How to synchronize ArrayList in Java with Example

Synchronization we have discussed in detail on java multithreading interview questions and answers.
Since ArrayList is non-synchronized and should not be used in multithreaded environment without explicit synchronization.In this tutorial we will learn about how to synchronize ArrayList in java.

Method 1 : Using Collections.synchronizedList()

One liner is using Collections.synchronizedList() we can make ArrayList synchronized.

import java.util.*;

 public class SynchronizedArrayList {
    public static void main(String args[]) {
       
       // Converting ArrayList to Synchronized ArrayList
       List<String> synchronizedarraylist = 
       Collections.synchronizedList(new ArrayList<String>());

       //Adding elements to synchronized ArrayList
       synchronizedarraylist.add("Basketball");
       synchronizedarraylist.add("Baseball");
       synchronizedarraylist.add("Football");

       System.out.println("Synchronized ArrayList Iteration:");
       synchronized(synchronizedarraylist) {
       Iterator<String> iterator = synchronizedarraylist.iterator(); 
       while (iterator.hasNext())
          System.out.println(iterator.next());
       }
  }
} 

Output

Synchronized ArrayList Iteration:
Basketball
Baseball
Football

Method 2 :Using CopyOnWriteArrayList

import java.util.*;
import java.util.concurrent.*;
 public class SynchronizedArrayList {
    public static void main(String args[]) {
    
    // Creating Synchronized ArrayList Object    
    CopyOnWriteArrayList<String> al = new CopyOnWriteArrayList<String>();

    //Adding elements to synchronized ArrayList
    al.add("Basketball");
    al.add("Baseball");
    al.add("Football");

    System.out.println("Showing synchronized ArrayList Elements:");
    //Synchronized block is not required in this method
    Iterator<String> iterator = al.iterator(); 
    while (iterator.hasNext())
       System.out.println(iterator.next());
  }
}

Output

Showing synchronized ArrayList Elements:
Basketball
Baseball
Football