Quick Answer
The correct answer is that the code compiles and runs, with `arr[0]` becoming `"one"`. This happens because `ArrayList.toArray()` returns an `Object[]` array, not an `Integer[]`, regardless of the list's generic type. Since the variable `arr` is declared as `Object[]`, the assignment is perfectly valid, and because the array's runtime type is `Object[]`, storing a `String` at index 0 is allowed without any compilation or runtime error. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of type erasure and the distinction between compile-time generic checks and runtime array types. A common trap is assuming `toArray()` respects the generic parameter, but it always produces a plain `Object[]`. Memory tip: "toArray() returns an Object array—no generics, no surprises."
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 writes: ArrayList<Integer> list = new ArrayList<>(); list.add(1); list.add(2); list.add(3); Object[] arr = list.toArray(); arr[0] = "one"; What happens?
Compiles and runs, but arr[0] becomes "one"
The array is Object[], so assigning a String is fine.
Cannot cast String to Integer at runtime
Why wrong: No cast occurs; the array is Object[], so String is directly assigned.
Compilation error because toArray returns Integer[]
Why wrong: toArray() without argument returns Object[], not Integer[].
Runtime error
Why wrong: No runtime error because array is Object[] and can hold String.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Compiles and runs, but arr[0] becomes "one"
The `ArrayList.toArray()` method returns an `Object[]` array, not an `Integer[]`. Since the reference variable `arr` is declared as `Object[]`, the assignment is valid. Assigning a `String` to `arr[0]` is allowed because `Object[]` can hold any object type. No compilation or runtime error occurs because the array's runtime type is `Object[]`, and the assignment is type-safe at compile time and runtime.
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.
Compiles and runs, but arr[0] becomes "one"
Why this is correct
The array is Object[], so assigning a String is fine.
Related concept
Read the scenario before looking for a memorised answer.
Cannot cast String to Integer at runtime
Why it's wrong here
No cast occurs; the array is Object[], so String is directly assigned.
Compilation error because toArray returns Integer[]
Why it's wrong here
toArray() without argument returns Object[], not Integer[].
Runtime error
Why it's wrong here
No runtime error because array is Object[] and can hold String.
Common exam traps
The trap here is that candidates mistakenly think `toArray()` returns a typed array (e.g., `Integer[]`) because of the generic declaration, but without an argument it always returns `Object[]`, making the subsequent assignment to a String perfectly valid.
Detailed technical explanation
The `toArray()` method is defined in the `Collection` interface to return `Object[]` due to type erasure; the generic type `Integer` is not reified at runtime. If the developer wanted an `Integer[]`, they would need to call `toArray(new Integer[0])`, which returns an array of the specified type. This distinction is critical when working with legacy code or APIs that expect `Object[]`.
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
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 — study guide chapter
Learn the concepts, then practise the questions
Working with Arrays and Collections practice questions
Targeted practice on this topic area only
All 1Z0-829 questions
509 questions across all exam domains
Oracle Certified Professional Java SE 17 Developer 1Z0-829 study guide
Full concept coverage aligned to exam objectives
1Z0-829 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Practise 1Z0-829 questions linked to Handling Date, Time, Text, Numeric and Boolean Values.
Practise 1Z0-829 questions linked to Controlling Program Flow.
Practise 1Z0-829 questions linked to Utilizing Java Object-Oriented Approach.
Practise 1Z0-829 questions linked to Handling Exceptions.
Practise 1Z0-829 questions linked to Working with Arrays and Collections.
Practise 1Z0-829 questions linked to Working with Streams and Lambda Expressions.
Practise 1Z0-829 questions linked to Java Platform Overview and Packaging.
Practise 1Z0-829 questions linked to Java I/O API and Securing Applications.
Practise 1Z0-829 questions linked to 1Z0-829 fundamentals.
Practise 1Z0-829 questions linked to 1Z0-829 scenario.
Practise 1Z0-829 questions linked to 1Z0-829 troubleshooting.
Practice this exam
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
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: Compiles and runs, but arr[0] becomes "one" — The `ArrayList.toArray()` method returns an `Object[]` array, not an `Integer[]`. Since the reference variable `arr` is declared as `Object[]`, the assignment is valid. Assigning a `String` to `arr[0]` is allowed because `Object[]` can hold any object type. No compilation or runtime error occurs because the array's runtime type is `Object[]`, and the assignment is type-safe at compile time and runtime.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Last reviewed: Jun 25, 2026
This 1Z0-829 practice question is part of Courseiva's free Oracle certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 1Z0-829 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.