Working with Arrays and Collections →hardMultiple ChoiceObjective-mapped

1Z0-829 Working with Arrays and Collections Practice Question

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?

⚠ Common exam trap

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.

Answer choices

Why each option matters

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.

Answer analysis

Option-by-option breakdown

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.
✗
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.

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1Z0-829 Working with Arrays and Collections Practice Question

Why each option matters

Option-by-option breakdown

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