Working with Streams and Lambda Expressions →mediumMultiple ChoiceObjective-mapped

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

A developer wants to collect elements from a stream into an immutable List. Which collector should be used?

⚠ Common exam trap

Test-takers frequently think `Collectors.toList()` returns an immutable list (since it's often used in read-only contexts), or they may overcomplicate the solution by chaining collectors when a direct method exists.

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

✓

Collectors.toUnmodifiableList()

`Collectors.toUnmodifiableList()` directly returns a collector that accumulates elements into an immutable `List`. This was introduced in Java 10 and guarantees that the resulting list cannot be modified, throwing `UnsupportedOperationException` on any mutation attempt.

Answer analysis

Option-by-option breakdown

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

✗
Collectors.toList()
Why it's wrong here
Returns a mutable list.
✗
Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)
Why it's wrong here
This works but is not the direct collector. The question asks 'which collector', so the direct one is toUnmodifiableList.
✗
Collectors.toList().stream().collect(Collectors.toUnmodifiableList())
Why it's wrong here
This is inefficient and not a single collector.
✓
Collectors.toUnmodifiableList()
Why this is correct
Correct: Returns an unmodifiable list.

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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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