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 THREE statements are true about HashSet in Java 17? (Choose three.)
Does not maintain insertion order
Order is not guaranteed.
Allows duplicate elements
Why wrong: No duplicates allowed.
Is thread-safe
Why wrong: Not synchronized.
Has constant-time performance for basic operations
Average O(1) for add, remove, contains.
Allows at most one null element
HashSet permits null.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Does not maintain insertion order
Option A is correct because HashSet is backed by a HashMap, which uses hash codes to store elements in buckets. The iteration order depends on the hash values and bucket distribution, which can change when the set is resized, so insertion order is not preserved.
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.
Does not maintain insertion order
Why this is correct
Order is not guaranteed.
Related concept
Read the scenario before looking for a memorised answer.
Allows duplicate elements
Why it's wrong here
No duplicates allowed.
Is thread-safe
Why it's wrong here
Not synchronized.
Has constant-time performance for basic operations
Why this is correct
Average O(1) for add, remove, contains.
Related concept
Read the scenario before looking for a memorised answer.
Allows at most one null element
Why this is correct
HashSet permits null.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates often confuse HashSet's lack of ordering with the ability to hold duplicates, or assume that 'constant-time performance' means guaranteed O(1) regardless of hash collisions, but the exam expects you to know that it is amortized constant-time under typical conditions.
Detailed technical explanation
Under the hood, HashSet uses a HashMap instance where each element is stored as a key with a constant dummy value. The constant-time performance for basic operations (add, remove, contains) relies on a good hash function and proper load factor; if many elements hash to the same bucket, performance degrades to O(n) in the worst case. In Java 17, the internal HashMap uses tree bins (converting linked lists to red-black trees) when bucket collisions exceed a threshold (TREEIFY_THRESHOLD = 8) to maintain O(log n) performance under high collision scenarios.
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: Does not maintain insertion order — Option A is correct because HashSet is backed by a HashMap, which uses hash codes to store elements in buckets. The iteration order depends on the hash values and bucket distribution, which can change when the set is resized, so insertion order is not preserved.
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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