Inheritance

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Figure 441. Guess who's inheriting the money Slide presentation
Guess who's inheriting the money

Figure 442. Biology and inheritance Slide presentation
Biology and inheritance

Figure 443. Duplicate code Slide presentation
public class Rectangle{
  // Center coordinate
  private double x; 
  private double y;

  public void move
    (double dx, double dy){ 
     x += dx;
     y += dy;
  }
  private double width, height;  ...
}
public class Circle {
  // Center coordinate
  private double x; 
  private double y;

  public void move
    (double dx, double dy){ 
     x += dx;
     y += dy;
  }
  private double radius;  ...
}

The center coordinate (x|y) appears both in Rectangle and Circle.

The move(...) method is being defined identically both in Rectangle and Circle!

  • width and height only appear in class Rectangle.

  • radius only appears in class Circle.


Figure 444. Idea: Centralize common code Slide presentation
  • Create a parent class Shape containing common code portions.

  • Relate both Rectangle and Circle to Shape.


Figure 445. Common and specific properties Slide presentation
Common Rectangle and Circle attributes:
double x;
double y;
Rectangle attributes Circle attributes
double width;
double height;
double radius;

Figure 446. Basic shape inheritance Slide presentation

Figure 447. Inheritance Slide presentation
  • Derived classes inherit state and behaviour.

  • Refinement, specialization.

  • is-A relationship:

    • A rectangle is a shape.

    • A circle is a shape.


Figure 448. Implementing Shape hierarchy Slide presentation
public class Shape {
  private double x, y;
}
public class Rectangle extends Shape {
  private double width;
  private double height;
}
public class Circle extends Shape {
  private double radius;
}

Figure 449. Creating instances Slide presentation
final double x = 2, y = 3;
final Shape shape = new Shape(x, y);

final double width = 2, height = 5;
final Rectangle r = new Rectangle(x, y , width, height);

final double radius = 2.5;
final Circle circle = new Circle(x, y , radius);

Figure 450. Shape constructor Slide presentation
/**
  * Creating a shape located at center coordinate.
  * @param x The center's x component.
  * @param y The center's y component.
  */
public Shape(double x,double y) {
  this.x = x;
  this.y = y;
}

Figure 451. Creating Rectangle instances Slide presentation
final Rectangle r = 
   new Rectangle(x, y ,
                 width, height );

Center coordinate components belonging to superclass Shape.

width and height belonging to class Rectangle.

Solution: Nested constructor call. Coming soon ...


Figure 452. Rectangle constructor Slide presentation
/**
  * Creating a rectangle at (x|y) of given width and height.
  * @param x Center's x component.
  * @param y Center's y component.
  * @param width Rectangle's width.
  * @param height Rectangle's height.
  */
public Rectangle(double x, double y,
             double width, double height) {
  super(x, y) ;
  this.width = width; this.height = height ;
}

Passing center coordinate to superclass Shape's constructor.

Note

This must be the first statement within Rectangle's constructor.

Processing width and h

eight in current Rectangle constructor.