A developer needs to concatenate two Stream into one. Which approach is most idiomatic?

new StreamBuilder().add(stream1).add(stream2).build()

Working with Streams and Lambda Expressions →easyMultiple ChoiceObjective-mapped

1Z0-829 Working with Streams and Lambda Expressions Practice Question

A developer needs to concatenate two Stream<String> into one. Which approach is most idiomatic?

⚠ Common exam trap

Many exam-takers confuse `flatMap` with concatenation or assume a non-existent `merge` method exists, because they recall similar operations from other APIs like `Map.merge()` or reactive streams, but the Java Stream API specifically provides `concat()` for this purpose.

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

✓

Stream.concat(stream1, stream2)

`Stream.concat(stream1, stream2)` is the idiomatic, built-in static method in the Java Stream API for concatenating two streams into a single sequential stream. It creates a lazily concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream, preserving encounter order.

Answer analysis

Option-by-option breakdown

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.
✗
stream1.merge(stream2)
Why it's wrong here
No such method.

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1Z0-829 Working with Streams and Lambda Expressions Practice Question

Why each option matters

Option-by-option breakdown

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