The answer is UnsupportedOperationException at runtime. This occurs because `Collectors.toUnmodifiableList()` produces an unmodifiable collection, meaning any attempt to structurally modify it—such as adding an element via `add()`—throws that exception, regardless of thread safety. The stream pipeline itself is thread-safe and compiles without error; the issue lies entirely in the immutability of the resulting list. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of the distinction between thread-safe operations during stream processing and the immutability guarantee of the collector’s output. A common trap is assuming that because streams can be parallel, the resulting collection is also modifiable or thread-safe for writes. In reality, `toUnmodifiableList` enforces read-only access, making it inherently safe for concurrent reads but vulnerable to modification attempts. Memory tip: “Unmodifiable means no add—expect UnsupportedOperationException if you try.”
1Z0-829 Working with Streams and Lambda Expressions Practice Question
This 1Z0-829 practice question tests your understanding of working with streams and lambda expressions. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
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?
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse `Collectors.toUnmodifiableList()` with `Collectors.toList()`, which returns a mutable list, and incorrectly assume the list can be modified by another thread.
Detailed technical explanation
How to think about this question
Under the hood, `Collectors.toUnmodifiableList()` (introduced in Java 10) wraps the collected elements in an unmodifiable list that rejects structural modifications via `add()`, `remove()`, etc., by throwing `UnsupportedOperationException`. This is different from `Collections.unmodifiableList()` which only provides a view; the collector returns a truly immutable list that also prevents null elements. In concurrent scenarios, this immutability guarantees thread safety for reads but does not prevent concurrent modification attempts from throwing exceptions.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 1Z0-829 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Working with Streams and Lambda Expressions — This question tests Working with Streams and Lambda Expressions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 1Z0-829 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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