AP CSA Interfaces and Abstract Classes 3

Question 1

Which of the following correctly demonstrates polymorphism using an interface reference?

Accepted Answer

Runnable r = new Thread();

Answer

Because Thread implements Runnable, a Runnable reference variable can hold a Thread object, demonstrating interface-based polymorphism.

Question 2

The Comparable interface in Java requires implementing which method?

Accepted Answer

compareTo(Object o)

Answer

The Comparable interface requires implementing `compareTo(Object o)`, which returns a negative, zero, or positive integer based on ordering.

Question 3

Given `interface Printable { void print(); }` and `abstract class Document implements Printable { }`, which statement is TRUE about Document?

Accepted Answer

Document can leave print() unimplemented since it is abstract

Answer

An abstract class is not required to implement inherited abstract interface methods; it can defer that responsibility to its concrete subclasses.

Question 4

Which of the following is a valid reason to choose an interface over an abstract class?

Accepted Answer

You want a class to inherit behavior from multiple unrelated types

Answer

Interfaces support a form of multiple inheritance of type, allowing a class to fulfill contracts from multiple unrelated types — something abstract classes cannot do.

Question 5

What does the following code print?

abstract class Animal {
  String name;
  Animal(String n) { name = n; }
  abstract String sound();
  public String toString() { return name + " says " + sound(); }
}
class Cat extends Animal {
  Cat(String n) { super(n); }
  String sound() { return "meow"; }
}
// In main:
Animal a = new Cat("Whiskers");
System.out.println(a);

Accepted Answer

Whiskers says meow

Answer

The Cat constructor passes "Whiskers" to the Animal constructor via super(n), and the overridden sound() returns "meow", so toString() produces "Whiskers says meow".

AP CSA - Advanced Placement Computer Science A Practice Test

AP CSA - Advanced Placement Computer Science A Practice Test

AP CSA Interfaces and Abstract Classes 3