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: TreeSet<Integer> set = new TreeSet<>(List.of(3,1,2)); set.add(2); System.out.println(set); What is the output?
[1,2]
Why wrong: Incorrect: all three distinct elements are present.
[1,3,2]
Why wrong: Incorrect: order is not this.
[3,1,2]
Why wrong: Incorrect: TreeSet sorts elements.
UnsupportedOperationException
Why wrong: Incorrect: no exception occurs.
[1,2,3]
Correct: sorted order, duplicates ignored.
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]
E is correct because TreeSet maintains elements in their natural sorted order (ascending for integers). The list [3,1,2] is sorted to [1,2,3], and adding 2 again has no effect since TreeSet does not allow duplicates. The output is [1,2,3].
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.
[1,2]
Why it's wrong here
Incorrect: all three distinct elements are present.
[1,3,2]
Why it's wrong here
Incorrect: order is not this.
[3,1,2]
Why it's wrong here
Incorrect: TreeSet sorts elements.
UnsupportedOperationException
Why it's wrong here
Incorrect: no exception occurs.
[1,2,3]
Why this is correct
Correct: sorted order, duplicates ignored.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates often confuse TreeSet with HashSet or List, expecting insertion order or allowing duplicates, but TreeSet enforces sorted order and uniqueness via its underlying tree structure.
Detailed technical explanation
TreeSet is backed by a TreeMap (a Red-Black tree), which ensures O(log n) time for add, remove, and contains operations. The natural ordering is determined by the Comparable interface (Integer implements Comparable). Duplicates are detected via the compareTo() method returning 0, so the second add(2) is ignored. This behavior is critical when using TreeSet for sorted, unique collections in scenarios like maintaining a sorted leaderboard.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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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] — E is correct because TreeSet maintains elements in their natural sorted order (ascending for integers). The list [3,1,2] is sorted to [1,2,3], and adding 2 again has no effect since TreeSet does not allow duplicates. The output is [1,2,3].
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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