1Z0-829 Working with Streams and Lambda Expressions Practice Question
This 1Z0-829 practice question tests your understanding of working with streams and lambda expressions. 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.
A team needs to process a large collection of orders to calculate total revenue per region. They decide to use parallel streams to improve performance. Which statement about using parallel streams for this task is true?
The stream() method returns a parallel stream by default.
Why wrong: stream() returns a sequential stream; parallelStream() returns a parallel stream.
B
Using a parallel stream with a stateful lambda operation can lead to incorrect results.
Stateful lambdas (e.g., accumulating into a non-thread-safe collection) cause race conditions in parallel pipelines.
C
Parallel streams always provide better performance than sequential streams.
Why wrong: Parallelism has overhead; it only improves performance if tasks are CPU-bound and splittable.
D
Parallel streams cannot be used with custom thread pools.
Why wrong: Parallel streams use the common ForkJoinPool by default, but custom pools can be used by submitting the stream task to your own ForkJoinPool.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Using a parallel stream with a stateful lambda operation can lead to incorrect results.
Option B is correct because parallel streams split the workload across multiple threads, and if the lambda operation is stateful (e.g., modifying a shared variable like a counter or a non-thread-safe collection), it can cause race conditions and produce incorrect results. The Streams API documentation explicitly warns against using stateful lambdas with parallel streams to avoid data integrity issues.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The stream() method returns a parallel stream by default.
Why it's wrong here
stream() returns a sequential stream; parallelStream() returns a parallel stream.
✓
Using a parallel stream with a stateful lambda operation can lead to incorrect results.
Why this is correct
Stateful lambdas (e.g., accumulating into a non-thread-safe collection) cause race conditions in parallel pipelines.
Related concept
Read the scenario before looking for a memorised answer.
✗
Parallel streams always provide better performance than sequential streams.
Why it's wrong here
Parallelism has overhead; it only improves performance if tasks are CPU-bound and splittable.
✗
Parallel streams cannot be used with custom thread pools.
Why it's wrong here
Parallel streams use the common ForkJoinPool by default, but custom pools can be used by submitting the stream task to your own ForkJoinPool.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume parallel streams are always faster (Option C) or that they cannot use custom thread pools (Option D), but the core exam focus is on the requirement for stateless, non-interfering lambdas to ensure correctness in parallel processing.
Detailed technical explanation
How to think about this question
Under the hood, parallel streams use the common ForkJoinPool by default, which has a parallelism level equal to the number of available processors minus one. Stateful lambdas, such as those that mutate a shared accumulator without proper synchronization, break the statelessness requirement of parallel stream pipelines and can lead to non-deterministic results due to interleaved thread execution. In real-world scenarios, this often manifests as incorrect totals or missing records when processing large datasets like order collections.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Working with Streams and Lambda Expressions — This question tests Working with Streams and Lambda Expressions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Using a parallel stream with a stateful lambda operation can lead to incorrect results. — Option B is correct because parallel streams split the workload across multiple threads, and if the lambda operation is stateful (e.g., modifying a shared variable like a counter or a non-thread-safe collection), it can cause race conditions and produce incorrect results. The Streams API documentation explicitly warns against using stateful lambdas with parallel streams to avoid data integrity issues.
What should I do if I get this 1Z0-829 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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