Inheritance

Figure 451. Guess who's inheriting the money

Figure 452. Biology and inheritance

Figure 453. Duplicate code

public class Rectangle{
  // Center coordinate
  private double x; ❶
  private double y;

  public void move
    (double dx, double dy){ ❷
     x += dx;
     y += dy;
  }
  private double width, height; ❸ ...
}

public class Circle {
  // Center coordinate
  private double x; ❶
  private double y;

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

❶	The center coordinate (`x`\|`y)` appears both in `Rectangle` and `Circle`.
❷	The move(...) method is being defined identically both in `Rectangle` and `Circle`!
❸	width and height only appear in class `Rectangle`. radius only appears in class Circle.

Figure 454. Idea: Centralize common code

Create a parent class Shape containing common code portions.
Relate both Rectangle and Circle to Shape.

Figure 455. Common and specific properties

Common `Rectangle` and `Circle` attributes:
	`double x; double y;`
`Rectangle` attributes		`Circle` attributes
`double width; double height;`		`double radius;`

Figure 456. Basic shape inheritance

‹›

Figure 457. Inheritance

Derived classes inherit state and behaviour.
Refinement, specialization.
“is-A” relationship:
- A rectangle is a shape.
- A circle is a shape.

Figure 458. Implementing Shape hierarchy

	`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 459. Creating instances

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 460. Shape constructor

/**
  * 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 461. Creating Rectangle instances

final Rectangle r = 
   new Rectangle(x, y ❶,
                 width, height ❷);

❶	Center coordinate components “belonging” to superclass `Shape`.
❷	width and height “belonging” to class `Rectangle`.

Solution: Nested constructor call. Coming soon ...

Figure 462. Rectangle constructor

/**
  * 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) ❶;
  this.width = width; this.height = height ❷;
}

❶	Passing center coordinate to superclass `Shape`'s constructor. Note This must be the first statement within `Rectangle`'s constructor.
❷	Processing `width` and `h`
❷	`eight` in “current” `Rectangle` constructor.

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