Question

PRG/421 Week One Analyze Assignment –

Analyzing a Java™Program Containing Abstract and Derived Classes

1.    What is the output of the program as it is written? (Program begins on p. 2)

2. Why would a programmer choose to define a method in an abstract class (such as the Animal constructor method or the getName()method in the code example) vs. defining a method as abstract (such as the makeSound()method in the example)?

/**********************************************************************

*           Program:          PRG/421 Week 1 Analyze Assignment

*           Purpose:         Analyze the coding for an abstract class

*                                   and two derived classes, including overriding methods

*           Programmer:    Iam A. Student

*           Class:               PRG/421r13, Java Programming II

* Comments:

* Notice that in the abstract Animal class shown here, one method is

* concrete (the one that returns an animal's name) because all animals can

* be presumed to have a name. But one method, makeSound(), is declared as

* abstract, because each concrete animal must define/override the makeSound() method

* for itself--there is no generic sound that all animals make.

/**********************************************************************

package mytest;

// Animal is an abstract class because "animal" is conceptual

// for our purposes. We can't declare an instance of the Animal class,

// but we will be able to declare an instance of any concrete class

// that derives from the Animal class.

abstract class Animal {

// All animals have a name, so store that info here in the superclass.

// And make it private so that other programmers have to use the

// getter method to access the name of an animal.

private final String animalName;

// One-argument constructor requires a name.

public Animal(String aName) {

animalName = aName;

}

// Return the name of the animal when requested to do so via this

// getter method, getName().

public String getName() {

return animalName;

}

// Declare the makeSound() method abstract, as we have no way of knowing

// what sound a generic animal would make (in other words, this

// method MUST be defined differently for each type of animal,

// so we will not define it here--we will just declare a placeholder

// method in the animal superclass so that every class that derives from

// this superclass will need to provide an override method

// for makeSound()).

public abstract String makeSound();           

};

// Create a concrete subclass named "Dog" that inherits from Animal.

// Because Dog is a concrete class, we can instantiate it.

class Dog extends Animal {

// This constructor passes the name of the dog to

// the Animal superclass to deal with.

public Dog(String nameOfDog) {

super(nameOfDog);

}

// This method is Dog-specific.

@Override

public String makeSound() {

return ("Woof");

}

}

// Create a concrete subclass named "Cat" that inherits from Animal.

// Because Cat is a concrete class, we can instantiate it.

class Cat extends Animal {

// This constructor passes the name of the cat on to the Animal

// superclass to deal with.

public Cat(String nameOfCat) {

super(nameOfCat);

}

// This method is Cat-specific.

@Override

public String makeSound() {

return ("Meow");

}

}

class Bird extends Animal {

// This constructor passes the name of the bird on to the Animal

// superclass to deal with.

public Bird (String nameOfBird) {

super(nameOfBird);

}

// This method is Bird-specific.

@Override

public String makeSound() {

return ("Squawk");

}

}

public class MyTest {

public static void main(String[] args) {

// Create an instance of the Dog class, passing it the name "Spot."  

// The variable aDog that we create is of type Animal.

Animal aDog = new Dog("Spot");

// Create an instance of the Cat class, passing it the name "Fluffy."  

// The variable aCat that we create is of type Animal.

Animal aCat = new Cat("Fluffy");

// Create an instance of (instantiate) the Bird class.

Animal aBird = new Bird("Tweety");

//Exercise two different methods of the aDog instance:

// 1) getName() (which was defined in the abstract Animal class)

// 2) makeSound() (which was defined in the concrete Dog class)

System.out.println("The dog named " + aDog.getName() + " will make this sound: " + aDog.makeSound());

//Exercise two different methods of the aCat instance:

// 1) getName() (which was defined in the abstract Animal class)

// 2) makeSound() (which was defined in the concrete Cat class)

System.out.println("The cat named " + aCat.getName() + " will make this sound: " + aCat.makeSound());

System.out.println("The bird named " + aBird.getName() + " will make this sound: " + aBird.makeSound());

}

}

Answer 1