- A
Use the collect method with a concurrent Collector such as toConcurrentMap().
Concurrent Collectors are designed for parallel reduction, using internal synchronization and efficient merging.
- B
Use forEach with AtomicInteger and update atomically.
Why wrong: AtomicInteger works for simple counters, but for complex accumulation, it may not be sufficient and can still cause contention.
- C
Use synchronized blocks inside the lambda expression.
Why wrong: Synchronization defeats the purpose of parallelism and introduces contention, reducing performance.
- D
Use ConcurrentHashMap for accumulation, but collect using toList().
Why wrong: ConcurrentHashMap provides thread-safe updates, but collecting into it via toList() is not concurrent; the combiner may cause race conditions.
Quick Answer
The answer is to use the collect method with a concurrent Collector such as toConcurrentMap(). This approach is correct because concurrent Collectors are specifically designed for parallel stream processing, using internal thread-safe data structures and atomic merges that eliminate race conditions without sacrificing parallelism. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of how parallel streams interact with mutable reduction—a common trap is assuming that ConcurrentHashMap alone solves the problem, but it does not guarantee atomicity for compound operations like merging map entries. Remember that toConcurrentMap and groupingByConcurrent are the only Collectors that safely combine results from multiple threads in a parallel stream without external synchronization. Memory tip: think "concurrent Collector for concurrent streams"—the Collector itself must be thread-safe, not just the underlying collection.
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 Java team is processing a large dataset with parallel streams. They notice inconsistent results due to non-atomic operations on shared mutable state. Which approach should they use to ensure thread-safety while maximizing performance?
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
Use the collect method with a concurrent Collector such as toConcurrentMap().
Option C is correct because using a concurrent Collector (like toConcurrentMap or groupingByConcurrent) ensures both thread-safety and efficiency in parallel processing. Option A is wrong because synchronized blocks inside lambdas can cause contention and undermine parallelism. Option B is wrong because ConcurrentHashMap alone does not guarantee atomicity of compound operations. Option D is wrong because forEach with AtomicInteger still requires careful coordination and may not be optimal.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Use the collect method with a concurrent Collector such as toConcurrentMap().
Why this is correct
Concurrent Collectors are designed for parallel reduction, using internal synchronization and efficient merging.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
Use forEach with AtomicInteger and update atomically.
Why it's wrong here
AtomicInteger works for simple counters, but for complex accumulation, it may not be sufficient and can still cause contention.
- ✗
Use synchronized blocks inside the lambda expression.
Why it's wrong here
Synchronization defeats the purpose of parallelism and introduces contention, reducing performance.
- ✗
Use ConcurrentHashMap for accumulation, but collect using toList().
Why it's wrong here
ConcurrentHashMap provides thread-safe updates, but collecting into it via toList() is not concurrent; the combiner may cause race conditions.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 1Z0-829 NAT questions on configuration and troubleshooting.
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Working with Streams and Lambda Expressions — study guide chapter
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FAQ
Questions learners often ask
What does this 1Z0-829 question test?
Working with Streams and Lambda Expressions — This question tests Working with Streams and Lambda Expressions — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Use the collect method with a concurrent Collector such as toConcurrentMap(). — Option C is correct because using a concurrent Collector (like toConcurrentMap or groupingByConcurrent) ensures both thread-safety and efficiency in parallel processing. Option A is wrong because synchronized blocks inside lambdas can cause contention and undermine parallelism. Option B is wrong because ConcurrentHashMap alone does not guarantee atomicity of compound operations. Option D is wrong because forEach with AtomicInteger still requires careful coordination and may not be optimal.
What should I do if I get this 1Z0-829 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 1Z0-829 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 24, 2026
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