Creating References Using Inheritance Hierarchies Practice

Sample questions

5 of 28 — sign in to practice the rest with adaptive difficulty and mastery tracking.

1. Sample 1difficulty 2/5

   public class Animal {}
   public class Dog extends Animal {}
   public class Cat extends Animal {}
   // Usage:
   Animal a = new Dog();
   System.out.print(a instanceof Dog);
   System.out.print(" ");
   System.out.print(a instanceof Animal);
   System.out.print(" ");
   System.out.print(a instanceof Cat);

   What is printed?

   • A

     false true false

   • B

     true true true

   • C

     true false false

   • D

     true true false

     check_circle

   Why

   a refers to a Dog. instanceof Dog is true; instanceof Animal is true (Dog is-a Animal); instanceof Cat is false (Dog is not a Cat).

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2. Sample 2difficulty 2/5

   public abstract class Vehicle {
       public abstract int wheels();
   }
   public class Bike extends Vehicle {
       public int wheels() { return 2; }
   }
   // Usage:
   Vehicle v;
   v = new Bike();
   System.out.print(v.wheels());

   What is printed?

   • A

     Runtime error

   • B

     Compile error: cannot declare Vehicle reference

   • C

     0

   • D

     2

     check_circle

   Why

   Abstract classes can be used as REFERENCE types even though they cannot be instantiated. v refers to a Bike, and dispatch returns 2.

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3. Sample 3difficulty 2/5

   public class Animal {}
   public class Dog extends Animal {
       public void bark() { System.out.println("Woof"); }
   }
   Animal a = new Dog();
   a.bark();

   What happens?

   • A

     Compile-time error: bark not in Animal

     check_circle
   • B

     Prints Woof

   • C

     Runtime error

   • D

     Prints nothing

   Why

   Method calls are checked against the declared (compile-time) type. Animal has no <code>bark</code>, so this fails to compile even though the object is a Dog.

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4. Sample 4difficulty 2/5

   public class Animal {}
   public class Cat extends Animal {}
   Animal a = new Cat();
   System.out.println(a instanceof Cat);
   System.out.println(a instanceof Animal);

   What is printed?

   • A

     true true

     check_circle
   • B

     true false

   • C

     false false

   • D

     false true

   Why

   The actual object is a Cat, which is also an Animal. Both checks return true.

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5. Sample 5difficulty 3/5

   public class Item {}
   public class Book extends Item {
       public String title() { return "Book"; }
   }
   public class Toy extends Item {
       public String name() { return "Toy"; }
   }
   public static String describe(Item i) {
       if (i instanceof Book) return ((Book) i).title();
       if (i instanceof Toy) return ((Toy) i).name();
       return "?";
   }
   // Usage:
   System.out.print(describe(new Toy()));

   What is printed?

   • A

     Book

   • B

     ?

   • C

     ClassCastException

   • D

     Toy

     check_circle

   Why

   instanceof Book is false; instanceof Toy is true; the cast succeeds and name() returns "Toy".

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