Question 42 of 513
1Z0-829 Working with Arrays and Collections Practice Question
Which TWO are true about the PriorityQueue class?
⚠ Common exam trap
A common mix-up: candidates confuse PriorityQueue with a FIFO queue (like LinkedList) and assume insertion order, or mistakenly think it allows nulls or provides O(1) contains() due to its heap structure.
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
✓
It is not thread-safe.
PriorityQueue is not thread-safe; it does not synchronize access, so concurrent modifications without external synchronization can lead to inconsistent behavior. Option D is correct because PriorityQueue can accept a custom Comparator at construction time to define the ordering of elements, overriding the natural order.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It allows null elements.
Why it's wrong here
Null not allowed.
- ✗
It orders elements based on insertion order.
Why it's wrong here
Orders by priority, not insertion.
- ✓
It is not thread-safe.
Why this is correct
Correct, not synchronized.
- ✓
It can be used with a custom Comparator.
Why this is correct
Correct.
- ✗
It provides O(1) time for contains().
Why it's wrong here
O(n) for contains.
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Last reviewed: Jun 11, 2026
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