Storing objects polymorphically in a Figure array

[Luke Stamper]

This is a homework I am having trouble on...We were given the base class by our professor and instructed to only worry about a circle and rectangle.

"In your derived classes, only implement draw and erase – not center. A rectangle needs length, height and position (x,y). A circle needs a radius and position. You are not going to actually draw anything – erase and draw should just write out that you are in them. Add all necessary additional constructors, variables and methods in the derived classes. Call super to get the base class constructor.

Write a test program to test your classes. Your test program must store your objects polymorphically in a Figure array and call center from each element. Your code should look like this for the polymorphic section:

         Figure[] f = new Figure[2];
   	  f[0] = c;
   	  f[1] = r;
   	  for(int i = 0; i < 2; i++ )
   	  	f[i].center();

Notice that we can store any derived type in a base class array! This means that no matter how many shapes we have, refreshing is simply:

for(int i=0 ; i <   num; i++) shapes[i].draw();

Here is sample output given by my professor...

Creating a rectangle.
R1 position (4,25) height = 5,width = 10
Creating a circle.
C1 position (5,5) radius = 44

Polymorphic Output:
In Figure, calling erase, draw
In Circle - doing erase
In Circle - doing draw
In Figure, calling erase, draw
In Rectangle - doing erase
In Rectangle - doing draw"



So far, my code gives me 2 compiler errors...

--------------------Configuration: <Default>--------------------
F:\Object Java\Rectangle.java:10: error: Rectangle is not abstract and does not override abstract method erase() in Figure
public class Rectangle extends Figure{
^
F:\Object Java\Circle.java:10: error: Circle is not abstract and does not override abstract method erase() in Figure
public class Circle extends Figure{
^
2 errors


Additionally, I am stuck on the idea of "Notice that we can store any derived type in a base class array! This means that no matter how many shapes we have, refreshing is simply: for(int i=0 ; i < num; i++) shapes[i].draw();.

I am also having trouble deciphering how to obtain the same output given by my professor.


Could anyone point me in the right direction or help me with some pseudo code so I can figure this out?

public abstract class Figure{
	private int X, Y;
    private String name;

	public Figure(String name, int X, int Y){
	   X = 0;
	   Y = 0;
       name = " ";
	}

	public abstract void draw();
        public abstract void erase();
        public void center(){
    	   System.out.println("In Figure, calling erase, draw");
    	   erase();
    	   draw();
    }
}


public class Circle extends Figure{

public double radius;

public Circle(){
	super("position", 5, 4);
}

public Circle(int r){
    super("position", 5, 4);
	setRadius(r);
    }

public void setRadius(int r){radius = r;}
public double getRadius(){return radius;}

    
}



public class Rectangle extends Figure{

public int length;
public int height;

public Rectangle(){
	super("position", 8, 7);
}

public Rectangle(int l, int h){
    super("position", 8, 7);
	setLength(l);
	setHeight(h);
    }

public void setLength(int l){length = l;}
public int getLength(){return length;}

public void setHeight(int h){height = h;}
public int getHeight(){return height;}

}



public class TestFigure {

    public static void main(String[] args) {

	Rectangle r1 = new Rectangle(10, 8);
	Circle c1 = new Circle(10);

	Figure[] f = new Figure[2];
   	  f[0] = c1;
   	  f[1] = r1;
   	  for(int i = 0; i < 2; i++ )
   	  	f[i].center();

   	Figure[] shapes = new Figure[2];
   	  for(int i=0 ; i < 2; i++)shapes[i].draw();


    }
}

[John Jai]

F:\Object Java\Rectangle.java:10: error: Rectangle is not abstract and does not override abstract method erase() in Figure 
public class Rectangle extends Figure{ 
^ 
F:\Object Java\Circle.java:10: error: Circle is not abstract and does not override abstract method erase() in Figure 
public class Circle extends Figure{ 
^ 
2 errors 

The errors are self explanatory. Any class that extends an abstract class should implement the abstract methods. So you have to implement the draw() and erase() methods in the sub classes Rectangle and Circle.

Read the tutorial - Abstract Methods and classes

[John Jai]

Additionally, I am stuck on the idea of "Notice that we can store any derived type in a base class array! This means that no matter how many shapes we have, refreshing is simply: for(int i=0 ; i < num; i++) shapes[i].draw();.

