The answer is `[A, B]`. This output results from using `flatMap` with `Optional` in streams, which transforms each `Optional<String>` into a stream of its value if present, or an empty stream if the Optional is empty, effectively flattening the nested structure and skipping any empty optionals. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this pattern tests your understanding of how `flatMap` differs from `map` when dealing with `Optional`—a common trap is using `map` instead, which would produce a `Stream<Optional<String>>` and retain the empty Optional as a null-like element. Remember that `flatMap` on an `Optional` acts as a filter: it unwraps the value only when present, making it ideal for chaining operations that might return empty results. A quick memory tip: think of `flatMap` as "flatten and map"—it both transforms and flattens, so `Optional.empty()` becomes a stream with nothing, not a stream containing an empty Optional.
1Z0-829 Working with Streams and Lambda Expressions Practice Question
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.
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);
```
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);
```
A
[A, B]
Correct: Only non-empty Optionals are flattened.
B
NullPointerException
Why wrong: Incorrect: No null values are present.
C
[Optional[A], Optional.empty, Optional[B]]
Why wrong: Incorrect: flatMap flattens out the Optional wrapper.
D
[]
Why wrong: Incorrect: There are non-empty Optionals.
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]`.
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, B]
Why this is correct
Correct: Only non-empty Optionals are flattened.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often 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.
Detailed technical explanation
How to think about this question
Under the hood, `flatMap` on an `Optional` calls `Optional.stream()` (introduced in Java 9), which returns a stream of one element if present, or an empty stream if empty. This avoids the need for explicit `filter(Optional::isPresent).map(Optional::get)` chains, making the code more concise and functional. In real-world scenarios, this pattern is commonly used when processing collections of optional values from external APIs or database queries where null-safe handling is critical.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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: [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]`.
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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