Working with Streams and Lambda Expressions →hardMultiple ChoiceObjective-mapped

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

Given: List<String> words = List.of("apple", "banana", "cherry");

var result = words.stream().filter(s -> s.length() > 5).collect(Collectors.toUnmodifiableList());

What happens if another thread tries to add to result?

⚠ Common exam trap

Candidates often confuse `Collectors.toUnmodifiableList()` with `Collectors.toList()`, which returns a mutable list, and incorrectly assume the list can be modified by another thread.

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

✓

UnsupportedOperationException at runtime.

B is correct because `Collectors.toUnmodifiableList()` returns an unmodifiable list, and any attempt to modify it (e.g., via `add()`) throws an `UnsupportedOperationException`. The stream pipeline itself is safe and compiles fine; the issue is the mutability of the resulting 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.

✗
Compilation error because var cannot be used with streams.
Why it's wrong here
var is perfectly valid here; the stream pipeline compiles.
✓
UnsupportedOperationException at runtime.
Why this is correct
The returned list is unmodifiable; calling add() throws UnsupportedOperationException.
✗
The thread successfully adds the element.
Why it's wrong here
The list is unmodifiable so add will fail.
✗
NullPointerException.
Why it's wrong here
NullPointerException would occur only if a null element is added, but here the list is unmodifiable.

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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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