A developer writes the following code using the Stream API: List list = List.of("a", "b", "c"); String result = list.stream().reduce("", (s1, s2) -> s1 + s2); System.out.println(result); What is the output?

Working with Streams and Lambda Expressions →easyMultiple ChoiceObjective-mapped

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

A developer writes the following code using the Stream API:

List<String> list = List.of("a", "b", "c"); String result = list.stream().reduce("", (s1, s2) -> s1 + s2); System.out.println(result);

What is the output?

⚠ Common exam trap

Many candidates confuse the `reduce` overloads, mistakenly thinking that all `reduce` operations return an `Optional`, or they forget that the identity value is included in the accumulation, leading them to expect an `Optional` wrapper or a reversed order.

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

✓

abc

The `reduce` method with an identity value (`""`) and a binary operator (`(s1, s2) -> s1 + s2`) accumulates the stream elements in encounter order. Starting from the identity, it concatenates each element: `"" + "a" = "a"`, then `"a" + "b" = "ab"`, then `"ab" + "c" = "abc"`. The result is a `String`, not an `Optional`, because an identity is provided.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✓
abc
Why this is correct
Correct concatenation.
✗
a
Why it's wrong here
The identity is not ignored; all elements are combined.
✗
Optional[abc]
Why it's wrong here
reduce with identity returns a value, not Optional.
✗
cba
Why it's wrong here
Order is preserved as sequential stream.

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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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