Question 180 of 513
1Z0-829 Working with Arrays and Collections Practice Question
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?
⚠ Common exam trap
Watch out — 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.
Answer choices
Why each option matters
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.
Answer analysis
Option-by-option breakdown
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.
- ✗
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.
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Last reviewed: Jun 25, 2026
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