This sentence refers to polymorphism. The shapes array variable is of the type Figure and hence can refer either a Rectangle or a Circle. So when you call the draw() method on the base reference (shape variable) you in turn call the respective draw() methods of the corresponding sub class object it refers to.

In this case either a Rectangle or a Circle. So the code is written generic to call the draw() method and you don't need to say whether it's Rectangle's draw() or Circle's draw() method. It gets detected during run time.

And note that draw() method is declared as abstract in the base Figure class. That guarantees you to implement a sub class specific draw() method in the Rectangle and the Circle class.

[Luke Stamper]

I added these to each of my subclasses (one for rectangle, one for draw).

public void draw(){
	System.out.println("In Rectangle - Doing draw");
}

public void erase(){
	System.out.println("In Rectangle - Doing erase");
}

This was my output...

--------------------Configuration: <Default>--------------------
In Figure, calling erase, draw
In Circle - Doing erase
In Circle - Doing draw
In Figure, calling erase, draw
In Rectangle - Doing erase
In Rectangle - Doing draw
Exception in thread "main" java.lang.NullPointerException
at TestFigure.main(TestFigure.java:24)

Process completed.

I'm not understanding how to call my circle/rectangle subclasses to print out position & radius for circle and position, height, & length for rectangle. If I try and do something like

System.out.println(r1);

it comes out like this...

Rectangle@1fee6fc

[Paul Clapham]

That's the sort of thing you get if you don't override the toString() method in your classes. If you have to have your Rectangle object be output in a specific way, then provide it with a toString() method which returns a suitable String.

[Luke Stamper]

I'm almost done. The idea is that the the new rectangle and circle will call the parameters from the super class and I will add the remaining parameters when I create a new object. Can anyone help?

public abstract class Figure{
	private int X, Y;
    private String name;

	public Figure(){
	   X = 0;
	   Y = 0;
       name = " ";
	}

	public Figure(String n, int x, int y) {
		this.X = x;
		this.Y = y;
		this.name = n;
	}

	public String getName() {
        return name;
    }

    public int getX() {
        return X;
    }

    public int getY() {
        return Y;
    }

	public abstract void draw();
        public abstract void erase();
        public void center(){
    	   System.out.println("In Figure, calling erase, draw");
    	   erase();
    	   draw();
    }
}

public class Circle extends Figure{

private double radius;

public Circle(String name, int x, int y, int radius){
    super("C1", 5, 5);
	this.setRadius(radius);
    }

public void setRadius(int r){this.radius = r;}
public double getRadius(){return this.radius;}

public int getSuperX()
   {
      return super.getX();
   }

public int getSuperY()
   {
      return super.getY();
   }

public String getSuperName()
   {
      return super.getName();
   }

public String toString(){

	return "Creating a circle. \n" + super.getName() + " position" + " (" + super.getX() + "," + super.getY() + ")" + " radius = " + radius + "\n";
   }

public void draw(){

	System.out.println("In Circle - Doing draw");
   }

public void erase(){

	System.out.println("In Circle - Doing erase");
   }

}

public class Rectangle extends Figure{

private int length;
private int height;

public Rectangle(String name, int x, int y, int length, int height){
    super("R1", 5, 4);
	this.setLength(length);
	this.setHeight(height);
    }

public void setLength(int l){this.length = l;}
public int getLength(){return this.length;}

public void setHeight(int h){this.height = h;}
public int getHeight(){return this.height;}

   public int getSuperX()
   {
      return super.getX();
   }

      public int getSuperY()
   {
      return super.getY();
   }

      public String getSuperName()
   {
      return super.getName();
   }

public String toString(){

	return "Creating a rectangle. \n" + super.getName() + " position" + " (" + super.getX() + "," + super.getY() + ")" + " height = " + height + " length = " + length+"\n";
}

public void draw(){
	System.out.println("In Rectangle - Doing draw");
}

public void erase(){
	System.out.println("In Rectangle - Doing erase");
}

}


public class TestFigure {

    public static void main(String[] args) {

	Rectangle r1 = new Rectangle(5,4);

	Circle c1 = new Circle(44);

	System.out.println(r1.toString());

	Figure[] f = new Figure[2];
   	  f[0] = c1;
   	  f[1] = r1;
   	  for(int i = 0; i < 2; i++ )
   	  	f[i].center();

   	Figure[] shapes = new Figure[2];
   	  for(int i=0 ; i < 2; i++)shapes[i].draw();

