1Z0-829 Working with Streams and Lambda Expressions Practice Question
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.
⚠ Common exam trap
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.
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.
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.
- ✓
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.
- ✗
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.
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