Exercise 6 w/ Solution

Advanced Class Features
Exercise #6: Use Interfaces and Abstract Classes
Objective
In this exercise you will create a hierarchy of animals that is rooted in an abstract class
Animal. Several of the animal classes will implement an interface called Pet. You will
experiment with variations of these animals, their methods, and polymorphism.
Directions
1. Create the Animal class, which is the abstract superclass of all animals.
1. Declare a protected integer attribute called legs, which records the
number of legs for this animal.
2. Define a protected constructor that initializes the legs attribute.
3. Declare an abstract method eat.
4. Declare a concrete method walk that prints out something about how the
animals walks (include the number of legs).
2. Create the Spider class.
1. The Spider class extends the Animal class.
2. Define a default constructor that calls the superclass constructor to specify
that all spiders have eight legs.
3. Implement the eat method.
3. Create the Pet interface specified by the UML diagram.
4. Create the Cat class that extends Animal and implements Pet.
1. This class must include a String attribute to store the name of the pet.
2. Define a constructor that takes one String parameter that specifies the
cat's name. This constructor must also call the superclass constructor to
specify that all cats have four legs.
3. Define another constructor that takes no parameters. Have this constructor
call the previous constructor (using the this keyword) and pass an empty
string as the argument.
4. Implement the Pet interface methods.
5. Implement the eat method.
5. Create the Fish class. Override the Animal methods to specify that fish can't walk
and don't have legs.
6. Create an TestAnimals program. Have the main method create and manipulate
instances of the classes you created above. Start with:
Fish d = new Fish();
Cat c = new Cat("Fluffy");
Animal a = new Fish();
Animal e = new Spider();
Pet p = new Cat();
Experiment by: a) calling the methods in each object, b) casting objects, c) using
polymorphism, and d) using super to call super class methods.

SOLUTION:

animal:

public abstract class Animal{
    protected int legs;
    protected Animal(int legs){
    this.legs=legs;
    }       
    public abstract void eat();
    public void walk()
    {
    System.out.println("Animals can walk.");
    }
}

cat:

public class Cat extends Animal implements Pet{
    public String name;
    public Cat(String name)
    {
        super(4);
        this.name=name;
    }
    public Cat()
    {
        super(4);
    }
    public String getName()
    {
        return name;
    }
    public void setName(String name)
    {
        this.name=name;
    }
    public void play(){
    }
    public void eat(){
    }
           
}

fish:

public class Fish extends Animal implements Pet{
    public String name;
    public Fish()
    {
        super(0);
    }   
    public String getName()
    {
        return name;
    }   
    public void setName(String name)
    {
        this.name=name;
    }   
    public void play(){
    }   
    public void walk()
    {
    System.out.println("Fish can't walk.");
    }
    public void eat(){
    }

}

pet:

public interface Pet{
public String getName();
public void setName(String name);
public void play();
    }

spider:

public class Spider extends Animal{
    Spider()
    {
        super(8);
    }
    public void eat()
    {
        System.out.print("Spiders can eat");
    }

}

TestAnimal:

public class TestAnimals{
    public static void main(String[] args)
    {
    Fish d=new Fish();
    Cat c=new Cat("Fluffy");
    Animal a=new Fish();
    Animal e=new Spider();
    Pet p=new Cat();
    System.out.println("Living in water");
    a.walk();   
    }
}

---end.