    }
}

--------------------Configuration: <Default>--------------------
F:\Object Java\TestFigure.java:14: error: constructor Rectangle in class Rectangle cannot be applied to given types;
Rectangle r1 = new Rectangle(5,4);
^
required: String,int,int,int,int
found: int,int
reason: actual and formal argument lists differ in length
F:\Object Java\TestFigure.java:16: error: constructor Circle in class Circle cannot be applied to given types;
Circle c1 = new Circle(44);
^
required: String,int,int,int
found: int
reason: actual and formal argument lists differ in length
2 errors

[Campbell Ritchie]

Your Rectangle constructor requires different parameters from what you are passing. Your rectangle constructor is badly designed and ought to be changed. You don’t want a “super” and a subclass name. You have a name which is set in the superclass, so you pass super(name). Where on earth you get “R1” and 4 and 5 from?
You pass name and x and y unchanged to the superclass’ constructor. Get rid of the getSuperXXX methods because they are unnecessary; the getXXX methods are inherited by all subclasses anyway. If you want to be clever, mark the getXXX methods in the Figure class final, because you don’t want them overridden.

[Luke Stamper]

I'm trying to reset x and y in each subclass with different values. That's where those came in.

[Luke Stamper]

Also, I am getting the error

Exception in thread "main" java.lang.NullPointerException
at TestFigure.main(TestFigure.java:28)

I am not sure how to remedy this.

[Luke Stamper]

I wanted to clarify what I am trying to do. I want the rectangle and circle class to each have a different (x,y) position. So in each class, I need to reset the values of X and Y from the superclass. I am having difficulty doing this.

[Campbell Ritchie]

Go through line 28 and see which objects you have put to the left of a . operator. One of them may be null. If you don’t work out which is null in a few minutes, post again with a few lines around no 28 in code tags.

[Luke Stamper]

I believe the shapes array was null so I added c1 and r1.

Figure[] shapes = new Figure[];
shapes[0] = c1;
shapes[1] = r1;
for(int i=0 ; i < 2; i++)shapes[i].draw();

Now my output is...

--------------------Configuration: <Default>--------------------
Creating a rectangle.
R1 position (0,0) height = 2 length = 3

Creating a circle.
C1 position (0,0) radius = 44.0

In Figure, calling erase, draw
In Circle - Doing erase
In Circle - Doing draw
In Figure, calling erase, draw
In Rectangle - Doing erase
In Rectangle - Doing draw
In Circle - Doing draw
In Rectangle - Doing draw

Process completed.


I am left trying to figure out how to ensure each shape gets a different position. X & Y in the super are set to 0, but I need to change for each subclass.

[Campbell Ritchie]

As for the null: whenever you create an array of reference types, it is filled with nulls as a default. If you don’t specifically fill the array with “real” references, you may suffer such an Exception.

[Luke Stamper]

Thank you. Could you also shed some light on my issue of resetting the X,Y from the super class for the rectangle and square so each object can have a different position?

[Campbell Ritchie]

You need to decide where you do and where you don’t want constructors.

public abstract class Vehicle
{
   private int speed;
   private int direction; // 0° = north, 90° = east, etc

   public Vehicle(int speed, int direction)
   {
       if (Math.abs(speed) > 672000000)
       {
           throw new EinsteinException(); // find out what 672000000mph is ;-)
       }
       this.speed = Math.abs(speed); // Positive speeds or 0 permitted.
       this.direction = direction % 360;
       if (this.direction < 0)
       {
          this.direction += 360); // Positive directions up to 359 permitted, or 0
       }
   }

   public int getSpeed()
   {
       return speed;
   }

   public int getDirection()
   {
       return direction;
   }

/**
 * ...
 *@param mph ... negative values should be used for slowing down
 */
   public void speedUp(int mph)
   {
      // work it out for yourself
   }

/**
 * ...
 *@param degrees ... positive values for turning right and negative values for turning left.
 */
   public void turn(int degrees)
   {
      // some arithmetic or other
   }

   @Override public String to String()
   {
       return String.format("A %s travelling at %dmph in the direction %d°",
                speed, direction);
   }
}

Now you can set speed and direction. Note the actions taken to ensure both speed and direction are within certain ranges.

