Demo of CopyOnWriteArrayList behaviour: while iterating through a previously nonempty list, clearing the list does not influence the iteration: all elements that existed when the iterator was created exist, despite the list being cleared while iterating.

CopyOnWriteArrayListBehaviour.java

package com.example;

import com.google.common.collect.Lists;

import java.util.List;
import java.util.concurrent.atomic.AtomicBoolean;

public class CopyOnWriteArrayListBehaviour {

  // Change this to a regular ArrayList to see what different behaviour the CopyOnWriteArrayList has
  private static final List<Integer> list = Lists.newCopyOnWriteArrayList();
  // private static final List<Integer> list = new ArrayList<>();

  public static void main(String[] args) {
    list.add(1);
    list.add(2);
    list.add(3);

    // Use AtomicBoolean for effectively final object access from multiple threads
    final AtomicBoolean isInIteratorLoop = new AtomicBoolean(false);

    final int theWait = 250; // milliseconds

    // Start a new thread for concurrent behaviour
    new Thread(
            () -> {
              for (Integer integer : list) {
                System.out.println("In iterator loop, list = " + list);
                isInIteratorLoop.set(true);

                try {
                  while (!list.isEmpty()) {
                    System.out.println("In iterator loop, waiting for list cleared");
                    Thread.sleep(theWait);
                  }
                  System.out.println(integer);

                } catch (InterruptedException e) {
                  throw new RuntimeException(e);
                }
              }
              System.out.println("In iterator loop, finished loop");
              isInIteratorLoop.set(false);
            })
        .start();

    try {
      while (!isInIteratorLoop.get()) {
        System.out.println("In main thread, waiting for iterator loop");
        Thread.sleep(theWait);
      }

      list.clear();
      System.out.println("In main thread, list cleared");

      while (isInIteratorLoop.get()) {
        System.out.println("In main thread, waiting for iterator loop to finish");
        Thread.sleep(theWait);
      }

      System.out.println("In main thread, list = " + list);

    } catch (InterruptedException e) {
      throw new RuntimeException(e);
    }
  }
}