Working with Streams and Lambda Expressions →easyMultiple ChoiceObjective-mapped

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

A developer wants to count how many strings in a list have length greater than 5. Which is the correct implementation?

⚠ Common exam trap

Candidates often confuse `count()` with a method that accepts a predicate, or mistakenly try to map booleans to numeric values using `mapToLong`, forgetting that `mapToLong` requires a `ToLongFunction` returning a primitive `long`, not a `boolean`.

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

✓

list.stream().filter(s -> s.length() > 5).count()

The Stream API provides the `filter` method to select elements matching a predicate, and `count` returns the number of elements in the resulting stream. This directly counts strings with length greater than 5 without needing any intermediate mapping or collection.

Answer analysis

Option-by-option breakdown

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

✗
list.stream().mapToLong(s -> s.length() > 5).sum()
Why it's wrong here
mapToLong expects a ToLongFunction, not a predicate; this would cause a compilation error.
✓
list.stream().filter(s -> s.length() > 5).count()
Why this is correct
Correct: filter then count gives number of elements satisfying predicate.
✗
list.stream().count(s -> s.length() > 5)
Why it's wrong here
count() does not accept a predicate; it returns the number of elements in the stream.
✗
list.stream().collect(Collectors.counting(s -> s.length() > 5))
Why it's wrong here
Collectors.counting() takes no argument; it counts all elements.

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

Why each option matters

Option-by-option breakdown

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