Quick Answer
The answer is that a ConcurrentModificationException is thrown. This occurs because the for-each loop in Java internally uses an Iterator to traverse the ArrayList, and calling list.remove() directly during iteration structurally modifies the list without updating the iterator’s internal state. The iterator’s fail-fast mechanism detects this mismatch via a modCount check and immediately throws the exception. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of fail-fast iterators and the distinction between using the collection’s own remove() versus the iterator’s remove() method—a classic trap where candidates mistakenly think the loop will skip elements or complete silently. Remember: when iterating an ArrayList with a for-each loop, never modify the list directly; always use the iterator’s own remove() if removal is needed. A useful memory tip: “For-each is read-only unless you use the iterator’s own remove.”
This 1Z0-829 practice question tests your understanding of working with arrays and collections. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Examine the code: List<String> list = new ArrayList<>(); list.add("A"); list.add("B"); for (String s : list) { list.remove(s); } What is the outcome?
A ConcurrentModificationException is thrown
Modifying list during iteration with for-each throws exception.
The loop completes successfully and list becomes empty
Why wrong: Exception prevents completion.
An IllegalStateException is thrown
Why wrong: ConcurrentModificationException is thrown.
The loop completes successfully and list contains [B]
Why wrong: Exception thrown.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
A ConcurrentModificationException is thrown
The for-each loop internally uses an Iterator to traverse the list. When the list is structurally modified (via `list.remove()`) during iteration without using the iterator's own `remove()` method, the iterator detects a concurrent modification and throws a `ConcurrentModificationException`. This is a fail-fast behavior of the `ArrayList` iterator.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
A ConcurrentModificationException is thrown
Why this is correct
Modifying list during iteration with for-each throws exception.
Related concept
Read the scenario before looking for a memorised answer.
The loop completes successfully and list becomes empty
Why it's wrong here
Exception prevents completion.
An IllegalStateException is thrown
Why it's wrong here
ConcurrentModificationException is thrown.
The loop completes successfully and list contains [B]
Why it's wrong here
Exception thrown.
Common exam traps
The trap here is that candidates often assume the for-each loop will safely remove elements one by one, not realizing that the implicit iterator throws an exception on structural modification.
Detailed technical explanation
Under the hood, `ArrayList` maintains a `modCount` field that tracks structural modifications. The iterator checks this count against its own `expectedModCount` on each `next()` call; if they differ, it throws `ConcurrentModificationException`. In this code, the first iteration removes "A", incrementing `modCount`, and the iterator's `next()` call on the second iteration detects the mismatch. This fail-fast mechanism is designed to catch bugs early in single-threaded code, but in multi-threaded scenarios it does not guarantee thread safety.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 1Z0-829 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Working with Arrays and Collections — This question tests Working with Arrays and Collections — Read the scenario before looking for a memorised answer..
The correct answer is: A ConcurrentModificationException is thrown — The for-each loop internally uses an Iterator to traverse the list. When the list is structurally modified (via `list.remove()`) during iteration without using the iterator's own `remove()` method, the iterator detects a concurrent modification and throws a `ConcurrentModificationException`. This is a fail-fast behavior of the `ArrayList` iterator.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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