Quick Answer
The answer is the `Function<String, Integer>` functional interface. This is correct because the `Function` interface is designed to accept one input of a specified type and produce an output of another type, perfectly matching the lambda `s -> s.length()` which takes a `String` and returns an `Integer`. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your ability to map lambda signatures to the correct functional interface from the `java.util.function` package, a common topic in the "Functional Interfaces and Lambda Expressions" objectives. A frequent trap is confusing `Function` with `UnaryOperator` (which requires the same input and output type) or `Consumer` (which returns void). Remember that `Function<T,R>` is your go-to for any transformation where the input and output types differ. A simple memory tip: think of "Function" as a "transformer" that takes one thing and gives you another, so for a string length, you are transforming a `String` into an `Integer`.
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.
A lambda that takes a String and returns its length is assigned to which functional interface?
UnaryOperator<String>
Why wrong: UnaryOperator takes and returns the same type.
Consumer<String>
Why wrong: Consumer takes a value and returns void.
Predicate<String>
Why wrong: Predicate returns a boolean.
Supplier<Integer>
Why wrong: Supplier takes no arguments and returns a value.
Function<String, Integer>
Correct. This functional interface accepts a String and returns an Integer.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Function<String, Integer>
Function<String, Integer> takes a String and returns an Integer. The lambda s -> s.length() matches this signature.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
UnaryOperator<String>
Why it's wrong here
UnaryOperator takes and returns the same type.
Consumer<String>
Why it's wrong here
Consumer takes a value and returns void.
Predicate<String>
Why it's wrong here
Predicate returns a boolean.
Supplier<Integer>
Why it's wrong here
Supplier takes no arguments and returns a value.
Function<String, Integer>
Why this is correct
Correct. This functional interface accepts a String and returns an Integer.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 1Z0-829 NAT questions on configuration and troubleshooting.
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FAQ
Working with Streams and Lambda Expressions — This question tests Working with Streams and Lambda Expressions — Static NAT maps one inside address to one outside address..
The correct answer is: Function<String, Integer> — Function<String, Integer> takes a String and returns an Integer. The lambda s -> s.length() matches this signature.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 1Z0-829 NAT questions on configuration and troubleshooting.
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 24, 2026
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