How to synchronize ArrayList in java with example and What is Collections.synchronizedlist ?

How to synchronize ArrayList in java with example and What is Collections.synchronizedlist ?

Description:
The ArrayList is not synchornized (not thread-safe). Collections.synchronizedList() method helps to get thread safe list. Collections.synchronizedList() method Returns a synchronized (thread-safe) list backed by the specified list. In order to guarantee serial access, it is critical that all access to the backing list is accomplished through the returned list.

here are two ways to synchronize explicitly:

1. Using Collections.synchronizedList() method
2. Using thread-safe variant of ArrayList: CopyOnWriteArrayList

Example 1: Collections.synchronizedList() method for Synchronizing ArrayList

In this example we are using Collections.synchronizedList() method. The important point to note here is that iterator should be in synchronized block in this type of synchronization as shown in the below example.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Collections;

public class Details {

   public static void main(String a[]){
       List<String> syncal = 
         Collections.synchronizedList(new ArrayList<String>());

       //Adding elements to synchronized ArrayList
       syncal.add("Pen");
       syncal.add("NoteBook");
       syncal.add("Ink");

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

Output:

Iterating synchronized ArrayList:
Pen
NoteBook
Ink

Method 2: Using CopyOnWriteArrayList

CopyOnWriteArrayList is a thread-safe variant of ArrayList.

package beginnersbook.com;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.Iterator;

public class Details {

 public static void main(String a[]){
    CopyOnWriteArrayList<String> al = new CopyOnWriteArrayList<String>();

    //Adding elements to synchronized ArrayList
    al.add("Pen");
    al.add("NoteBook");
    al.add("Ink");

    System.out.println("Displaying 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:

Displaying synchronized ArrayList Elements:
Pen
NoteBook
Ink

Live Demo