Runtime Error

What This Error Means

This error occurs when a program attempts to modify a collection (like an ArrayList or HashSet) while it is being iterated over by a foreach loop, iterator, or lambda expression.

Why It Happens

This error often happens when multiple threads are accessing the same collection concurrently, or when a program is using a collection in a multithreaded environment without proper synchronization.

How to Fix It

1. 1To fix this error, you can use the following solutions:
2. 21. Use an Iterator to remove elements while iterating over a collection.
3. 32. Create a temporary collection to store the elements to be removed.
4. 43. Use a thread-safe collection, such as CopyOnWriteArrayList or ConcurrentHashMap.
5. 54. Synchronize access to the collection using a lock or a semaphore.

Example Code Solution

import java.util.ArrayList;
import java.util.Iterator;

public class ConcurrencyErrorExample {
  public static void main(String[] args) {
    ArrayList<String> list = new ArrayList<>();
    list.add("A");
    list.add("B");
    list.add("C");

    for (String s : list) {
      if (s.equals("B")) {
        list.remove(s);
      }
    }
  }
}

import java.util.ArrayList;
import java.util.Iterator;

public class ConcurrencyErrorExample {
  public static void main(String[] args) {
    ArrayList<String> list = new ArrayList<>();
    list.add("A");
    list.add("B");
    list.add("C");

    Iterator<String> iterator = list.iterator();
    while (iterator.hasNext()) {
      String s = iterator.next();
      if (s.equals("B")) {
        iterator.remove();
      }
    }
  }
}