<codeclass Dog extends Animal { }</code declares

Answer: Dog as a subclass of Animal <codeextends</code indicates inheritance: Dog inherits from Animal.

Answer: 10 x is protected, so Child can access it directly. x = 5, so doubleX() returns 10.

Answer: Compile error: Animal is abstract; cannot be instantiated Abstract classes cannot be directly instantiated with new. The compiler reports an error.

AP Computer Science A · Topic 9.1

Creating Superclasses and Subclasses Practice

Q: <codeclass Dog extends Animal { }</code declares

Answer: Dog as a subclass of Animal <codeextends</code indicates inheritance: Dog inherits from Animal.

Q: What is printed?

Answer: 10 x is protected, so Child can access it directly. x = 5, so doubleX() returns 10.

Q: What occurs?

Answer: Compile error: Animal is abstract; cannot be instantiated Abstract classes cannot be directly instantiated with new. The compiler reports an error.

Part of Inheritance.

Practice questions

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Sample questions

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Sample 1difficulty 2/5
$<code> class Dog extends Animal { } </code> declares$
- A
  $Dog as a parent of Animal$
- B
  $Dog as a subclass of Animal$
  check_circle
- C
  $Animal containing a Dog$
- D
  $Compile error$
Why
$<code> extends </code> indicates inheritance: Dog inherits from Animal.$
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Sample 2difficulty 2/5
```
public class Parent {
    protected int x = 5;
}
public class Child extends Parent {
    public int doubleX() { return x * 2; }
}
// Usage:
Child c = new Child();
System.out.print(c.doubleX());
```
$What is printed?$
- A
  $0$
- B
  $Compile error: x is private$
- C
  $5$
- D
  $10$
  check_circle
Why
$x is protected, so Child can access it directly. x = 5, so doubleX() returns 10.$
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Sample 3difficulty 2/5
```
public abstract class Animal {
    public abstract String sound();
}
// Usage:
Animal a = new Animal();
```
$What occurs?$
- A
  $Compiles fine$
- B
  $Compiles and creates an Animal with null sound$
- C
  $Compile error: Animal is abstract; cannot be instantiated$
  check_circle
- D
  $Runtime error when sound() is called$
Why
$Abstract classes cannot be directly instantiated with new. The compiler reports an error.$
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Sample 4difficulty 2/5
```
public class A {
    public int doubleIt(int n) { return n * 2; }
}
public class B extends A { }
System.out.println(new B().doubleIt(7));
```
$What is printed?$
- A
  $Compile-time error$
- B
  $7$
- C
  $14$
  check_circle
- D
  $0$
Why
$B inherits doubleIt from A.$
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Sample 5difficulty 2/5
```
public class Engine { }
public class Car {
    private Engine e;
    public Car() { e = new Engine(); }
}
```
$What relationship does Car have with Engine?$
- A
  $has-a (composition)$
  check_circle
- B
  $is-a (inheritance)$
- C
  $neither$
- D
  $is-a abstract$
Why
$Car CONTAINS an Engine as a field — has-a (composition). is-a would require <code> Car extends Engine </code> .$
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