Java.Collections.Write a single-threaded program that causes a collection to throw a ConcurrentModificationException.

💣 Here’s a minimal and crystal-clear single-threaded Java program that throws a ConcurrentModificationException by modifying a collection during iteration — a classic trap!

✅ Java Program That Throws ConcurrentModificationException

import java.util.*;

public class ConcurrentModificationExample {
    public static void main(String[] args) {
        List<String> colors = new ArrayList<>();
        colors.add("red");
        colors.add("green");
        colors.add("blue");

        // This will throw ConcurrentModificationException
        for (String color : colors) {
            if (color.equals("green")) {
                colors.remove(color); // ❌ Modifying list while iterating
            }
        }
    }
}

💥 Output at runtime:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:xxx)
    ...

🔍 Why this happens:

• The for-each loop uses an internal Iterator.
• When you call colors.remove(color), the collection is modified directly, but the iterator isn’t aware of that change.
• On the next loop iteration, it checks if the internal structure has changed → detects it → boom → throws ConcurrentModificationException.

🛡️ Want to avoid it?

Use the Iterator‘s own remove() method:

Iterator<String> iterator = colors.iterator();
while (iterator.hasNext()) {
    String color = iterator.next();
    if (color.equals("green")) {
        iterator.remove(); // ✅ Safe removal
    }
}