1Z0-829 Working with Streams and Lambda Expressions Practice Question
Exhibit
Refer to the exhibit.
```java
List<Optional<String>> list = Arrays.asList(
Optional.of("A"),
Optional.empty(),
Optional.of("B"));
List<String> result = list.stream()
.flatMap(Optional::stream)
.collect(Collectors.toList());
System.out.println(result);
```What is the output?
⚠ Common exam trap
A common mix-up: candidates confuse `map` with `flatMap` on optionals, expecting `map` to produce a stream of optionals, but `flatMap` is required to unwrap and flatten the optionals into a stream of their contained values.
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
✓
[A, B]
The code uses `flatMap` on a stream of `Optional<String>` values. `flatMap` flattens nested optionals, so `Optional.of("A")` yields a stream containing "A", `Optional.empty()` yields an empty stream (effectively skipping it), and `Optional.of("B")` yields a stream containing "B". The terminal operation `collect(Collectors.toList())` collects the non-empty strings into a list, resulting in `[A, B]`.
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, B]
Why this is correct
Correct: Only non-empty Optionals are flattened.
- ✗
NullPointerException
Why it's wrong here
Incorrect: No null values are present.
- ✗
[Optional[A], Optional.empty, Optional[B]]
Why it's wrong here
Incorrect: flatMap flattens out the Optional wrapper.
- ✗
[]
Why it's wrong here
Incorrect: There are non-empty Optionals.
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