The answer is that the code prints the doubled numbers in a non-deterministic order and results may contain missing or duplicate entries. This occurs because the lambda in the map operation is stateful—it modifies an external, non-thread-safe results list—and when used in a parallel stream, multiple threads concurrently access and mutate that shared list without synchronization. Such stateful operations in parallel streams break the stream’s contract of non-interference and statelessness, leading to race conditions that corrupt the output. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of why stateful lambdas are forbidden in parallel streams, even when followed by forEachOrdered, which only guarantees encounter order for consumption, not safety for upstream mutations. A common trap is assuming forEachOrdered fixes all ordering or safety issues. Memory tip: “Stateful in parallel? Expect chaos—missing, duplicate, or scrambled data.”
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.
Exhibit
Refer to the exhibit.
```java
List<Integer> source = new ArrayList<>(Arrays.asList(1, 2, 3, 4));
List<Integer> results = new ArrayList<>();
source.parallelStream()
.map(n -> { results.add(n); return n * 2; })
.forEachOrdered(System.out::println);
```
Assuming the code runs in a multithreaded environment, which statement best describes the behavior?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Refer to the exhibit.
```java
List<Integer> source = new ArrayList<>(Arrays.asList(1, 2, 3, 4));
List<Integer> results = new ArrayList<>();
source.parallelStream()
.map(n -> { results.add(n); return n * 2; })
.forEachOrdered(System.out::println);
```
A
The code will compile but throw an exception at runtime because of the use of parallel stream with stateful lambda.
Why wrong: No runtime exception occurs directly from the stateful lambda, but data corruption may occur.
B
It prints the doubled numbers in order (2, 4, 6, 8) and results contains [1,2,3,4] in that order.
Why wrong: Incorrect: The map operation is performed in parallel, so results list may not maintain order and may have race conditions.
C
It may produce a ConcurrentModificationException because results is modified while iterating.
Why wrong: ConcurrentModificationException is not thrown because the stream does not iterate over results; it's a separate collection. However, race conditions are possible.
D
It prints the doubled numbers in a non-deterministic order and results may contain missing or duplicate entries.
Correct: Parallel stream processes elements concurrently, so the order of map execution is non-deterministic. The shared results list is not thread-safe, leading to data corruption.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It prints the doubled numbers in a non-deterministic order and results may contain missing or duplicate entries.
The lambda in map is stateful (modifying results list) and not thread-safe. This can cause race conditions and non-deterministic output. Additionally, forEachOrdered does not make the preceding stateful operation safe.
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 code will compile but throw an exception at runtime because of the use of parallel stream with stateful lambda.
Why it's wrong here
No runtime exception occurs directly from the stateful lambda, but data corruption may occur.
✗
It prints the doubled numbers in order (2, 4, 6, 8) and results contains [1,2,3,4] in that order.
Why it's wrong here
Incorrect: The map operation is performed in parallel, so results list may not maintain order and may have race conditions.
✗
It may produce a ConcurrentModificationException because results is modified while iterating.
Why it's wrong here
ConcurrentModificationException is not thrown because the stream does not iterate over results; it's a separate collection. However, race conditions are possible.
✓
It prints the doubled numbers in a non-deterministic order and results may contain missing or duplicate entries.
Why this is correct
Correct: Parallel stream processes elements concurrently, so the order of map execution is non-deterministic. The shared results list is not thread-safe, leading to data corruption.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 1Z0-829 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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: It prints the doubled numbers in a non-deterministic order and results may contain missing or duplicate entries. — The lambda in map is stateful (modifying results list) and not thread-safe. This can cause race conditions and non-deterministic output. Additionally, forEachOrdered does not make the preceding stateful operation safe.
What should I do if I get this 1Z0-829 question wrong?
Identify which 1Z0-829 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
medium
A.The stream() method returns a parallel stream by default.
✓ B.Using a parallel stream with a stateful lambda operation can lead to incorrect results.
C.Parallel streams always provide better performance than sequential streams.
D.Parallel streams cannot be used with custom thread pools.
Why B: 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.
Variation 2. A stream pipeline uses sorted() with a Comparator that calls a thread-unsafe method on each element. The pipeline is parallel. What is the likely outcome?
hard
✓ A.The pipeline may produce incorrect ordering or throw a ConcurrentModificationException.
B.The pipeline will throw an exception because the comparator is stateful.
C.The pipeline will sort correctly because sorted() is a sequential operation.
D.The pipeline will run sequentially because the stream source is not ordered.
Why A: The sorted operation in parallel streams uses multiple threads and requires the comparator to be stateless and thread-safe. A thread-unsafe comparator can cause unpredictable results or exceptions.
Last reviewed: Jun 24, 2026
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