- A
The 'reduce()' operation can produce a result of a different type than the stream elements.
The `reduce()` operation can produce a result of a different type than the stream elements. For example, `stream.reduce(0, (sum, i) -> sum + i, Integer::sum)` reduces an `IntStream` to an `Integer`.
- B
The 'flatMap()' operation is a terminal operation.
Why wrong: The `flatMap()` operation is an intermediate operation, not a terminal operation. It transforms each element into a stream and then flattens them.
- C
The 'findFirst()' operation is a short-circuiting terminal operation.
The `findFirst()` operation is a short-circuiting terminal operation. It returns an `Optional` describing the first element of the stream, and it can terminate early if a match is found.
- D
The 'collect()' operation is an intermediate operation.
Why wrong: The `collect()` operation is a terminal operation, not an intermediate one. It consumes the stream and accumulates elements into a mutable container.
- E
Streams can be reused after a terminal operation.
Why wrong: Streams cannot be reused after a terminal operation. Attempting to perform another operation on a consumed stream throws an `IllegalStateException`.
Stream reduce() with Different Result Type — Oracle Certified Professional Java SE 17 Explained
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.
Which two statements about the Stream API are true? (Choose two.)
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
The 'reduce()' operation can produce a result of a different type than the stream elements.
Option A is correct because the `reduce()` operation in the Stream API uses a combiner function that can transform the stream elements into an accumulated result of a different type. For example, `stream.reduce(0, (sum, i) -> sum + i, Integer::sum)` returns an `Integer` from an `IntStream`, or you can reduce a `Stream<String>` to a `StringBuilder`. This is explicitly allowed by the `reduce(U identity, BiFunction<U,? super T,U> accumulator, BinaryOperator<U> combiner)` signature.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The 'reduce()' operation can produce a result of a different type than the stream elements.
Why this is correct
The `reduce()` operation can produce a result of a different type than the stream elements. For example, `stream.reduce(0, (sum, i) -> sum + i, Integer::sum)` reduces an `IntStream` to an `Integer`.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The 'flatMap()' operation is a terminal operation.
Why it's wrong here
The `flatMap()` operation is an intermediate operation, not a terminal operation. It transforms each element into a stream and then flattens them.
- ✓
The 'findFirst()' operation is a short-circuiting terminal operation.
Why this is correct
The `findFirst()` operation is a short-circuiting terminal operation. It returns an `Optional` describing the first element of the stream, and it can terminate early if a match is found.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The 'collect()' operation is an intermediate operation.
Why it's wrong here
The `collect()` operation is a terminal operation, not an intermediate one. It consumes the stream and accumulates elements into a mutable container.
- ✗
Streams can be reused after a terminal operation.
Why it's wrong here
Streams cannot be reused after a terminal operation. Attempting to perform another operation on a consumed stream throws an `IllegalStateException`.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Oracle Java certification exams often test the distinction between intermediate and terminal operations. The trap here is that candidates confuse `flatMap()` (intermediate) with a terminal operation or assume `collect()` is intermediate because it doesn't produce a single value like `reduce()`, but it is terminal because it consumes the stream.
Detailed technical explanation
How to think about this question
The `reduce()` operation is a specialized reduction that can change the type via its identity and accumulator parameters, enabling patterns like summing integers from strings. Under the hood, the Stream API uses a `Spliterator` to traverse elements, and terminal operations trigger the pipeline's evaluation; intermediate operations like `flatMap()` are chained and only executed when a terminal operation is invoked. Short-circuiting terminal operations like `findFirst()` may not process the entire stream, which is critical for infinite streams.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 1Z0-829 question test?
Working with Streams and Lambda Expressions — This question tests Working with Streams and Lambda Expressions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The 'reduce()' operation can produce a result of a different type than the stream elements. — Option A is correct because the `reduce()` operation in the Stream API uses a combiner function that can transform the stream elements into an accumulated result of a different type. For example, `stream.reduce(0, (sum, i) -> sum + i, Integer::sum)` returns an `Integer` from an `IntStream`, or you can reduce a `Stream<String>` to a `StringBuilder`. This is explicitly allowed by the `reduce(U identity, BiFunction<U,? super T,U> accumulator, BinaryOperator<U> combiner)` signature.
What should I do if I get this 1Z0-829 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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