The old way of re-using existing programs

  • Suppose we need to write a Rectangle class and we found the Circle class that looks similar: 

      public class Circle
{
    private String color;

    private double radius;

    Circle(String col, double r)
    {
        color = col; radius = r;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }
    public double getRadius()         { return radius;}
    public void   setRadius(double r) { radius = r; }

    public double getArea()
    {
        return 3.14159*radius*radius;
    }
}




      public class Rectangle
{
   

   

    
    
     We need to write this class                       
  


  
  
  
  
   

    
   
       
   
}

Comment: I have simplified the example in the text book (uses fewer variables and methods)

The old way of re-using existing programs

  • The old way:   (1) copy the variables and methods in the Circle class: 

      public class Circle
{
    private String color;

    private double radius;

    Circle(String col, double r)
    {
        color = col; radius = r;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }
    public double getRadius()         { return radius;}
    public void   setRadius(double r) { radius = r; }

    public double getArea()
    {
        return 3.14159*radius*radius;
    }
}




      public class Rectangle
{
    private String color;
   
    private double radius;

    // Do not copy constructors
    // Make new one(s) !!
  
  

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }
    public double getRadius()         { return radius;}
    public void   setRadius(double r) { radius = r; }



    public double getArea()       
    {
        return 3.14159*radius*radius;
    }
}

Comment: I have simplified the example in the text book (uses fewer variables and methods)

The old way of re-using existing programs

  • The old way:   (2) make changes to the variables and methods: 

      public class Circle
{
    private String color;

    private double radius;

    Circle(String col, double r)
    {
        color = col; radius = r;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }
    public double getRadius()         { return radius;}
    public void   setRadius(double r) { radius = r; }

    public double getArea()
    {
        return 3.14159*radius*radius;
    }
}




      public class Rectangle
{
    private String color;
    private double width;
    private double height;

    Rectangle(String col, double w, double h)
    {
        color = col; width = w; height = h;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }
    public double getWidth()          { return width;}
    public void   setWidth(double r)  { width = r; }
    public double getHeight()         { return height;}
    public void   setHeight(double r) { height = r; }

    public double getArea()       
    {
        return width*height;
    }
}

Comment: I have simplified the example in the text book (uses fewer variables and methods)

The old way of re-using existing programs

  • Notice:   there are duplication of variable(s) and/or methods in the programs: 

      public class Circle
{
    private String color;

    private double radius;

    Circle(String col, double r)
    {
        color = col; radius = r;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }   
    public double getRadius()         { return radius;}
    public void   setRadius(double r) { radius = r; }

    public double getArea()
    {
        return 3.14159*radius*radius;
    }
}




      public class Rectangle
{
    private String color;
    private double width;
    private double height;

    Rectangle(String col, double w, double h)
    {
        color = col; width = w; height = h;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }   
    public double getWidth()          { return width;}
    public void   setWidth(double r)  { width = r; }
    public double getHeight()         { return height;}
    public void   setHeight(double r) { height = r; }

    public double getArea()       
    {
        return width*height;
    }
}

Duplication makes program maintainance difficult (e.g., changing setColor( )  ⇒  make changes in 2 places)

Test program on Circle and Rectangle objects

  

public class myProg
{
    public static void main(String[] args)
    {
        Circle c1 = new Circle("red", 2);
        Circle c2 = new Circle("blue", 4);
        
        Rectangle r1 = new Rectangle("green", 2, 3);
        Rectangle r2 = new Rectangle("red", 1, 4);        

        System.out.println( c1.getColor() + " " + c1.getArea() );
        System.out.println( c2.getColor() + " " + c2.getArea() );
        System.out.println( r1.getColor() + " " + r1.getArea() );
        System.out.println( r2.getColor() + " " + r2.getArea() );

    }
}

DEMO: demo/13-inheritance/04-copy+paste

Let's see how we can define a suitable superclass for Circle and Rectangle that allow us to remove duplication !

Applying OOP: an object oriented design example

  • Find the superclass: we find the common (= shared) properties(= variables) and actions (= methods) in the classes: 

      public class Circle
{
    private String color;

    private double radius;

    Circle(String col, double r)
    {
        color = col; radius = r;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }   
    public double getRadius()         { return radius;}
    public void   setRadius(double r) { radius = r; }

    public double getArea()
    {
        return 3.14159*radius*radius;
    }
}




      public class Rectangle
{
    private String color;
    private double width;
    private double height;

    Rectangle(String col, double w, double h)
    {
        color = col; width = w; height = h;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }   
    public double getWidth()          { return width;}
    public void   setWidth(double r)  { width = r; }
    public double getHeight()         { return height;}
    public void   setHeight(double r) { height = r; }

    public double getArea()       
    {
        return width*height;
    }
}

• We define a super class containing all these common members

Applying OOP: an object oriented design example

  • The superclass that contains the common (shared) properties and actions in the classes Circle and Rectangle are: 

      public class GeometricObject  
{
    private String color;

    GeometricObject( ) { }          // Constructor 1

    GeometricObject( String col )   // Constructor 2
    {
        color = col;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }   

    public double getArea()  // Dummy method !!
    {
        return 0;  // Some default value
    }
}

Comment:   some methods (e.g.: getArea( )) in the superclass may not have a useful method body --- it's OK.
                     We will learn (later) that this it is very useful for the polymorphism mechanism !!

