Page 239 - Beginning PHP 5.3

Chapter 8: Objects
Here ’ s how you might use ShapeInfo to display the color and size of a square:

$mySquare = new Square;
$mySquare- > setColor( “green” );
$mySquare- > makeHollow();
$mySquare- > setSideLength( 3 );
$info = new ShapeInfo();
$info- > setShape( $mySquare );

$info- > showInfo(); // Displays “The shape’s color is green, and its area is 9.”
You ’ re probably wondering what this has to do with abstract classes. Well, imagine another programmer
comes along and creates a new child class, Rectangle , based on your Shape class:
class Rectangle extends Shape {
private $_width = 0;
private $_height = 0;

public function getWidth() {
return $this- > _width;
}

public function getHeight() {
return $this- > _height;
}

public function setWidth( $width ) {
$this- > _width = $width;
}

public function setHeight( $height ) {
$this- > _height = $height;
}
}

Notice anything missing? What happens if you try to use a Rectangle object with the ShapeInfo

class ’ s showInfo() method?
$myRect = new Rectangle;
$myRect- > setColor( “yellow” );
$myRect- > fill();
$myRect- > setWidth( 4 );
$myRect- > setHeight( 5 );
$info = new ShapeInfo();
$info- > setShape( $myRect );
$info- > showInfo();

The answer is that you get the following error:

Call to undefined method Rectangle::getArea()
Our intrepid programmer has forgotten to create a getArea() method in his Rectangle class. Or
maybe he didn ’ t realize he was supposed to. After all, how was he to know that Rectangle objects
needed to work with your ShapeInfo class?

201

c08.indd 201 9/21/09 9:03:43 AM
c08.indd 201
9/21/09 9:03:43 AM