involves multi-threading tasks then prefer CopyOnWriteArraySet over HashSet.
Otherwise , always use HashSet .

Recap : Difference between HashSet and CopyOnWriteArraySet in Java

Throws ConcurrentModificationException Yes No
Iterator typeFail fast iteratorFail safe iterator
PerformanceFastSlow in comparision
Added Java 1.2 Java 1.5 

If you have any other questions or doubts regarding the difference between HashSet and CopyOnWriteArraySet in Java , please mention in comments.

About The Author

Subham Mittal has worked in Oracle for 3 years .
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