- A
A stream can be traversed multiple times.
Why wrong: This statement is false. A stream can only be traversed once. Attempting to traverse it a second time throws an IllegalStateException.
- B
The peek() method is an intermediate operation.
This statement is true. peek() is an intermediate operation that returns the same stream elements after performing an action.
- C
The findFirst() method returns an Optional.
This statement is true. findFirst() returns an Optional containing the first element, or an empty Optional if the stream is empty.
- D
The collect() method is an intermediate operation.
Why wrong: This statement is false. collect() is a terminal operation; it consumes the stream and produces a result.
- E
The map() method returns a stream of the same type.
Why wrong: This statement is false. map() applies a function to each element and can change the element type; the resulting stream type is determined by the function's return type.
Stream Intermediate and Terminal Operations (peek, findFirst) — Oracle Certified Professional Java SE 17 Explained
This 1Z0-829 practice question tests your understanding of working with streams and lambda expressions. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 correct? (Choose two.) A. A stream can be traversed multiple times. B. The peek() method is an intermediate operation. C. The findFirst() method returns an Optional. D. The collect() method is an intermediate operation. E. The map() method returns a stream of the same type.
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 peek() method is an intermediate operation.
The correct statements are B and C. Option B is correct because peek() is an intermediate operation that allows performing an action on each element without modifying the stream. Option C is correct because findFirst() returns an Optional describing the first element, or an empty Optional if the stream is empty. Option A is incorrect because a stream can only be consumed once; it cannot be traversed multiple times. Option D is incorrect because collect() is a terminal operation, not an intermediate operation. Option E is incorrect because map() can change the element type; for example, mapping a Stream<String> to Stream<Integer> is common.
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.
- ✗
A stream can be traversed multiple times.
Why it's wrong here
This statement is false. A stream can only be traversed once. Attempting to traverse it a second time throws an IllegalStateException.
- ✓
The peek() method is an intermediate operation.
Why this is correct
This statement is true. peek() is an intermediate operation that returns the same stream elements after performing an action.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
The findFirst() method returns an Optional.
Why this is correct
This statement is true. findFirst() returns an Optional containing the first element, or an empty Optional if the stream is empty.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The collect() method is an intermediate operation.
Why it's wrong here
This statement is false. collect() is a terminal operation; it consumes the stream and produces a result.
- ✗
The map() method returns a stream of the same type.
Why it's wrong here
This statement is false. map() applies a function to each element and can change the element type; the resulting stream type is determined by the function's return type.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The main trap in this question is confusing intermediate and terminal operations. Candidates often mistake collect() as intermediate or think that peek() is terminal. Another trap is assuming streams can be reused, but they are single-use only.
Detailed technical explanation
How to think about this question
Under the hood, intermediate operations like `peek()` and `map()` are added to a pipeline of operations that are evaluated lazily. The `peek()` method is primarily intended for debugging, as it allows you to observe elements as they pass through the pipeline, but it should not be used for side effects in production code because the Stream API does not guarantee that `peek()` will be invoked for every element in all scenarios (e.g., with short-circuiting operations). The `findFirst()` method returns an `Optional` to safely handle the case where the stream is empty, avoiding null pointer exceptions.
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
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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 peek() method is an intermediate operation. — The correct statements are B and C. Option B is correct because peek() is an intermediate operation that allows performing an action on each element without modifying the stream. Option C is correct because findFirst() returns an Optional describing the first element, or an empty Optional if the stream is empty. Option A is incorrect because a stream can only be consumed once; it cannot be traversed multiple times. Option D is incorrect because collect() is a terminal operation, not an intermediate operation. Option E is incorrect because map() can change the element type; for example, mapping a Stream<String> to Stream<Integer> is common.
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: Jun 30, 2026
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