- A
ConcurrentModificationException is thrown
Fail-fast behavior.
- B
Compilation error
Why wrong: Code compiles correctly.
- C
[B]
Why wrong: Exception thrown before completing iteration.
- D
[A, B] unchanged
Why wrong: Modification causes exception.
Question 133 of 519
1Z0-829 Working with Arrays and Collections Practice Question
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);
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
✓
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
Option-by-option breakdown
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
Common exam trap: answer the scenario, not the keyword
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
How to think about this question
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.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 1Z0-829 question test?
Working with Arrays and Collections — This question tests Working with Arrays and Collections — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
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.
What should I do if I get this 1Z0-829 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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