Need for concurrent Collections

Need : (3 Major Reasons)

1. Collections like ArrayList and LinkedList can be accessed by multiple Threads but they create DataInconsistency problems. (Not Thread Safe)
2. Collections like Vector, Hashtable and Stack are Thread safe because their methods are synchronized. Hence multiple threads can’t work on them concurrently hence we can’t improve the performance of a Java job even when we use multithreading. Also (synchronizedList(), synchronizedSet(), synchronizedMap())
   • For these ThreadSafe collections even for read operation, only one thread is allowed to execute at a time.
3. If one thread is trying to iterate a collection at the same time another thread is trying to modify the collection immediately we will get ConucrrentModificationException

Example of ConcurrentModificationException

Simple Example :

public class ConcurrentModificationExceptionDemo {
	
	static List<Integer> tempList= new ArrayList<>();

	public static void main(String[] args) {
		tempList.add(1);
		tempList.add(2);
		tempList.add(3);
		Iterator<Integer> itr = tempList.iterator();
		while(itr.hasNext()) {
			System.out.println(itr.next());
			tempList.add(4);
		}
	}
}

Complex Example with Threads

package com.demo;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.TimeUnit;

public class ConcurrentModificationExceptionDemo extends Thread{
	
	static List<Integer> tempList= new ArrayList<>();
	public void run() {
		try {
			TimeUnit.SECONDS.sleep(2);
			tempList.add(4);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
	
	public static void main(String[] args) {
		tempList.add(1);
		tempList.add(2);
		tempList.add(3);
		ConcurrentModificationExceptionDemo t = new ConcurrentModificationExceptionDemo();
		t.start();
		Iterator<Integer> itr = tempList.iterator();
		while(itr.hasNext()) {
			try {
				TimeUnit.SECONDS.sleep(3);
			} catch (Exception e) {
				e.printStackTrace();
			}
			System.out.println(itr.next());
		}
	}
}