Answer: 15 Autoboxing converts int 7 and 8 into Integer objects when added. Auto-unboxing converts them back to int when adding, producing 15.

Answer: 0 0 The list is empty so the for-each loop never executes, sum stays 0, and size is 0.

Answer: 15 The loop adds 1, 2, 3, 4, 5 to the list. Summing them with the for-each loop gives 1+2+3+4+5 = 15. Autoboxing converts each int to an Integer when adding.

AP Computer Science A · Topic 7.1

Introduction to ArrayList Practice

Part of ArrayList.

Practice questions

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Sample questions

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Sample 1difficulty 2/5
```
ArrayList<Integer> list = new ArrayList<>();
list.add(7);
list.add(8);
int x = list.get(0) + list.get(1);
System.out.println(x);
```
$What is printed?$
- A
  $Compile-time error$
- B
  $7$
- C
  $15$
  check_circle
- D
  $78$
Why
$Autoboxing converts int 7 and 8 into Integer objects when added. Auto-unboxing converts them back to int when adding, producing 15.$
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Sample 2difficulty 2/5
```
ArrayList<Integer> list = new ArrayList<>();
int sum = 0;
for (int x : list) sum += x;
System.out.println(sum + " " + list.size());
```
$What is printed?$
- A
  $Runtime error$
- B
  $0 1$
- C
  $1 0$
- D
  $0 0$
  check_circle
Why
$The list is empty so the for-each loop never executes, sum stays 0, and size is 0.$
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Sample 3difficulty 2/5
```
ArrayList<Integer> list = new ArrayList<>();
for (int i = 1; i <= 5; i++) list.add(i);
int s = 0;
for (int x : list) s += x;
System.out.println(s);
```
$What is printed?$
- A
  $Compile-time error$
- B
  $5$
- C
  $10$
- D
  $15$
  check_circle
Why
$The loop adds 1, 2, 3, 4, 5 to the list. Summing them with the for-each loop gives 1+2+3+4+5 = 15. Autoboxing converts each int to an Integer when adding.$
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Sample 4difficulty 2/5
$Which is a correct ArrayList declaration?$
- A
  $ArrayList list = new ArrayList(int);$
- B
  $ArrayList <Integer> list = new ArrayList<>();$
  check_circle
- C
  $ArrayList <int> list = new ArrayList <int> ();$
- D
  $ArrayList <int> list = new ArrayList<>();$
Why
$Generic type parameter must be a reference type; use <code> Integer </code> (not <code> int </code>).$
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Sample 5difficulty 3/5
$<code> ArrayList <int> </code> is$
- A
  $Compile error (can only use reference types)$
  check_circle
- B
  $Valid (will autobox)$
- C
  $Slow but works$
- D
  $Compiles only with --enable-experimental$
Why
$Generic type parameters must be reference types; use Integer, Double, etc.$
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