Quick Answer
The answer is [1,2,3]. This is correct because while a HashSet does not maintain any inherent order, converting it to an ArrayList preserves the elements, and then Collections.sort() applies a stable, natural ascending order sort to the list, resulting in the sorted sequence 1, 2, 3. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of the distinction between collection types and the behavior of utility methods like Collections.sort() when you sort a HashSet after converting to ArrayList. A common trap is assuming the HashSet’s iteration order will persist, but the sort operation overrides any arbitrary order, so the output is always the sorted list. Memory tip: “HashSet is unordered, but sort brings order—just convert and sort.”
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.
Given: Set<Integer> set = new HashSet<>(List.of(1,2,3)); List<Integer> list = new ArrayList<>(set); Collections.sort(list); System.out.println(list); What is the output?
[3,2,1]
Why wrong: Incorrect: sorting produces ascending order.
Compilation fails
Why wrong: The code compiles successfully.
[1,2,3]
Correct: after sorting, the list is in ascending order.
[1,2]
Why wrong: Incorrect: all three elements are present.
[1,3,2]
Why wrong: Incorrect: sorted list is [1,2,3].
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
[1,2,3]
The code creates a HashSet from a List of 1, 2, 3, then copies it into an ArrayList. HashSet does not guarantee order, but Collections.sort() sorts the list in natural ascending order (1, 2, 3). The output is [1,2,3], making option C correct.
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.
[3,2,1]
Why it's wrong here
Incorrect: sorting produces ascending order.
Compilation fails
Why it's wrong here
The code compiles successfully.
[1,2,3]
Why this is correct
Correct: after sorting, the list is in ascending order.
Related concept
Read the scenario before looking for a memorised answer.
[1,2]
Why it's wrong here
Incorrect: all three elements are present.
[1,3,2]
Why it's wrong here
Incorrect: sorted list is [1,2,3].
Common exam traps
The trap here is that candidates assume HashSet preserves insertion order (like LinkedHashSet) and thus expect unsorted output, forgetting that Collections.sort() explicitly sorts the list into ascending order.
Detailed technical explanation
Under the hood, Collections.sort() uses a modified mergesort (TimSort) on the list's internal array, which is O(n log n) and stable. The HashSet constructor with a Collection copies elements in iteration order, which is unpredictable for HashSet (based on hash buckets), but sorting eliminates that unpredictability. In real-world scenarios, relying on HashSet order for sorted output is a common pitfall; always use TreeSet or explicit sorting when order matters.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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: [1,2,3] — The code creates a HashSet from a List of 1, 2, 3, then copies it into an ArrayList. HashSet does not guarantee order, but Collections.sort() sorts the list in natural ascending order (1, 2, 3). The output is [1,2,3], making option C correct.
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
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