Quick Answer
The answer is Stream.concat(stream1, stream2). This is the most idiomatic approach because the Stream API provides a dedicated static factory method, Stream.concat, specifically designed to merge two streams into a single sequential stream, preserving the order of elements from the first stream followed by the second. Under the hood, it creates a concatenated stream that is lazy and efficient, avoiding the overhead of collecting or flattening intermediate structures. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your familiarity with the standard Stream utility methods and your ability to distinguish idiomatic solutions from less efficient alternatives, such as using flatMap with a list or manually iterating. A common trap is attempting to use Stream.of(stream1, stream2).flatMap(s -> s), which works but is less readable and introduces unnecessary complexity. Remember the memory tip: when you need to join two streams end-to-end, think “concat” as in concatenating strings—it’s the direct, one-call solution that examiners expect.
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 developer needs to concatenate two Stream<String> into one. Which approach is most idiomatic?
stream1.flatMap(s -> stream2)
Why wrong: This would flatten each element of stream1 into the entire stream2, not concatenation.
new StreamBuilder().add(stream1).add(stream2).build()
Why wrong: Does not exist in standard API.
Stream.concat(stream1, stream2)
Correct: Static method that returns a concatenated stream.
stream1.merge(stream2)
Why wrong: No such method.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Stream.concat(stream1, stream2)
Stream.concat is the standard method to concatenate two streams.
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
For each option: why learners choose it and why it is or isn't the right answer here.
stream1.flatMap(s -> stream2)
Why it's wrong here
This would flatten each element of stream1 into the entire stream2, not concatenation.
new StreamBuilder().add(stream1).add(stream2).build()
Why it's wrong here
Does not exist in standard API.
Stream.concat(stream1, stream2)
Why this is correct
Correct: Static method that returns a concatenated stream.
Related concept
Static NAT maps one inside address to one outside address.
stream1.merge(stream2)
Why it's wrong here
No such method.
Common exam traps
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
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.
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
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
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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FAQ
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..
The correct answer is: Stream.concat(stream1, stream2) — Stream.concat is the standard method to concatenate two streams.
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.
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 24, 2026
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