Applying OOP: an object oriented design example

  • We change the Circle and Rectangle classes and make them extend the superclass GeometricObject: 

      public class Circle extends GeometricObject
{
    private String color;

    private double radius;

    Circle(String col, double r)
    {
        color = col; radius = r;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }   
    public double getRadius()         { return radius;}
    public void   setRadius(double r) { radius = r; }

    public double getArea()
    {
        return 3.14159*radius*radius;
    }
}




      public class Rectangle extends GeometricObject
{
    private String color;
    private double width;
    private double height;

    Rectangle(String col, double w, double h)
    {
        color = col; width = w; height = h;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }   
    public double getWidth()          { return width;}
    public void   setWidth(double r)  { width = r; }
    public double getHeight()         { return height;}
    public void   setHeight(double r) { height = r; }

    public double getArea()       
    {
        return width*height;
    }
}

• The classes will inherit:   (1) color, (2) getColor(), (3) setColor() and (4) getArea()

Applying OOP: an object oriented design example

  • The String color variable inherited from the superclass (GeometricObject) is the same and we can remove its definition: 

      public class Circle extends GeometricObject
{
  

    private double radius;

    Circle(String col, double r)
    {
        color = col; radius = r;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }   
    public double getRadius()         { return radius;}
    public void   setRadius(double r) { radius = r; }

    public double getArea()
    {
        return 3.14159*radius*radius;
    }
}




      public class Rectangle extends GeometricObject
{
 
    private double width;
    private double height;

    Rectangle(String col, double w, double h)
    {
        color = col; width = w; height = h;
    }

    public String getColor()          { return color; }
    public void   setColor(String c)  { color = c; }   
    public double getWidth()          { return width;}
    public void   setWidth(double r)  { width = r; }
    public double getHeight()         { return height;}
    public void   setHeight(double r) { height = r; }

    public double getArea()       
    {
        return width*height;
    }
}

• The classes will inherit:   (1) color, (2) getColor(), (3) setColor() and (4) getArea()

Applying OOP: an object oriented design example

  • The getColor() and setColor() methods inherited are also the same and we can remove their definitions: 

      public class Circle extends GeometricObject
{
  

    private double radius;

    Circle(String col, double r)
    {
        color = col; radius = r;
    }

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

    public double getArea()
    {
        return 3.14159*radius*radius;
    }
}




      public class Rectangle extends GeometricObject
{
 
    private double width;
    private double height;

    Rectangle(String col, double w, double h)
    {
        color = col; width = w; height = h;
    }

  
   
    public double getWidth()          { return width;}
    public void   setWidth(double r)  { width = r; }
    public double getHeight()         { return height;}
    public void   setHeight(double r) { height = r; }

    public double getArea()       
    {
        return width*height;
    }
}

• The classes will inherit:   (1) color, (2) getColor(), (3) setColor() and (4) getArea()

Applying OOP: an object oriented design example

  • The getArea() method is different from the GeometricObject, so we override it with a new getArea() method in the subclass: 

      public class Circle extends GeometricObject
{
  

    private double radius;

    Circle(String col, double r)
    {
        color = col; radius = r;
    }

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

    public double getArea() // Overrides
    {
        return 3.14159*radius*radius;
    }
}




      public class Rectangle extends GeometricObject
{
 
    private double width;
    private double height;

    Rectangle(String col, double w, double h)
    {
        color = col; width = w; height = h;
    }

  
   
    public double getWidth()          { return width;}
    public void   setWidth(double r)  { width = r; }
    public double getHeight()         { return height;}
    public void   setHeight(double r) { height = r; }

    public double getArea() // Overrides       
    {
        return width*height;
    }
}

• The classes will inherit:   (1) color, (2) getColor(), (3) setColor() and (4) getArea()

Applying OOP: an object oriented design example

  • There is one problem: (can you spot it ?) 

      public class Circle extends GeometricObject
{
  

    private double radius;

    Circle(String col, double r)
    {
        color = col; radius = r;
    }

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

    public double getArea() // Overrides
    {
        return 3.14159*radius*radius;
    }
}




      public class Rectangle extends GeometricObject
{
 
    private double width;
    private double height;

    Rectangle(String col, double w, double h)
    {
        color = col; width = w; height = h;
    }

  
   
    public double getWidth()          { return width;}
    public void   setWidth(double r)  { width = r; }
    public double getHeight()         { return height;}
    public void   setHeight(double r) { height = r; }

    public double getArea() // Overrides       
    {
        return width*height;
    }
}

• Problem:   color is private variable in GeometricObject and cannot be accessed directly

Applying OOP: an object oriented design example

  • Replace all direct access to private members in superclass with calls to accessor/mutator methods: 

      public class Circle extends GeometricObject
{
  

    private double radius;

    Circle(String col, double r)
    {
        setColor(col); radius = r;
    }

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

    public double getArea() // Overrides
    {
        return 3.14159*radius*radius;
    }
}




      public class Rectangle extends GeometricObject
{
 
    private double width;
    private double height;

    Rectangle(String col, double w, double h)
    {
        setColor(col); width = w; height = h;
    }

  
   
    public double getWidth()          { return width;}
    public void   setWidth(double r)  { width = r; }
    public double getHeight()         { return height;}
    public void   setHeight(double r) { height = r; }

    public double getArea() // Overrides       
    {
        return width*height;
    }
}

• Notice:   now there is no duplication of variable defintions or methods !

Test program on Circle and Rectangle objects

  

public class myProg
{
    public static void main(String[] args)
    {
        Circle c1 = new Circle("red", 2);
        Circle c2 = new Circle("blue", 4);
        
        Rectangle r1 = new Rectangle("green", 2, 3);
        Rectangle r2 = new Rectangle("red", 1, 4);

        System.out.println( c1.getColor() + " " + c1.getArea() );
        System.out.println( c2.getColor() + " " + c2.getArea() );
        System.out.println( r1.getColor() + " " + r1.getArea() );
        System.out.println( r2.getColor() + " " + r2.getArea() );
    }
}

DEMO: demo/13-inheritance/05-OOD

This test program works like before !!!