CopyOnWriteArraySet in java

CopyOnWriteArraySet is a Set that uses an internal CopyOnWriteArrayList for all of its operations. It is introduced in JDK 1.5, we can say that it is thread-safe version of Set.

It share some properties of Set and also has its own properties:

The internal implementation of CopyOnWriteArraySet is CopyOnWriteArrayList only.
Insertion order is preserved and duplicates are allowed.
Multiple Threads are able to perform update operation simultaneously but for every update operation a separate cloned copy is created. As for every update a new cloned copy will be created which is costly. Hence if multiple update operation are required then it is not recommended to use CopyOnWriteArraySet.
While one thread iterating the Set, other threads can perform updation, here we wont get any runtime exception like ConcurrentModificationException.
Iterator of CopyOnWriteArraySet class can perform only read only and wont perform deletion, otherwise we will get Run-time exception UnsupportedOperationException.

Constructors of CopyOnWriteArraySet:

CopyOnWriteArraySet c = new CopyOnWriteArraySet(): Creates an empty set.
CopyOnWriteArraySet c = new CopyOnWriteArraySet(Collection c): Creates a set containing all of the elements of the specified collection.

Methods in CopyOnWriteArraySet:

add(E e): This method Adds the specified element to this set if it is not already present.
addAll(Collection c): This method Adds all of the elements in the specified collection to this set if they’re not already present.
clear(): This method Removes all of the elements from this set.
contains(Object o): This method Returns true if this set contains the specified element.
containsAll(Collection c): This method Returns true if this set contains all of the elements of the specified collection.
equals(Object o): This method Compares the specified object with this set for equality.
forEach(Consumer action): This method Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
isEmpty(): This method Returns true if this set contains no elements.
iterator(): This method Returns an iterator over the elements contained in this set in the order in which these elements were added.
remove(Object o): This method Removes the specified element from this set if it is present.
removeAll(Collection c): This method Removes from this set all of its elements that are contained in the specified collection.
removeIf(Predicate filter): This method Removes all of the elements of this collection that satisfy the given predicate.
retainAll(Collection c): This method Retains only the elements in this set that are contained in the specified collection.
size(): This method Returns the number of elements in this set.
spliterator(): This method Returns a Spliterator over the elements in this set in the order in which these elements were added.
toArray(): This method Returns an array containing all of the elements in this set.
toArray(T[] a): This method Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.

Example:

// Java program to illustrate  
// CopyOnWriteArraySet class 
import java.util.concurrent.*; 
import java.util.*; 
  
class ConcurrentDemo extends Thread { 
  
    static CopyOnWriteArraySet l = new CopyOnWriteArraySet(); 
  
    public void run() 
    { 
        // Child thread trying to add 
        // new element in the Set object 
        l.add("D"); 
    } 
      
    public static void main(String[] args)  
    { 
        l.add("A"); 
        l.add("B"); 
        l.add("C"); 
  
        // We create a child thread  
        // that is going to modify  
        // CopyOnWriteArraySet l. 
        ConcurrentDemo t = new ConcurrentDemo(); 
        t.start(); 
  
        // Wait for the thread to  
        // add the element. 
        try { 
            Thread.sleep(2000); 
        } 
        catch (InterruptedException e) { 
            System.out.println("child going to add element"); 
        } 
          
        System.out.println(l); 
          
        // Now we iterate through the  
        // CopyOnWriteArraySet and we 
        // wont get exception. 
        Iterator itr = l.iterator(); 
        while (itr.hasNext())  
        { 
            String s = (String)itr.next(); 
            System.out.println(s); 
          
            if(s.equals("C")) 
            { 
                // Here we will get 
                // RuntimeException 
                itr.remove(); 
            } 
        } 
    } 
} 

Output:

[A, B, C, D]
A
B
C
Exception in thread "main" java.lang.UnsupportedOperationException

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