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 TWO are true about the PriorityQueue class?
It allows null elements.
Why wrong: Null not allowed.
It orders elements based on insertion order.
Why wrong: Orders by priority, not insertion.
It is not thread-safe.
Correct, not synchronized.
It can be used with a custom Comparator.
Correct.
It provides O(1) time for contains().
Why wrong: O(n) for contains.
Answer choices
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.
Option C is correct because 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.
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.
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.
Related concept
Read the scenario before looking for a memorised answer.
It can be used with a custom Comparator.
Why this is correct
Correct.
Related concept
Read the scenario before looking for a memorised answer.
It provides O(1) time for contains().
Why it's wrong here
O(n) for contains.
Common exam traps
The trap here is that candidates often 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.
Detailed technical explanation
PriorityQueue is implemented as a binary heap (typically a min-heap) stored in an array, where the head is the smallest element according to the ordering. The contains() method iterates through the array, yielding linear time complexity. In real-world scenarios, using PriorityQueue for thread-safe operations requires wrapping it with synchronized collections or using PriorityBlockingQueue.
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: It is not thread-safe. — Option C is correct because 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.
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 11, 2026
This 1Z0-829 practice question is part of Courseiva's free Oracle certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 1Z0-829 exam.
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