CopyOnWriteArrayList

Constructors

Methods

Basic operations

• Add/Update elements
   

import java.util.*;
import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) {
        CopyOnWriteArrayList<Integer> numbers= new CopyOnWriteArrayList<>();

        // Using the add() method
        numbers.add(2);
        numbers.add(4);
        
        numbers.addIfAbsent(6);

        System.out.println("list: " + numbers);

        CopyOnWriteArrayList alphanum = new CopyOnWriteArrayList();
        alphanum.add("a");
        alphanum.add("b");

        alphanum.addAll(numbers);
        System.out.println("New list: " + alphanum);
        alphanum.set(2,"c");
        System.out.println("Final list after updation:" + alphanum);
    }
}

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Output:

list: [2, 4, 6]
New list: [a, b, 2, 4, 6]
Final list after updation:[a, b, c, 4, 6]

Try it on Java Online Compiler.
 

• Access elements
   

import java.util.*;
import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) {
        CopyOnWriteArrayList<Integer> numbers= new CopyOnWriteArrayList<>();

        // Using the add() method
        numbers.add(2);
        numbers.add(4);
        numbers.add(6);
        numbers.addIfAbsent(8);

        System.out.println("list: " + numbers);

        System.out.println("Element at index 2: " + numbers.get(2));
        
        System.out.println("list after updation:");
        numbers.forEach((e) -> {
            e = e * 10;
          System.out.print(e + " ");
        });
      System.out.println("\nSublist 1-3:  " + numbers.subList(0,3));
    }
}

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Output:

list: [2, 4, 6, 8]
Element at index 2: 6
list after updation:
20 40 60 80 
Sublist 1-3:  [2, 4, 6]

• Remove elements
   

import java.util.*;
import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) {
        CopyOnWriteArrayList<Integer> numbers= new CopyOnWriteArrayList<>();

        // Using the add() method
        numbers.add(2);
        numbers.add(4);
        numbers.add(6);
        numbers.addIfAbsent(8);

        System.out.println("list: " + numbers);
        
        System.out.println("Removing Element at index 3..  ");
        numbers.remove(3);
        
        System.out.println("Removing Elements less than 5..  ");
        numbers.removeIf(e -> e<5);
        
        System.out.println("Final list :  " + numbers);
        
    }
}

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Output:

list: [2, 4, 6, 8]
Removing Element at index 3..  
Removing Elements less than 5..  
Final list :  [6]

Check out this problem - Duplicate Subtree In Binary Tree

FAQs

1. What is the difference between Synchronized ArrayList and CopyOnWriteArrayList?
   
   Synchronised Arraylist iterators are fail-fast and must be contained within the synchronised block. CopyOnWriteArrayList iterators are fail-safe and can be used outside the synchronised block. The entire ArrayList is blocked for both read and write operations in a Synchronised ArrayList. The ArrayList is blocked only for write operations in a CopyOnWriteArrayList.
    
2. What is the advantage of using CopyOnWriteArrayList?
   
   Making a fresh copy of an array consumes both time and memory overhead. To avoid this, developers often resort to using a synchronised ArrayList instead. However, every time you iterate across the contents of the collection, there is a need to synchronise all operations, including read and write, to ensure consistency. The CopyOnWriteArrayList easily solves this problem.

Key Takeaways

This article extensively discusses CopyOnWriteArrayList. We hope that this blog has helped you enhance your knowledge about the different methods and ways of implementing the CopyOnWriteArrayList class in Java. If you would like to learn more, check out our articles on Data Structures in Java, Multithreading in Java and lists vs Arrays.

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