In this article, we will discuss CopyOnWriteArrayList class – the implementation class for List interface in detail

This is the thread-safe version of ArrayList, where modify operation is performed on a separate cloned copy and finally JVM merges both original and cloned copies

Apart from thread-safety difference between ArrayList and CopyOnWriteArrayList, all properties of ArrayList are applicable to CopyOnWriteArrayList

1. CopyOnWriteArrayList:

• CopyOnWriteArrayList is implementation class of List interface (i.e.; CopyOnWriteArrayList implements List)
• For every modify/update operation, a new separate cloned copy is created and modification is performed on cloned copy; while other threads can iterate over original copy
• After modification/updation, JVM takes care of merging both copies (i.e.; original and cloned copy) –> so that we get latest copy with all updation/modification
• Since, every time a new separate cloned copy is created for updation/modification. Therefore, it is suited for multi-threaded environment where there are more number of read/get operation and comparatively less update/modify operation
• While one thread iterating over original copy, other threads can modify with separate cloned copy and compiler won’t throw any ConcurrentModificationException; which isn’t case with ArrayList
• It never throws ConcurrentModificationException while 2 or more threads operating simultaneously i.e.; it is fail-safe iterator
• But, there are certain limitation too with CopyOnWriteArrayList which isn’t case with ArrayList like, while iterating CopyOnWriteArrayList, remove operation isn’t possible and compiler throws UnsupportedOperationException
• Other than above discussed points, all other properties of ArrayList are applicable for CopyOnWriteArrayList too i.e.;
• Insertion-order is maintained
• Duplicate objects are allowed
• Null insertion is possible
• This is introduced in Java 1.5 version
• Present in java.util.concurrent package and implements java.util.List
• Also, implements java.util.RandomAccess, java.lang.Cloneable, java.io.Serializable marker interfaces which provides special ability to CopyOnWriteArrayList (provided by JVM at run time) like,
• java.util.RandomAccess: to access any random element/objects with same speed
• java.lang.Cloneable: to create a duplicate object or to clone an object
• java.io.Serializable: to transfer objects across network

Source: Team BenchResources.Net

2. CopyOnWriteArrayList constructors:

2.1 CopyOnWriteArrayList cowal = new CopyOnWriteArrayList();

• creates an empty CopyOnWriteArrayList object

2.2 CopyOnWriteArrayList cowal = new CopyOnWriteArrayList(Collection c);

• creates an equivalent CopyOnWriteArrayList object for the specified collection
• it is basically used for inter-conversion between collection objects

2.3 CopyOnWriteArrayList cowal = new CopyOnWriteArrayList(Object[] a);

• creates an equivalent CopyOnWriteArrayList object for the specified Object array

3. CopyOnWriteArrayList method:

Note: Apart from above mentioned CopyOnWriteArrayList specific methods, CopyOnWriteArrayList class inherits all methods from List interface

4. CopyOnWriteArrayList Examples:

1. Using addAllAbsent() method
2. Using combination of addAll() and addAllAbsent() methods

4.1 CopyOnWriteArrayList :

CopyOnWriteArrayListDemo.java

package in.bench.resources.concurrent.collection;

import java.util.ArrayList;
import java.util.concurrent.CopyOnWriteArrayList;

public class CopyOnWriteArrayListDemo {

	public static void main(String[] args) {

		// COWAL of All capital
		CopyOnWriteArrayList<String> allCapital =
				new CopyOnWriteArrayList<String>();
		allCapital.addIfAbsent("Berlin");
		allCapital.addIfAbsent("Tokyo");
		allCapital.addIfAbsent("Moscow");
		allCapital.addIfAbsent("Tokyo");// again, adding Tokyo

		// AL of European capital
		ArrayList<String> euroCapital = new ArrayList<String>();
		euroCapital.add("Moscow");
		euroCapital.add("London");
		euroCapital.add("Paris");

		// adding European capital to all capital using addAllAbsent()
		allCapital.addAllAbsent(euroCapital);

		// print to console
		System.out.println(allCapital);
	}
}

Output:

[Berlin, Tokyo, Moscow, London, Paris]

Explanation:

• 1^st list is of type CopyOnWriteArratList which uses addIfAbsent() method to add new elements; only if it isn’t already present in the invoking list (i.e.; COWAL)
• From above output, it is clear though Tokyo is added 2^nd time using addIfAbsent() method and it isn’t get added to list
• 2^nd list of type ArrayList which is general one and adds new elements using add() method
• There is one final operation being performed in the above demo example; which is adding all elements of ArrayList to CopyOnWriteArrayList using addAllAbsent() method
• Which checks in the invoking COWAL list before adding any elements from specified AL list
• From the output, it is clear that Moscow is already present so it isn’t added to the invoking list

4.2 CopyOnWriteArrayList :

CopyOnWriteArrayListExample.java

package in.bench.resources.concurrent.collection;

import java.util.ArrayList;
import java.util.concurrent.CopyOnWriteArrayList;

public class CopyOnWriteArrayListExample {

	public static void main(String[] args) {

		// AL of European capital
		ArrayList<String> euroCapital = new ArrayList<String>();
		euroCapital.add("Moscow");
		euroCapital.add("London");
		euroCapital.add("Paris");

		// AL of Asian capital
		ArrayList<String> asiaCapital = new ArrayList<String>();
		asiaCapital.add("Tokyo");
		asiaCapital.add("Beijing");
		asiaCapital.add("Singapore");

		// COWAL of All capital
		CopyOnWriteArrayList<String> allCapital =
				new CopyOnWriteArrayList<String>();
		allCapital.add("Berlin");
		allCapital.add("Tokyo");
		allCapital.add("Moscow");

		// adding European capital to all capital using addAll()
		allCapital.addAll(euroCapital);

		// print to console
		System.out.println(allCapital);

		// adding Asian capital to all capital using addAllAbsent()
		allCapital.addAllAbsent(asiaCapital);

		// print to console
		System.out.println(allCapital);
	}
}

Output:

[Berlin, Tokyo, Moscow, Moscow, London, Paris]
[Berlin, Tokyo, Moscow, Moscow, London, Paris, Beijing, Singapore]

Explanation:

• We have created 2 ArrayList objects of European capitals and Asian capitals
• And also created all mix of capitals of type CopyOnWriteArrayList object
• 1^st operation: adding ArrayList to CopyOnWriteArrayList using addAll(Collection) method
• This operation is performed without checking already elements present in the specified list with the invoking list
• As a result, there are duplicates elements in the invoking list after add operation
• From the output, it is seen that there are 2 Moscow elements (i.e.; Moscow present in both specified list and invoking list)
• 2^nd operation: adding ArrayList to CopyOnWriteArrayList using addAllAbsent(Collection) method
• This operation is performed after checking invoking list with the specified liste.; if the invoking list already contains any of elements present in the specified list then it is neglected and rest all are added to invoking list
• From the output, it is clear that Tokyo doesn’t get added to the invoking list as it is already present

Related Articles:

• Java 5 – Introduction to Concurrent Collection
• Java 5 – CopyOnWriteArrayList class with example
• Java 5 – CopyOnWriteArrayList with Read and Update operations simultaneously
• Java 5 – Remove operation with CopyOnWriteArrayList and ArrayList
• Java 5 – ArrayList v/s CopyOnWriteArrayList
• Java 5 – CopyOnWriteArrayList v/s SynchronizedList
• Java 5 – Concurrent Collection Interview question and answers

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/essential/concurrency/collections.html
• https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CopyOnWriteArrayList.html

Happy Coding !!
Happy Learning !!