Polymorphic behavior provided by inheritance programming assignment

[Joniela Geldenhuys]

Hey guys can someone please help me!

this is what the assignment want from me

1. Create an abstract base class, Ball. the single constructor requires a String to indicate the type of ball, which then is stored in an instance variable. The instance method getType(), returns a new String containing the type of ball. the class declares an abstract method play() which returns void
2. Create an abstract class Bounceable which inherits from Ball. Bouncealbe has a single abstract method, bounce() which returns a String there are no instance varibles
3. create 2 concrete classes Baseball and Bowlingball which inherit from Ball. No instance variables or additional methods are required only implementation of any inherited abstract methods
4. create 2 concrete classes Tennisball and Basketball which inherit from Bounceable. No instance variables or additonal methods are required only implementation of any inherited abstract methods

The class PolyTest source code they gave me
Here is all my classes

public abstract class Ball
{
	public String type;

	public Ball(String type)
	{
		this.type=type;
	}

	abstract public void play();

	public String getType()
	{
		return type;
	}
}

public class Baseball extends Ball
{
	public Baseball()
	{
		super("Baseball");
	}

	public void play()
	{
		System.out.println("The American pasttime - batter up!");
		}
}

public class Basketball extends Bounceable
{
	public Basketball()
	{
		super("Basketball");
	}

	public void play()
	{
		System.out.println("Basketball begins with a tipoff.");
	}
	//the bounce method returns the string
	public String bounce()
	{
		String bounce = "dribble...";
		return bounce;
	}
}

public abstract class Bounceable extends Ball
{
	public Bounceable()
	{
		super("Bounceable");
	}

	abstract public void bounce();
}

public class Bowlingball extends Ball
{
	public Bowlingball()
	{
		super("Bowlingball");
	}

	public void play()
	{
		System.out.println("Set up the ons. Do you know how to score?");
	}
}

public class Tennisball extends Bounceable
{
	public Tennisball()
	{
		super("Tennisball");
	}
	public void play()
	{
		System.out.println("All set for a match? I love this game!");
	}
	public String bounce()
	{
		String bounce = "Boing!";
		return bounce;
	}
}

public class PolyTest
{
	public static void main(String[] args)
	{
		Ball[] ball = {new Baseball(),new Basketball(),new Bowlingball(),new Tennisball()};

		System.out.println("For each type of ball...\n");

		for(int i =0; i < ball.length; ++i)
		{
			System.out.println("Ball #" + (i+1)+" is a " +ball[i].getType());

			System.out.print("Playing:  ");
			ball[i].play();
			//checks to see if it is an object of type Bounceable
			if( ball[i] instanceof Bounceable)
			// if so, call bounce method of Bounceable object
			System.out.println("Bouncing " +((Bounceable)ball[i]).bounce());
			else
			System.out.println("You can't bounce this ball!");

			System.out.println("\n");
		}
		System.out.println("\nEnd of program");
	}
}

These are the errors

PolyTest.java: void type not allowed here
system.out.println("Ball #" +(i+1)+ "is a "+ball[i].getType());

Basketball.java: Basketball is not an abstract and does not override abstract method bounce() in Bounceable
public class Basketball extends Bounceable

Basketball.java: bounce() in Basketball cannot override bounce() in Bounceable; attempting to use incompatible return type
found: java.lang.string
required: void
public String bounce()

Basketball.java: Cannot find symbol
symbol: Constructor Bounceable(java.lang.string)
location: class Bounceable
super("Basketball");

Tennisball.java: Tennisball is not an abstract and does not override abstract method bounce() in Bounceable
public class Tennisball extends Bounceable

Tennisball.java: bounce() in Tennisball cannot override bounce() in Bounceable; attepmting to use incompatible return type
found: java.lang.string
required: void
public String bounce()

Tennisball.java: Cannot find symbol
Symbol: Constructor Bounceable(java.lang.string)
Location: class Bounceable
super("Tennisball");

Please tell me what I'm doing wrong

[Winston Gutkowski]

Joniela Geldenhuys wrote:Please tell me what I'm doing wrong

When you get a compiler error you need to read it very carefully and then look at your code. The messages are usually quite specific. For example:
Basketball.java: bounce() in Basketball cannot override bounce() in Bounceable; attempting to use incompatible return type
found: java.lang.string
required: void
public String bounce()
is telling you that there is an incompatibility between the method bounce() in Basketball, and the one in Bounceable. Have a look at both methods carefully. Do you see the problem now? If not, come back; otherwise work through each message with the same care.

Nobody ever said programming was easy .

Winston

[Harsha Smith]

change the return type of the method bounce to String. insert a constructor that takes a String as argument in the Bounceable class.

 abstract class Bounceable extends Ball  
 {  
     public Bounceable(String type)  
     {  
         super(type);  
     }  
   
     abstract public String bounce();  
 }  

[Rob Spoor]

Joniela Geldenhuys wrote:PolyTest.java: void type not allowed here
system.out.println("Ball #" +(i+1)+ "is a "+ball[i].getType());

I don't get that error.

Basketball.java: Basketball is not an abstract and does not override abstract method bounce() in Bounceable
public class Basketball extends Bounceable

Basketball.java: bounce() in Basketball cannot override bounce() in Bounceable; attempting to use incompatible return type
found: java.lang.string
required: void
public String bounce()

These two are related. Bounceable specifies method public void bounce(), yet Basketball has method public String bounce(). The return types must match. Just change the void into String in Bounceable.

Basketball.java: Cannot find symbol
symbol: Constructor Bounceable(java.lang.string)
location: class Bounceable
super("Basketball");

Class Bounceable has no constructor that takes a String. Add one:

public Bounceable(String type)
{
    super(type);
}

Tennisball.java: Tennisball is not an abstract and does not override abstract method bounce() in Bounceable
public class Tennisball extends Bounceable

Tennisball.java: bounce() in Tennisball cannot override bounce() in Bounceable; attepmting to use incompatible return type
found: java.lang.string
required: void
public String bounce()

Tennisball.java: Cannot find symbol
Symbol: Constructor Bounceable(java.lang.string)
Location: class Bounceable
super("Tennisball");

The same issues as with Basketball.

After you've solved these errors there is a bit of advice I would like to give. I think that Bounceable doesn't belong into this hierarchy. It seems better to make it an interface:

public interface Bounceable
{
    // no constructors allowed

    public String bounce(); // automatically abstract
}

Basketball and Tennisball then only need minimal change:

public class Basketball extends Ball implements Bounceable

public class Tennisball extends Ball implements Bounceable

The rest of your classes can remain the same. And as a bonus, you can create a class BouncingCastle that also implements Bounceable without having to be a Ball!