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

Runnable r = new Runnable();

Answer

Thread r = new Runnable();

Answer

interface r = new Thread();

Question 2

The Comparable interface in Java requires implementing which method?

Accepted Answer

compareTo(Object o)

Answer

equals(Object o)

Answer

hashCode()

Answer

toString()

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

Document must implement print() immediately

Answer

Document cannot implement Printable because it is abstract

Answer

Document automatically gets a default print() from Printable

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

You want to share fields (instance variables) across related classes

Answer

You want to provide a partial default implementation

Answer

You want to prevent direct instantiation with a constructor

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

Animal says meow

Answer

Whiskers says null

Answer

A compile-time error

Question 6

In AP CSA, when a method parameter is declared with an interface type, what objects can be passed to it?

Accepted Answer

Any object whose class implements that interface

Answer

Only objects of the exact interface type

Answer

Only objects of the first class that implemented that interface

Answer

Only abstract class instances that implement that interface

Question 7

Consider two interfaces: `interface Flyable { void fly(); }` and `interface Swimmable { void swim(); }`. A class Duck implements both. Which statement about Duck is correct?

Accepted Answer

Duck can be assigned to variables of type Flyable, Swimmable, or Duck

Answer

Duck can only be referred to as a Flyable or Swimmable, never as a Duck

Answer

Duck must be declared abstract because it implements two interfaces

Answer

Java does not allow a class to implement two interfaces