Quick Answer
The answer is 4, because the ArrayList constructor that accepts a Collection copies each element from the source into a new, fully mutable ArrayList, even when the source is an immutable list returned by List.of(). When you write new ArrayList<>(List.of("A","B","C")), the fixed-size, unmodifiable list from List.of() is passed to the ArrayList copy constructor, which iterates over its elements and populates a new backing array that supports structural modification. This means the subsequent call to list.add("D") works without throwing an UnsupportedOperationException, and the list grows to size 4. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of the distinction between the immutable factory methods (List.of, Set.of, Map.of) and the mutable collection constructors—a classic trap is assuming that wrapping an immutable list somehow preserves immutability. Remember the copy-constructor rule: any Collection passed to a standard mutable constructor becomes a fresh, modifiable copy. Memory tip: "List.of gives you a frozen pizza; the ArrayList constructor bakes it into a fresh, sliceable pie."
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.
Given the code snippet: List<String> list = new ArrayList<>(List.of("A","B","C")); list.add("D"); System.out.println(list.size()); What is the result?
5
Why wrong: Only 4 elements are present.
UnsupportedOperationException
Why wrong: No exception because the ArrayList is mutable.
4
Correct: the new ArrayList is mutable, so add works, size is 4.
Compilation fails
Why wrong: The code compiles successfully.
3
Why wrong: The list has 4 elements after adding.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
4
Option C is correct because the code creates an ArrayList from a fixed-size list returned by List.of() using the ArrayList constructor, which creates a new mutable list containing the elements "A", "B", "C". The subsequent call to list.add("D") successfully appends the element, resulting in a list of size 4. The ArrayList constructor copies the elements, so the original immutable list is not modified.
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.
5
Why it's wrong here
Only 4 elements are present.
UnsupportedOperationException
Why it's wrong here
No exception because the ArrayList is mutable.
4
Why this is correct
Correct: the new ArrayList is mutable, so add works, size is 4.
Related concept
Read the scenario before looking for a memorised answer.
Compilation fails
Why it's wrong here
The code compiles successfully.
3
Why it's wrong here
The list has 4 elements after adding.
Common exam traps
The trap here is that candidates mistakenly think the list returned by List.of() is directly used, leading them to expect an UnsupportedOperationException, but the ArrayList constructor creates a separate mutable copy, so the add() succeeds.
Detailed technical explanation
The List.of() factory method (introduced in Java 9) returns an unmodifiable list, but the ArrayList constructor performs a shallow copy of its elements, creating a new, mutable list backed by a dynamic array. The add() method on ArrayList has amortized O(1) time complexity, and the size is tracked internally via the size field. This pattern is commonly used to initialize a mutable list from a fixed set of values while retaining the ability to modify it later.
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: 4 — Option C is correct because the code creates an ArrayList from a fixed-size list returned by List.of() using the ArrayList constructor, which creates a new mutable list containing the elements "A", "B", "C". The subsequent call to list.add("D") successfully appends the element, resulting in a list of size 4. The ArrayList constructor copies the elements, so the original immutable list is not modified.
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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