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Working with Arrays and CollectionshardMultiple SelectObjective-mapped

1Z0-829 Working with Arrays and Collections Practice Question

Which THREE statements are true about HashSet in Java 17? (Choose three.)

⚠ Common exam trap

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.

Answer choices

Why each option matters

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

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.

Answer analysis

Option-by-option breakdown

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.

  • 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.

  • Allows at most one null element

    Why this is correct

    HashSet permits null.

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