Inheritance

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Figure 446. Guess who's inheriting the money Slide presentation Create comment in forum
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Figure 447. Biology and inheritance Slide presentation Create comment in forum
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Figure 448. Duplicate code Slide presentation Create comment in forum
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 449. Idea: Centralize common code Slide presentation Create comment in forum
  • Create a parent class Shape containing common code portions.

  • Relate both Rectangle and Circle to Shape.


Figure 450. Common and specific properties Slide presentation Create comment in forum
Common Rectangle and Circle attributes:
double x;
double y;
Rectangle attributes Circle attributes
double width;
double height;
double radius;

Figure 451. Basic shape inheritance Slide presentation Create comment in forum

Figure 452. Inheritance Slide presentation Create comment in forum
  • Derived classes inherit state and behaviour.

  • Refinement, specialization.

  • is-A relationship:

    • A rectangle is a shape.

    • A circle is a shape.


Figure 453. Implementing Shape hierarchy Slide presentation Create comment in forum
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 454. Creating instances Slide presentation Create comment in forum
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 455. Shape constructor Slide presentation Create comment in forum
/**
  * 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 456. Creating Rectangle instances Slide presentation Create comment in forum
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 457. Rectangle constructor Slide presentation Create comment in forum
/**
  * 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.