Quick Answer
The correct answer is that both `List.copyOf(Arrays.asList("a", "b"))` and `Collections.unmodifiableList()` are valid ways to create an immutable list in Java 17. `List.copyOf()` creates a truly immutable list from an existing collection, throwing a `NullPointerException` if any element is null, while `Collections.unmodifiableList()` wraps a mutable list in an unmodifiable view that prevents structural modifications at runtime. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your ability to distinguish between factory methods that produce deeply immutable lists and wrapper methods that only provide an unmodifiable view—a common trap is assuming `Collections.unmodifiableList()` creates an immutable list when the underlying list can still be changed through its original reference. To remember the difference, think of `List.copyOf()` as a "snapshot" that cannot be altered, whereas `Collections.unmodifiableList()` is a "guard" that can be bypassed if the original list is accessible.
This 1Z0-829 practice question tests your understanding of working with arrays and collections. 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.
Which TWO are valid ways to create an immutable list in Java 17? (Choose two.)
Arrays.asList("a", "b")
Why wrong: Returns fixed-size but mutable via set.
List.of(List.of("a", "b"))
Why wrong: Creates a list containing a list, not a flat immutable list.
Collections.unmodifiableList(new ArrayList<>(List.of("a", "b")))
Wraps list to be unmodifiable.
List.copyOf(Arrays.asList("a", "b"))
List.copyOf returns an immutable list.
new ArrayList<>(List.of("a", "b"))
Why wrong: Creates a mutable ArrayList.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Collections.unmodifiableList(new ArrayList<>(List.of("a", "b")))
Option C is correct because `Collections.unmodifiableList()` wraps a mutable list in an unmodifiable view, preventing any structural modifications. Option D is correct because `List.copyOf()` creates an immutable list from an existing collection, throwing `NullPointerException` if any element is null. Both produce lists that cannot be modified after creation.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Arrays.asList("a", "b")
Why it's wrong here
Returns fixed-size but mutable via set.
List.of(List.of("a", "b"))
Why it's wrong here
Creates a list containing a list, not a flat immutable list.
Collections.unmodifiableList(new ArrayList<>(List.of("a", "b")))
Why this is correct
Wraps list to be unmodifiable.
Related concept
Read the scenario before looking for a memorised answer.
List.copyOf(Arrays.asList("a", "b"))
Why this is correct
List.copyOf returns an immutable list.
Related concept
Read the scenario before looking for a memorised answer.
new ArrayList<>(List.of("a", "b"))
Why it's wrong here
Creates a mutable ArrayList.
Common exam traps
Oracle often tests the distinction between an unmodifiable view (which can be circumvented if the backing list is accessible) and a truly immutable list (which cannot be changed by any means), and candidates mistakenly think `Arrays.asList()` or `new ArrayList<>()` are immutable.
Detailed technical explanation
Under the hood, `List.copyOf()` (introduced in Java 10) returns an unmodifiable list that is not a view — it copies the elements into a new internal array, guaranteeing immutability even if the source list is later modified. `Collections.unmodifiableList()` returns a wrapper that delegates all mutator methods to throw `UnsupportedOperationException`, but the underlying mutable list can still be changed through its reference, breaking the immutability contract. In real-world scenarios, prefer `List.of()` or `List.copyOf()` for true immutability, especially in concurrent or API design contexts.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Working with Arrays and Collections — This question tests Working with Arrays and Collections — Read the scenario before looking for a memorised answer..
The correct answer is: Collections.unmodifiableList(new ArrayList<>(List.of("a", "b"))) — Option C is correct because `Collections.unmodifiableList()` wraps a mutable list in an unmodifiable view, preventing any structural modifications. Option D is correct because `List.copyOf()` creates an immutable list from an existing collection, throwing `NullPointerException` if any element is null. Both produce lists that cannot be modified after creation.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Same concept, more angles
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Variation 1. A method returns a List<Integer>. The caller wants to ensure the list cannot be modified. Which is the best approach?
mediumWhy D: Option D is correct because `Collections.unmodifiableList()` returns a read-only view of the specified list. Any attempt to modify the returned list (e.g., via `add`, `remove`, `set`) will throw an `UnsupportedOperationException`. This is the standard, recommended approach in the Java Collections Framework to provide an unmodifiable wrapper without copying the underlying data.
Variation 2. What is the result of executing this code?
hardWhy C: Option C is correct. Collectors.toUnmodifiableList returns an immutable list, so calling add throws UnsupportedOperationException at runtime. Option A is wrong because the code compiles. Options B and D are wrong because the add operation fails.
Variation 3. What is the result of the following code? List<String> list = List.of("A", "B"); list.add("C"); System.out.println(list);
easyWhy C: The `List.of()` factory method returns an immutable list. Calling `add()` on an immutable list throws an `UnsupportedOperationException` at runtime, so the code does not compile or run successfully. Option C is correct because the exception is thrown when the `add` method is invoked.
Last reviewed: Jun 30, 2026
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