Quick Answer
The answer is "abc". This is correct because the Stream reduce operation with an identity value of an empty string and the binary operator (s1, s2) -> s1 + s2 performs an associative accumulation of the stream elements in encounter order, starting from the identity and applying the operator sequentially: first "" + "a" yields "a", then "a" + "b" yields "ab", and finally "ab" + "c" yields "abc". On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this tests your understanding of the reduce method’s behavior when an identity is provided—a common trap is forgetting that the identity must be an identity value for the combiner (here, "" works because "" + any string equals that string), and that providing an identity means the result is never an Optional. A key memory tip: when you see reduce with an identity, think "starting point for accumulation"—the identity is the seed, not an extra element, and the result type matches the identity type.
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 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?
abc
Correct concatenation.
a
Why wrong: The identity is not ignored; all elements are combined.
Optional[abc]
Why wrong: reduce with identity returns a value, not Optional.
cba
Why wrong: Order is preserved as sequential stream.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
abc
The correct answer is A because 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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
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.
Related concept
Read the scenario before looking for a memorised answer.
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.
Common exam traps
The trap here is that candidates often 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.
Detailed technical explanation
Under the hood, `Stream.reduce(identity, accumulator)` is a terminal operation that performs a reduction on the elements using associative accumulation. The identity must be an identity for the accumulator function, meaning `accumulator.apply(identity, x)` equals `x`. For string concatenation, `""` satisfies this. This operation is equivalent to a mutable reduction using `collect(Collectors.joining())`, but `reduce` is less efficient for strings because it creates intermediate `String` objects, whereas `Collectors.joining()` uses a `StringBuilder` internally.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
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FAQ
Working with Streams and Lambda Expressions — This question tests Working with Streams and Lambda Expressions — Read the scenario before looking for a memorised answer..
The correct answer is: abc — The correct answer is A because 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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