The answer is to replace `Collectors.toList()` with `Collectors.toUnmodifiableList()`. This is correct because the method’s contract only requires returning a sorted list without modifying the original collection, and an unmodifiable list provides immutability, which improves memory efficiency and thread safety without any performance overhead from defensive copying. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of Java 17’s enhanced `Collectors` and the principle of returning immutable collections when mutability is not needed. A common trap is assuming `Collectors.toList()` is always fine, but the exam emphasizes that `Collectors.toUnmodifiableList()` is the optimal choice for `stream sorted()` pipelines that produce a final result. Remember the mnemonic: “Sorted and sealed—unmodifiable is the ideal.”
1Z0-829 Working with Arrays and Collections Practice Question
This 1Z0-829 practice question tests your understanding of working with arrays and collections. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 is writing a method that takes a Collection<Integer> and returns a List<Integer> containing the same elements in sorted order. The method should not modify the original collection. The developer tries the following code: public List<Integer> sortCollection(Collection<Integer> col) { return col.stream().sorted().collect(Collectors.toList()); } The code compiles and runs, but the team lead says it is not optimal. What improvement should be made?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Return an unmodifiable list using collect(Collectors.toUnmodifiableList())
Option D is correct because the current code creates a mutable list via `Collectors.toList()`, but the method contract does not require mutability. Using `Collectors.toUnmodifiableList()` returns an unmodifiable list, which is more memory-efficient and thread-safe, and it aligns with the principle of returning immutable collections when modification is not needed. This improvement does not affect sorting correctness but optimizes the result for immutability.
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.
✗
Use parallelStream() for better performance
Why it's wrong here
Overhead of parallel may outweigh benefit for small collections.
✗
Use a TreeSet and then convert to List
Why it's wrong here
TreeSet removes duplicates, which may not be desired.
✗
Convert the collection to an ArrayList first, then call Collections.sort() on it
Why it's wrong here
This creates an extra intermediate list and modifies it, not necessarily better.
✓
Return an unmodifiable list using collect(Collectors.toUnmodifiableList())
Why this is correct
Ensures result cannot be modified, good practice for API design.
Related concept
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 often focus on performance improvements like parallelism or sorting efficiency, but the actual optimization is about immutability and API design, which is a common subtlety in Java collections questions.
Detailed technical explanation
How to think about this question
Under the hood, `Collectors.toList()` makes no guarantees about the mutability, serializability, or thread-safety of the returned list, whereas `Collectors.toUnmodifiableList()` (introduced in Java 10) returns a list that is structurally immutable, backed by the stream pipeline's internal buffer. In real-world scenarios, returning an unmodifiable list prevents accidental modification by callers, which is especially important in multi-threaded environments or when the list is exposed as part of an API contract. Additionally, the `sorted()` intermediate operation uses a `TimSort` algorithm internally, which is stable and efficient for partially ordered data.
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 Arrays and Collections — This question tests Working with Arrays and Collections — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Return an unmodifiable list using collect(Collectors.toUnmodifiableList()) — Option D is correct because the current code creates a mutable list via `Collectors.toList()`, but the method contract does not require mutability. Using `Collectors.toUnmodifiableList()` returns an unmodifiable list, which is more memory-efficient and thread-safe, and it aligns with the principle of returning immutable collections when modification is not needed. This improvement does not affect sorting correctness but optimizes the result for immutability.
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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