Quick Answer
The correct answer is `List.of("a", "b")` and `Collections.unmodifiableList()`, as both produce a list that cannot be structurally modified. `List.of()` creates a truly immutable list from the ground up, rejecting null elements and throwing an `UnsupportedOperationException` on any mutation attempt, while `Collections.unmodifiableList()` wraps an existing list in a read-only view that delegates all mutator methods to throw the same exception. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of the distinction between unmodifiable views and immutable collections, a common pitfall being that `Collections.unmodifiableList()` still allows the underlying list to change if its reference is retained elsewhere. A frequent trap is confusing `Arrays.asList()` (which returns a fixed-size but mutable list) with these immutable options. Remember the mnemonic: "Of is off-limits, unmodifiable is a shield—both block structural change, but only `List.of()` is fully sealed."
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?
Stream.of("a").collect(Collectors.toList())
Why wrong: Returns mutable list.
Collections.unmodifiableList(new ArrayList<>())
Returns immutable view.
Arrays.asList("a", "b")
Why wrong: Returns fixed-size but mutable list.
new ArrayList<>()
Why wrong: Mutable list.
List.of("a", "b")
Returns immutable list.
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<>())
Option B is correct because `Collections.unmodifiableList()` returns a view of the specified list that cannot be structurally modified; any attempt to add, remove, or set elements will throw an `UnsupportedOperationException`. Option E is correct because `List.of()` (introduced in Java 9) directly creates an immutable list that disallows `null` elements and throws `UnsupportedOperationException` on mutation attempts.
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.
Stream.of("a").collect(Collectors.toList())
Why it's wrong here
Returns mutable list.
Collections.unmodifiableList(new ArrayList<>())
Why this is correct
Returns immutable view.
Related concept
Read the scenario before looking for a memorised answer.
Arrays.asList("a", "b")
Why it's wrong here
Returns fixed-size but mutable list.
new ArrayList<>()
Why it's wrong here
Mutable list.
List.of("a", "b")
Why this is correct
Returns immutable list.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates often confuse 'fixed-size' (as in `Arrays.asList()`) with 'immutable', or assume that `Collectors.toList()` returns an immutable list, when in fact it returns a mutable `ArrayList`.
Detailed technical explanation
Under the hood, `List.of()` returns an instance of `ImmutableCollections.ListN` or `List12`, which are private inner classes that store elements in a compact array and throw `UnsupportedOperationException` for all mutator methods. `Collections.unmodifiableList()` wraps the original list in a `UnmodifiableList` inner class that delegates read operations but throws exceptions on mutation; however, if the underlying list is still referenced elsewhere, it can be mutated through that reference, making it only 'view-level' immutable. In contrast, `List.of()` guarantees true immutability because the backing array is never exposed.
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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Working with Arrays and Collections — study guide chapter
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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<>()) — Option B is correct because `Collections.unmodifiableList()` returns a view of the specified list that cannot be structurally modified; any attempt to add, remove, or set elements will throw an `UnsupportedOperationException`. Option E is correct because `List.of()` (introduced in Java 9) directly creates an immutable list that disallows `null` elements and throws `UnsupportedOperationException` on mutation attempts.
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. Which of the following will compile without error and produce an unmodifiable list containing three elements?
hardWhy C: Option C is correct because `List.of("A","B","C")` returns an unmodifiable list containing exactly three elements, as specified by the Java 9+ static factory method. It compiles without error and any attempt to modify the list will throw an `UnsupportedOperationException`.
Last reviewed: Jun 11, 2026
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