How to Use a Synchronized List in Java with a For Loop

How can I use a synchronized list in Java while iterating using a for loop?

List<String> syncList = Collections.synchronizedList(new ArrayList<String>());

In Java, a synchronized list is a thread-safe version of a list which prevents concurrent modification exceptions when accessed by multiple threads. This is particularly important when you need to iterate over a list while ensuring that other threads do not modify the list concurrently.

import java.util.*;

public class SynchronizedListExample {
    public static void main(String[] args) {
        List<String> syncList = Collections.synchronizedList(new ArrayList<String>());
        syncList.add("Item 1");
        syncList.add("Item 2");
        syncList.add("Item 3");

        synchronized(syncList) {
            for (String item : syncList) {
                System.out.println(item);
            }
        }
    }
}

Causes

• Multiple threads accessing and modifying the list without proper synchronization.
• Concurrent modifications leading to exceptions during iteration.

Solutions

• Use Collections.synchronizedList() method to create a synchronized list.
• Wrap the forEach operation in a synchronized block when iterating over the list.

Mistake: Not synchronizing the block while iterating the synchronized list.

Solution: Wrap the iteration block within a synchronized block to ensure thread safety.

Mistake: Assuming that just using Collections.synchronizedList() is sufficient for safe iteration without locks.

Solution: Always use a synchronized block or synchronized methods when iterating to avoid ConcurrentModificationException.