Quick Answer
The answer is to use HashSet and sort when iteration is needed. This is correct because HashSet provides O(1) average-time performance for contains() checks, while TreeSet requires O(log n) time for the same operation—a critical difference when handling up to 100,000 IDs with frequent lookups. Since sorted iteration is required only once per hour, the O(n log n) cost of sorting the HashSet into a list is negligible compared to the daily savings on contains() calls. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your ability to balance data structure trade-offs: HashSet optimizes for fast membership checks, while TreeSet guarantees sorted order at the cost of slower lookups. A common trap is assuming TreeSet is always better when sorted output is needed, but the exam rewards recognizing when sorting on demand is more efficient. Memory tip: "Hash for check, sort on request"—prioritize the dominant operation.
This 1Z0-829 practice question tests your understanding of working with arrays and collections. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
An application needs to maintain a set of unique customer IDs (type String) and frequently check if an ID is already present. The set is expected to contain up to 100,000 IDs. The current implementation uses a TreeSet, but performance tests show that the contains() operation is slower than desired. The developer considers switching to a HashSet. However, the business requires that when iterating the set, IDs must appear in sorted order. The developer proposes to convert the HashSet to a sorted list each time iteration is needed. Iteration occurs rarely (once per hour). What is the best approach?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Use ConcurrentSkipListSet
Why wrong: Overkill for single-threaded; similar performance to TreeSet.
Use LinkedHashSet
Why wrong: Preserves insertion order, not sorted order.
Keep the TreeSet because it maintains sorted order
Why wrong: contains() is O(log n); slower than HashSet.
Use HashSet and sort when iteration is needed
Fast contains() and sorting once per hour is fine.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Use HashSet and sort when iteration is needed
Option D is correct because the primary performance bottleneck is the contains() operation, which is O(log n) for TreeSet but O(1) average for HashSet. Since iteration with sorted order is required only once per hour, the cost of sorting the HashSet into a list (O(n log n)) is negligible compared to the frequent contains() checks. This trade-off optimizes for the dominant use case while still meeting the sorted iteration requirement.
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.
Use ConcurrentSkipListSet
Why it's wrong here
Overkill for single-threaded; similar performance to TreeSet.
Use LinkedHashSet
Why it's wrong here
Preserves insertion order, not sorted order.
Keep the TreeSet because it maintains sorted order
Why it's wrong here
contains() is O(log n); slower than HashSet.
Use HashSet and sort when iteration is needed
Why this is correct
Fast contains() and sorting once per hour is fine.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates often assume sorted iteration must be maintained at all times, overlooking the fact that the requirement is only for rare iteration, making the conversion cost acceptable in exchange for faster contains().
Trap categories for this question
Similar concept trap
Overkill for single-threaded; similar performance to TreeSet.
Detailed technical explanation
Under the hood, HashSet uses a hash table with bucket arrays and relies on hashCode() and equals() for O(1) average lookup, while TreeSet uses a Red-Black tree with O(log n) operations. Sorting a HashSet into a list uses Collections.sort() which implements TimSort (a hybrid of merge sort and insertion sort) with O(n log n) time complexity. For 100,000 elements, sorting once per hour is far less costly than performing thousands of O(log n) contains() checks per second.
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: Use HashSet and sort when iteration is needed — Option D is correct because the primary performance bottleneck is the contains() operation, which is O(log n) for TreeSet but O(1) average for HashSet. Since iteration with sorted order is required only once per hour, the cost of sorting the HashSet into a list (O(n log n)) is negligible compared to the frequent contains() checks. This trade-off optimizes for the dominant use case while still meeting the sorted iteration requirement.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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