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.
Which is true about CopyOnWriteArrayList?
It is not thread-safe.
Why wrong: It is thread-safe.
Iterators reflect concurrent modifications.
Why wrong: Iterator works on snapshot.
Modifications are expensive as they create a new array.
Each modification copies the underlying array.
It is optimized for frequent modifications.
Why wrong: Optimized for read-heavy scenarios.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Modifications are expensive as they create a new array.
CopyOnWriteArrayList is a thread-safe variant of ArrayList where all mutative operations (add, set, remove, etc.) are implemented by making a fresh copy of the underlying array. This design makes modifications expensive because each modification requires copying the entire array, but it provides safe iteration without locking. Option C correctly identifies that modifications are expensive due to the array copy.
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.
It is not thread-safe.
Why it's wrong here
It is thread-safe.
Iterators reflect concurrent modifications.
Why it's wrong here
Iterator works on snapshot.
Modifications are expensive as they create a new array.
Why this is correct
Each modification copies the underlying array.
Related concept
Read the scenario before looking for a memorised answer.
It is optimized for frequent modifications.
Why it's wrong here
Optimized for read-heavy scenarios.
Common exam traps
The trap here is that candidates often assume 'thread-safe' collections like CopyOnWriteArrayList allow iterators to see concurrent modifications (like ConcurrentHashMap does), but CopyOnWriteArrayList iterators are snapshot-based and do not reflect modifications after creation.
Trap categories for this question
Scenario analysis trap
Optimized for read-heavy scenarios.
Detailed technical explanation
Under the hood, CopyOnWriteArrayList maintains a volatile reference to an array. On each mutative operation, it acquires a lock (ReentrantLock), copies the entire array into a new array of length n+1 (or adjusted size), performs the modification on the new array, and then sets the volatile reference to the new array. This ensures that readers never see a partially modified array and can iterate without synchronization. A real-world scenario is a listener list in an event system where listeners are rarely added or removed but events are fired frequently; the copy-on-write approach avoids locking 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: Modifications are expensive as they create a new array. — CopyOnWriteArrayList is a thread-safe variant of ArrayList where all mutative operations (add, set, remove, etc.) are implemented by making a fresh copy of the underlying array. This design makes modifications expensive because each modification requires copying the entire array, but it provides safe iteration without locking. Option C correctly identifies that modifications are expensive due to the array copy.
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
This 1Z0-829 practice question is part of Courseiva's free Oracle certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 1Z0-829 exam.
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