Quick Answer
The answer is 6. This is correct because the single-argument `reduce` operation in Java streams applies a `BinaryOperator` to accumulate the stream elements sequentially, starting with the first two elements: here, 1 + 2 = 3, then 3 + 3 = 6. Unlike the three-argument `reduce` with an identity value, this overload returns an `Optional<Integer>` to safely handle empty streams, and calling `get()` on a non-empty `Optional` extracts the result. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of the `reduce` method’s return type and its behavior with non-empty versus empty streams—a common trap is forgetting that `get()` on an empty `Optional` throws `NoSuchElementException`. To reduce errors, remember: no identity means an `Optional` is returned, so always check `isPresent()` or use `orElse()` in production code. Memory tip: “One arg, Optional bag; three args, identity tag.”
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:
List<Integer> list = List.of(1, 2, 3); Optional<Integer> opt = list.stream().reduce((a, b) -> a + b); System.out.println(opt.get());
What is the result?
6
Correct output.
Optional[6]
Why wrong: Println would print Optional[6] only if opt is printed directly, but opt.get() returns Integer.
Optional.empty
Why wrong: Stream is non-empty.
NoSuchElementException
Why wrong: Stream has elements.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
6
The `reduce` method with a single argument (a `BinaryOperator`) returns an `Optional` because the stream might be empty. Here, the stream contains three integers, so the reduction computes `1 + 2 = 3`, then `3 + 3 = 6`, yielding `Optional[6]`. Calling `get()` on a non-empty `Optional` returns the contained value `6`, which is printed.
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.
6
Why this is correct
Correct output.
Related concept
Read the scenario before looking for a memorised answer.
Optional[6]
Why it's wrong here
Println would print Optional[6] only if opt is printed directly, but opt.get() returns Integer.
Optional.empty
Why it's wrong here
Stream is non-empty.
NoSuchElementException
Why it's wrong here
Stream has elements.
Common exam traps
The trap here is that candidates may forget that `reduce` with one argument returns an `Optional` and incorrectly assume the output is `Optional[6]` or that `get()` throws an exception, overlooking that the stream is non-empty.
Detailed technical explanation
The `reduce(BinaryOperator)` method performs a left-associative reduction on the stream elements, using the accumulator function. Under the hood, it uses the `ReduceOps` internal class to create a terminal operation that combines elements sequentially. In real-world scenarios, this is commonly used for summing, concatenating, or finding the maximum/minimum, but care must be taken with empty streams to avoid calling `get()` without checking `isPresent()`.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 1Z0-829 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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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: 6 — The `reduce` method with a single argument (a `BinaryOperator`) returns an `Optional` because the stream might be empty. Here, the stream contains three integers, so the reduction computes `1 + 2 = 3`, then `3 + 3 = 6`, yielding `Optional[6]`. Calling `get()` on a non-empty `Optional` returns the contained value `6`, which is printed.
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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Same concept, more angles
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Variation 1. Refer to the exhibit. What is the output?
mediumWhy B: The reduce method starts with identity 0 and applies the accumulator function to sum all elements: 0+1=1, 1+2=3, 3+3=6, 6+4=10, 10+5=15.
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Last reviewed: Jun 11, 2026
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