Synchronization of ArrayList in Java

Implementation of ArrayList is not synchronized by default. It means if a thread modifies it structurally and multiple threads access it concurrently, it must be synchronized externally. Structural modification implies the addition or deletion of element(s) from the list or explicitly resizes the backing array. Changing the value of an existing element is not a structural modification.

There are two ways to create a Synchronized ArrayList. 

1. Collections.synchronizedList() method. 
2. Using CopyOnWriteArrayList.

Method 1: Using Collections.synchronizedList() method

To do serial access, all access to the backing list must be accomplished through the returned list. It is imperative that the user manually synchronizes on the returned list when iterating over it. 

public static  List<T> synchronizedList(List<T> list)

• Accepts a List which could be the implementation of the List interface. e.g. ArrayList, LinkedList.
• Returns a Synchronized(thread-safe) list backed by the specified list.
• The parameter list is the list to be wrapped in a synchronized list.
• T represents generic

// Java program to demonstrate working of
// Collections.synchronizedList

import java.util.*;

class GFG
{
    public static void main (String[] args)
    {
        List<String> list =
           Collections.synchronizedList(new ArrayList<String>());

        list.add("practice");
        list.add("code");
        list.add("quiz");

        synchronized(list)
        {
            // must be in synchronized block
            Iterator it = list.iterator();

            while (it.hasNext())
                System.out.println(it.next());
        }
    }
}

practice
code
quiz

Method 2: Using CopyOnWriteArrayList

CopyOnWriteArrayList<T> threadSafeList = new CopyOnWriteArrayList<T>();

• Create an empty List.
• It implements List interface.
• It is a thread-safe variant of ArrayList.
• T represents generic

A thread-safe variant of ArrayList in which all mutative operations (e.g., add, set, remove..) are implemented by creating a separate copy of an underlying array. It achieves thread safety by creating a separate copy of the List which is different than vector or other collections used to provide thread-safety. 

• It is useful when you can't or don't want to synchronize the traversal yet need to prevent interference among concurrent threads.
• It is costly as it involves separate Array copy with every write operation(e.g., add, set, remove.)
• It is very efficient when you have List and need to traverse its elements and don't modify it often.

The iterator does not throw ConcurrentModificationException even if copyOnWriteArrayList is modified once the iterator is created. The iterator is iterating over the separate copy of ArrayList while a write operation is happening on another copy of ArrayList.

// Java program to illustrate the thread-safe ArrayList.

import java.io.*;
import java.util.Iterator;
import java.util.concurrent.CopyOnWriteArrayList;

class GFG
{
    public static void main (String[] args)
    {
        // creating a thread-safe Arraylist.
        CopyOnWriteArrayList<String> threadSafeList
            = new CopyOnWriteArrayList<String>();

        // Adding elements to synchronized ArrayList
        threadSafeList.add("geek");
        threadSafeList.add("code");
        threadSafeList.add("practice");

        System.out.println("Elements of synchronized ArrayList :");

        // Iterating on the synchronized ArrayList using iterator.
        Iterator<String> it = threadSafeList.iterator();

        while (it.hasNext())
            System.out.println(it.next());
    }
}

Elements of synchronized ArrayList :
geek
code
practice

What happens if we try to modify CopyOnWriteArrayList through the iterator's method? 

It throws UnsupportedOperationException if you try to modify CopyOnWriteArrayList through iterator's own method(e.g. add(), set(), remove()). 

// Java program to illustrate the thread-safe ArrayList

import java.io.*;
import java.util.Iterator;
import java.util.concurrent.CopyOnWriteArrayList;

class GFG
{
    public static void main (String[] args)
    {
        // creating a thread-safe Arraylist.
        CopyOnWriteArrayList<String> threadSafeList =
            new CopyOnWriteArrayList<String>();

        // Adding elements to synchronized ArrayList
        threadSafeList.add("geek");
        threadSafeList.add("code");
        threadSafeList.add("practice");

        System.out.println("Elements of synchronized ArrayList :");

        // Iterating on the synchronized ArrayList using iterator.
        Iterator<String> it = threadSafeList.iterator();

        while (it.hasNext())
        {
            String str = it.next();
            it.remove();
        }
    }
}

Runtime Error: 

Exception in thread "main" java.lang.UnsupportedOperationException
    at java.util.concurrent.CopyOnWriteArrayList$COWIterator.remove
        (CopyOnWriteArrayList.java:1176)
    at GFG.main(File.java:28)

Other constructors of CopyOnWriteArrayList 

1. CopyOnWriteArrayList(Collection<? extends E> c): Creates a list containing the elements of the specified collection, in the order, they are returned by the collection's iterator. 

2. CopyOnWriteArrayList(E[] toCopyIn): Creates a list holding a copy of the given array.

Why use ArrayList when the vector is synchronized?

1. Performance: Vector is synchronized and thread-safe, and because of this, it is slightly slower than ArrayList.
2. Functionality: Vector synchronizes at the level of each individual operation. Generally, a programmer likes to synchronize a whole sequence of operations. Synchronizing individual operations is both less safe and slower.
3. Vectors obsolete: Vectors are considered obsolete and unofficially deprecated in java. Also, the vector synchronizes on each individual operation which is almost never done. Mostly java programmers prefer using ArrayList since they will probably synchronize the arrayList explicitly anyway if they need to synchronize.

Must Read: Vector vs ArrayList in Java