Working with Streams and Lambda Expressions →easyMultiple ChoiceObjective-mapped

1Z0-829 Working with Streams and Lambda Expressions Practice Question

Exhibit

Refer to the exhibit.

Error log:
Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
	The method flatMap(Function<? super String,? extends Stream<? extends String>>) in the type Stream<String> is not applicable for the arguments (String::chars)

Code snippet:
List<String> words = List.of("hello", "world");
List<Character> letters = words.stream()
    .flatMap(String::chars)
    .mapToObj(c -> (char) c)
    .collect(Collectors.toList());

The code fails to compile. What is the reason?

⚠ Common exam trap

Candidates often assume `String::chars` returns a `Stream<Character>` because it deals with characters, but it actually returns an `IntStream`, leading to a type mismatch when used with `flatMap`.

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 flatMap method requires a function that returns a Stream, but String::chars returns an IntStream.

The `chars()` method on `String` returns an `IntStream`, not a `Stream<Character>`. The `flatMap` method on a `Stream<String>` expects a function that returns a `Stream<?>`, but `String::chars` returns an `IntStream`, which is a primitive stream and not a subtype of `Stream<?>`. This type mismatch causes a compilation error.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
The collect method is not applicable for a Stream<Character>.
Why it's wrong here
Collectors.toList() works fine on Stream<Character>.
✗
The method reference String::chars is not a valid method reference.
Why it's wrong here
String::chars is a valid method reference; it returns an IntStream.
✗
The chars() method returns a Stream<Character>, but flatMap requires a function that returns a Stream<Integer>.
Why it's wrong here
chars() returns IntStream, not Stream<Character>.
✓
The flatMap method requires a function that returns a Stream, but String::chars returns an IntStream.
Why this is correct
flatMap expects a Function returning a Stream<? extends R>, but chars() returns IntStream, which is not a Stream.

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1Z0-829 Working with Streams and Lambda Expressions Practice Question

Exhibit

Why each option matters

Option-by-option breakdown

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