List Fail-fast Behavior Quiz

What’s the output of the below Java code?

The correct answer is:

ConcurrentModificationException

Let's break it down:

What’s happening?

• `list.stream()` creates a stream view of the original list. Inside `.map()`, we're modifying the same list `(list.add("x"))` while iterating over it.

• This breaks the fail-fast behavior of the stream — the ArrayList internally tracks structural modifications and throws a `ConcurrentModificationException` if a change is detected during iteration.

If you are interested, here is the logic in the Java SDK

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