😵 Why ConcurrentModificationException is so often caught by Juns

When you first start writing in Java, one of the first mistakes that beginners face is a terrible beast with a long name: ConcurrentModificationException. This error looks scary, but the reason for it is trivial. Let's figure out why it occurs and how to avoid it.

What's going on?

List<String> names = new ArrayList<>();
names.add("Anya");
names.add("Boris");
names.add("Vika");

for (String name : names) {
    if (name.equals("Vika")) {
        names.remove(name); // 💥 ConcurrentModificationException!
    }
}

But instead of a result, you get an exception.

🤛 Why is this happening?

Because for-each uses iterator inside, and you are changing the list itself at this time. It's like you're walking across a bridge, and someone's dismantling the boards behind you. JVM is like: "STOP! It's not safe!" 🚨

🔍 How to understand what exactly causes the error

• list.remove() inside for-each

• map.put() inside for by map.entrySet()

• Modification through a collection inside Stream

✅ How to do it right

🛠 Method 1: Use a regular for

for (int i = 0; i < names.size(); i++) {
    if (names.get(i).equals("Vika")) {
        names.remove(i);
        i--;
    }
}

🛠 Method 2: Use Iterator

Iterator<String> it = names.iterator();
while (it.hasNext()) {
    if (it.next().equals("Vika")) {
        it.remove();
    }
}

🛠 Method 3: Use removeIf

names.removeIf(name -> name.equals("Vika"));

🤯 Why Juns often make mistakes

• ❌ They don't know that for-each uses Iterator inside

• ❌ Confused remove() in the collection and in the iterator

• ❌ They are afraid to use Iterator because it seems "old"

• ❌ They don't read the trace stack carefully

💬 How it sounds in the interview

— Why do you have ConcurrentModificationException in your code?
— I just deleted the element inside for-each...

ConcurrentModificationException is not a bug, it is protection against potentially dangerous behavior. And if you caught it — rejoice! You are now one step closer to understanding Java at a deep level 🧩

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