Question

Assume that Animal extends Object (and assume Animal is an abstract class, no other abstract classes) and that

FarmAnimal extends Animal

Cow extends FarmAnimal

Chicken extends FarmAnimal

Assume all the above classes have a default constructor.

Which of these statements compile?

Animal a1 = new Animal(); ______
Animal a2 = new Cow(); ______
Cow a3 = new Animal(); ______
Cow a4 = new FarmAnimal(); ______
FarmAnimal a5 = new Cow(); ______
Cow a6 = new Cow(); ______
Cow a7 = new Chicken(); ______
Chicken a8 = new Cow(); _____
Chicken a9 = new FarmAnimal(); ______

Answer 1

Animal a1 = new Animal(); Animal a2 = new Cow(); FarmAnimal a5 = new Cow();

Answer 2

*assuming this is Java and the rules of polymorphism is the question

Answer 3

http://docs.oracle.com/javase/tutorial/java/IandI/polymorphism.html This tutorial should help clear up any confusion

Answer 4

Animal a1 = new Animal(); doesn't compile! Animal is an abstract class and can't be instantiated.

Animal a1 = new Animal(); Does not compile, Animal is abstract
Animal a2 = new Cow(); Compiles, Cow is a subclass of Animal and is not abstract
Cow a3 = new Animal(); Does not compile, Animal is abstract
Cow a4 = new FarmAnimal(); Does not compile, FarmAnimal isn't a subclass of cow (it's superclass)
FarmAnimal a5 = new Cow(); Compiles, Cow is a subclass of FarmAnimal
Cow a6 = new Cow(); Compiles
Cow a7 = new Chicken(); Does not compile, Chicken isn't a subclass of Cow
Chicken a8 = new Cow(); Does not compile, Cow isn't a subclass of Chicken
Chicken a9 = new FarmAnimal(); Does not compile, FarmAnimal isn't a subclass of Chicken