Yes, this is relevant, because you now have a superclass with two int fields. You can forget about the Exception and Math.abs and the 360°, since you have no limits to the values you permit.
Now rewrite the Vehicle class, miss out all the bits you don’t need, and compare it with your Figure class.
You want constructors which will set up those two fields, so you write a constructor which sets up those two fields. You don’t want default values, so you get rid of any other constructors. You may have to amend other code which uses those constructors, which you are getting rid of. You do not want a constructor like this, because you do not want to default to all your vehicles doing 97mph eastward:

/**
 * defaults to 97mph due East
 */
public Vehicle()
{
   speed = 97;
   direction = 90;
}

/**
 * Bus class follows soon: scroll down a little.
 * Empty constructor which simply calls the Vehicle() constructor, not realising that means this
 * Bus will be going due East at 97mph ;-)
 */
public Bus()
{
}

If you found a constructor like that in the Vehicle class, and called new Bus(); anywhere, you get the default values. So you don’t want either of those constructors. If you have a constructor which gives you defaults you don’t want, get rid of it. That’s the only way to ensure nobody calls it by mistake.

Now let’s look at a possible subclass

public class Bus extends Vehicle
{
   private int passengers;

   public Bus(int speed, int direction, int passengers)
   {
       super(speed, direction);
       this.passengers = Math.max(0, passengers); // You can have an empty bus, but never < 0 passengers!
   }

/**
 * ...
 *@param howMany ... use negative numbers to represent passengers alighting
 */
   public void pickUpPassengers(int howMany)
   {
      passengers = Math.max(0, passengers + howMany);
   }

   @Override public String toString()
   {
      return String.format("%s, with %d passengers.", super.toString(), passengers);
   }
}

Note there is no turn method, nor speedUp, nor getSpeed, nor getDirection. Those are inherited from the Vehicle class. The only place you use super. is in the toString method, so you are adding “A Bus travelling at 97mph in direction 90°” to “with 127 passengers.”
And yes, it will say Bus, not Vehicle.

So we have a Bus class which inherits those features from its superclass. Now see if you can’t turn my Bus class into your Circle class. Only give it the constructors you want. Then you must pass parameters to those constructors, defining the x, y locations

[Luke Stamper]

Got it! Thanks for your help, it is much appreciated.

public abstract class Figure{
	private int X, Y;
    private String name;

	public Figure(String name, int X, int Y){
	   this.X = X;
	   this.Y = Y;
       this.name = name;
	}

	public String getName() {
        return name;
    }

    public int getX() {
        return X;
    }

    public int getY() {
        return Y;
    }

    public String toString()
    {
    	return String.format("Creating a " + name + "\nposition " + "(" + X + "," + Y + ")");
    }

	public abstract void draw();
        public abstract void erase();
        public void center(){
    	   System.out.println("In Figure, calling erase, draw");
    	   erase();
    	   draw();
    }
}


public class Circle extends Figure{

private double radius;

public Circle(String name, int X, int Y, int radius){
    super(name, X, Y);
	this.radius = radius;
    }

public String toString(){

	return super.toString() + " radius = " + radius + "\n";
}

public void draw(){

	System.out.println("In Circle - Doing draw");
   }

public void erase(){

	System.out.println("In Circle - Doing erase");
   }


}


public class Rectangle extends Figure{

private int length;
private int height;

public Rectangle(String name, int X, int Y, int length, int height){
    super(name, X, Y);
	this.length = length;
	this.height = height;
    }

public String toString(){

	return super.toString() + " height = " + height + " length = " + length+"\n";
}

public void draw(){
	System.out.println("In Rectangle - Doing draw");
}

public void erase(){
	System.out.println("In Rectangle - Doing erase");
}

}



public class TestFigure {

    public static void main(String[] args) {

	Rectangle r1 = new Rectangle("Rectangle", 4, 25, 3, 2);

	Circle c1 = new Circle("Circle", 5, 5, 44);

	System.out.println(r1.toString());
	System.out.println(c1.toString());

	Figure[] f = new Figure[2];
   	  f[0] = c1;
   	  f[1] = r1;
   	  for(int i = 0; i < 2; i++ )
   	  	f[i].center();

   	Figure[] shapes = new Figure[2];
   	  shapes[0] = c1;
   	  shapes[1] = r1;
   	  for(int i=0 ; i < 2; i++)shapes[i].draw();

    }
}

[Campbell Ritchie]

In your centre() method, are you supposed to alter the x, y position?
By the way: it should be x and y, not X and Y, for the field names.

You can see how much of an improvement that latest code is if you compare it to the previous attempt. You have saved thirty lines Well done.