Quick Answer
The answer is that a ConcurrentModificationException is thrown at runtime. This occurs because the enhanced for-each loop internally uses an Iterator to traverse the list, and when you call `list.remove(s)` directly on the ArrayList, you are structurally modifying the list outside of the iterator’s own `remove` method. The iterator checks a `modCount` field against its own expected count on each iteration; any discrepancy triggers 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 contract between a collection and its iterator. A common trap is assuming that removing an element during iteration is safe simply because the loop continues—it is not, unless you use the iterator’s `remove()` method or an explicit `Iterator` loop. Memory tip: “For-each is a read-only tour guide; if you change the map, the tour crashes.”
This 1Z0-829 practice question tests your understanding of working with arrays and collections. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Which of the following correctly describes the behavior of the following code? List<String> list = new ArrayList<>(); list.add("A"); list.add("B");
for (String s : list) {
if (s.equals("A")) {list.remove(s);
} }
System.out.println(list);
ConcurrentModificationException is thrown
Fail-fast behavior.
Compilation error
Why wrong: Code compiles correctly.
[B]
Why wrong: Exception thrown before completing iteration.
[A, B] unchanged
Why wrong: Modification causes exception.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
ConcurrentModificationException is thrown
The code uses an enhanced for-each loop to iterate over an ArrayList while calling `list.remove(s)` when the element equals "A". This modifies the list structurally during iteration, which causes the iterator (used internally by the for-each loop) to detect the concurrent modification and throw a `ConcurrentModificationException` at runtime. The exception is thrown because the iterator's `modCount` check fails when the list is modified outside of the iterator's own `remove` method.
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.
ConcurrentModificationException is thrown
Why this is correct
Fail-fast behavior.
Related concept
Read the scenario before looking for a memorised answer.
Compilation error
Why it's wrong here
Code compiles correctly.
[B]
Why it's wrong here
Exception thrown before completing iteration.
[A, B] unchanged
Why it's wrong here
Modification causes exception.
Common exam traps
The trap here is that candidates often think the for-each loop will safely remove the element and continue, or that the exception only occurs with multiple threads, but in fact any structural modification (add, remove, clear) to the underlying collection during single-threaded iteration triggers the fail-fast mechanism.
Detailed technical explanation
Under the hood, the enhanced for-each loop uses an `Iterator` obtained from `list.iterator()`. The `ArrayList` iterator maintains a `modCount` field that tracks structural modifications; when `list.remove(s)` is called directly on the list (not via the iterator), the `modCount` is incremented, but the iterator's expected `modCount` is not updated. On the next iteration, the iterator's `checkForComodification()` method detects the mismatch and throws `ConcurrentModificationException`. This is a fail-fast behavior designed to prevent unpredictable results during iteration.
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: ConcurrentModificationException is thrown — The code uses an enhanced for-each loop to iterate over an ArrayList while calling `list.remove(s)` when the element equals "A". This modifies the list structurally during iteration, which causes the iterator (used internally by the for-each loop) to detect the concurrent modification and throw a `ConcurrentModificationException` at runtime. The exception is thrown because the iterator's `modCount` check fails when the list is modified outside of the iterator's own `remove` method.
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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Variation 1. What is the result of executing the code in the exhibit?
hardWhy C: The code uses an ArrayList and iterates over it with an enhanced for-each loop while simultaneously removing an element via the list's remove() method. This structural modification directly from the list (not via the iterator's remove()) triggers a ConcurrentModificationException because the iterator's modCount check fails. The exception is thrown at the next iteration attempt after the removal.
Last reviewed: Jun 25, 2026
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