The answer is `Collectors.toUnmodifiableList()`. This collector directly produces an immutable list from a stream, meaning any attempt to modify the result—such as adding, removing, or replacing elements—will throw an `UnsupportedOperationException`. Introduced in Java 10, it is the cleanest and most intentional way to collect stream elements into an immutable list, as opposed to wrapping a mutable list with `Collections.unmodifiableList()` after collection. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your knowledge of Java 10+ collector enhancements and the distinction between truly immutable collections and unmodifiable views. A common trap is confusing `Collectors.toList()` (which returns a mutable list) or `Collectors.toUnmodifiableList()` with older approaches like `collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList))`. For a memory tip, think "U for Unchangeable"—`toUnmodifiableList()` gives you a list you cannot touch after creation.
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.
A developer wants to collect elements from a stream into an immutable List. Which collector should be used?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Collectors.toUnmodifiableList()
Option D is correct because `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.
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.
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may 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.
Detailed technical explanation
How to think about this question
Under the hood, `Collectors.toUnmodifiableList()` uses a specialized accumulator that builds the list in a mutable `ArrayList` during collection, then wraps it in an unmodifiable view via `List.copyOf()` (or an internal equivalent) before returning. This ensures that the returned list is truly immutable, not just wrapped, and avoids the overhead of an extra wrapping step. In real-world scenarios, using immutable collections improves thread safety and prevents accidental modification in shared code paths.
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: Collectors.toUnmodifiableList() — Option D is correct because `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.
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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