Which is a reference (object) type in Java?

Answer: String <codeString</code is a class. Objects are stored by reference.

What happens when this code runs?

Answer: NullPointerException because names[0] is null A new String[] is filled with null references. Calling .length() on null throws NullPointerException.

AP Computer Science A · Topic 2.1

Objects: Instances of Classes Practice

Q: Which is a reference (object) type in Java?

Answer: String <codeString</code is a class. Objects are stored by reference.

Q: What is printed?

Answer: 42 a and b refer to the same object; modifying via b is visible via a.

Q: What happens when this code runs?

Answer: NullPointerException because names[0] is null A new String[] is filled with null references. Calling .length() on null throws NullPointerException.

Part of Using Objects.

Practice questions

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Sample questions

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Sample 1difficulty 2/5
$Which is a reference (object) type in Java?$
- A
  $boolean$
- B
  $String$
  check_circle
- C
  $int$
- D
  $double$
Why
$<code> String </code> is a class. Objects are stored by reference.$
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Sample 2difficulty 3/5

public class Holder {
    private int n;
    public Holder(int n) { this.n = n; }
    public int getN() { return n; }
    public void setN(int n) { this.n = n; }
}
// ...
Holder a = new Holder(1);
Holder b = a;
b.setN(42);
System.out.println(a.getN());

What is printed?

A
$0$
B
$1$
C
$42$
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D
$Compile-time error$

Why

a and b refer to the same object; modifying via b is visible via a.

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Sample 3difficulty 3/5
```
String[] names = new String[3];
System.out.println(names[0].length());
```
$What happens when this code runs?$
- A
  $NullPointerException because names[0] is null$
  check_circle
- B
  $Prints 0$
- C
  $Prints empty string$
- D
  $Compile error$
Why
$A new String[] is filled with null references. Calling .length() on null throws NullPointerException.$
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Sample 4difficulty 3/5
$Calling a method on a <code> null </code> reference causes$
- A
  $Returns null$
- B
  $Compile error$
- C
  $Returns 0$
- D
  $Throws NullPointerException at runtime$
  check_circle
Why
$<code> String s = null; s.length(); </code> \to NPE at runtime.$
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Sample 5difficulty 3/5
$What is the default value for an uninitialized object reference (instance field)?$
- A
  $Empty string$
- B
  $null$
  check_circle
- C
  $false$
- D
  $0$
Why
$Reference types default to <code> null </code> .$
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