Why do I need to synchronize a list returned by Collections.synchronizedList

Question

i found this at dos.oracle.com

public static List synchronizedList(List list)

Returns a synchronized (thread-safe) list backed by the spe

Answer 1

I was a bit confused by this topic at first because most example I found were without a context. I finally found this blog post that cleared things up for me: http://netjs.blogspot.de/2015/09/how-and-why-to-synchronize-arraylist-in-java.html

From the example above it looks like I just have to convert my list with Collections.synchronizeList() and then I can add and remove items without worrying about thread safety. But here it is important to note that you have to synchronize the list before it is passed to the different threads because otherwise the lists access is not mutually exclusive.

So a complete example would be:

public class SynchroProblem implements Runnable{
  private List myList;

  //Constructor
  public SynchroProblem(List myList){
    this.myList = myList;
  }

  @Override
  public void run() {
    // Do stuff with the list .add(), .remove(), ...
    myList.add(5);

    // Even if mylist is synchronized the iterator is not, 
    // so for using the iterator we need the synchronized block
    synchronized (myList){
      // do stuff with iterator e.g.
      Iterator iterator = myList.iterator();
      while (iterator.hasNext()){
        int number = iterator.next();
        if (number == 123){
          iterator.remove();
        }
      }

    }
  }

  public static void main(String[] args) {

    List originalList = new ArrayList();

    // Synchronize list
    List syncList = Collections.synchronizedList(originalList);

    // Create threads and pass the synchronized list
    Thread t1 = new Thread(new SynchroProblem(syncList));
    Thread t2 = new Thread(new SynchroProblem(syncList));

    t1.start();
    t2.start();

  }
}