Quick Answer
The answer is [2, 4, 6]. The `replaceAll` method in Java, introduced in the `List` interface, applies a given `UnaryOperator` to every element in the list, replacing each element with the result of that operation. In this code, the lambda expression `x -> x * 2` multiplies each integer by 2, transforming the original list `[1, 2, 3]` into `[2, 4, 6]`. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of functional interfaces and default methods in the Collections Framework, specifically how `replaceAll` modifies the list in place rather than creating a new one. A common trap is confusing `replaceAll` with `replace` (which replaces a specific value) or forgetting that the lambda must match the `UnaryOperator<T>` functional interface. Memory tip: think "replaceAll applies the action to ALL elements, one by one, transforming them in place."
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.
What does the following code print? List<Integer> list = new ArrayList<>(List.of(1,2,3)); list.replaceAll(x -> x * 2); System.out.println(list);
[2,3,4]
Why wrong: Incorrect: not the result of doubling.
[2,4,6]
Correct: each element is doubled.
Compilation fails
Why wrong: The code compiles and runs.
[1,4,9]
Why wrong: Incorrect: wrong operation.
[1,2,3]
Why wrong: Incorrect: replaceAll modifies the 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
[2,4,6]
The `replaceAll` method on a `List` applies the given `UnaryOperator` to each element, replacing it with the result. Here, `x -> x * 2` multiplies each integer by 2, so the list becomes `[2, 4, 6]`. Option B is correct because the code compiles and runs, producing that output.
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.
[2,3,4]
Why it's wrong here
Incorrect: not the result of doubling.
[2,4,6]
Why this is correct
Correct: each element is doubled.
Related concept
Read the scenario before looking for a memorised answer.
Compilation fails
Why it's wrong here
The code compiles and runs.
[1,4,9]
Why it's wrong here
Incorrect: wrong operation.
[1,2,3]
Why it's wrong here
Incorrect: replaceAll modifies the list.
Common exam traps
The trap here is that candidates may think `replaceAll` doesn't exist on `List` or that it requires a `Comparator`, or they may confuse it with `replaceAll` on `Map` or `String`, leading them to incorrectly choose compilation failure (Option C).
Detailed technical explanation
Under the hood, `List.replaceAll` was introduced in Java 8 as a default method on the `List` interface. It uses an internal `ListIterator` to set each element to the result of the function, which is efficient for `ArrayList` (O(n) time). A subtle behavior: if the list is immutable (e.g., from `List.of`), calling `replaceAll` directly would throw `UnsupportedOperationException`, but wrapping it in `new ArrayList<>()` creates a mutable copy, avoiding that trap. In real-world scenarios, this is useful for in-place transformations like applying discounts to a list of prices.
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: [2,4,6] — The `replaceAll` method on a `List` applies the given `UnaryOperator` to each element, replacing it with the result. Here, `x -> x * 2` multiplies each integer by 2, so the list becomes `[2, 4, 6]`. Option B is correct because the code compiles and runs, producing that output.
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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Last reviewed: Jun 25, 2026
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