Working with Arrays and Collections →hardMultiple SelectObjective-mapped

1Z0-829 Working with Arrays and Collections Practice Question

Which TWO are true about HashSet and TreeSet?

⚠ Common exam trap

Test-takers frequently confuse the performance guarantees of HashSet (constant-time average) with TreeSet (logarithmic time), or mistakenly think both maintain insertion order, when in fact only LinkedHashSet does.

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

✓

HashSet uses equals() and hashCode() for uniqueness, TreeSet uses compareTo() or Comparator.

HashSet relies on the equals() and hashCode() methods to determine element uniqueness, while TreeSet uses the natural ordering (compareTo()) or a provided Comparator to maintain sorted order and check for duplicates. This distinction is fundamental to how these Set implementations enforce uniqueness.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
HashSet maintains elements in insertion order.
Why it's wrong here
LinkedHashSet does.
✓
HashSet uses equals() and hashCode() for uniqueness, TreeSet uses compareTo() or Comparator.
Why this is correct
Correct.
✗
Both implement the List interface.
Why it's wrong here
They implement Set.
✗
Both guarantee constant-time performance for basic operations (add, remove, contains).
Why it's wrong here
TreeSet is O(log n).
✓
TreeSet requires elements to be mutually comparable, unless a Comparator is provided.
Why this is correct
Otherwise ClassCastException.

Go deeper

1Z0-829 Working with Arrays and Collections Practice Question

Why each option matters

Option-by-option breakdown

Related to this question