In this article, we will discuss how to get synchronized version of List using Collections class’s utility synchronizedList() method

Q) How to make Synchronized List ?

• From original collection framework introduced in Java 1.2 version, by default ArrayList/LinkedList classes aren’t thread-safe i.e.; non-synchronized
• But we can make it thread using Collections utility synchronizedList(list) method
• While iterating synchronized List, make sure to iterate inside synchronized block;
• otherwise we may face non-deterministic behavior

From Java doc,

* It is imperative that the user manually synchronize on the returned
* list when iterating over it:

   List list = Collections.synchronizedList(new ArrayList());
           ...
       synchronized (list) {
           Iterator i = list.iterator(); // Must be in synchronized block
           while (i.hasNext())
               foo(i.next());
       }

* Failure to follow this advice may result in non-deterministic behavior.

1. To get Synchronized version of ArrayList

Method signature:

public static List synchronizedList(List<Object> list);

SynchronizedVersionOfList.java

package in.bench.resources.java.collection;

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

public class SynchronizedVersionOfList {

	public static void main(String[] args) {

		// creating ArrayList object of type String
		ArrayList<String> unSynchronizedList = new ArrayList<String>();

		// adding elements to ArrayList object
		unSynchronizedList.add("Facebook");
		unSynchronizedList.add("LinkedIn");
		unSynchronizedList.add("YouTube");
		unSynchronizedList.add("Google");

		// to get synchronized ArrayList
		List<String> synchronizedList = Collections
				.synchronizedList(unSynchronizedList);

		System.out.println("Iterating through synchronized ArrayList\n");

		// synchronized block
		synchronized(synchronizedList) {

			Iterator<String> iterator = synchronizedList.iterator();
			while (iterator.hasNext())
				System.out.println(iterator.next());
		}
	}
}

Output:

Iterating through synchronized ArrayList

Facebook
LinkedIn
YouTube
Google

Note: similarly, we can make thread-safe for LinkedList/Vector class

Related Articles:

• Java – How to get synchronized version of List ?
• Java – How to get synchronized version of Set ?
• Java – How to get synchronized version of Map ?
• Java – How to get synchronized version Collection ?

References:

• https://docs.oracle.com/javase/tutorial/collections/intro/
• https://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Collection.html
• https://docs.oracle.com/javase/tutorial/collections/interfaces/list.html
• https://docs.oracle.com/javase/7/docs/api/java/util/List.html
• https://docs.oracle.com/javase/tutorial/collections/interfaces/order.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html
• https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html

Happy Coding !!
Happy Learning !!