Page 241 - Beginning PHP 5.3

Chapter 8: Objects
public function setColor( $color ) {
$this- > _color = $color;
}

public function isFilled() {
return $this- > _filled;
}

public function fill() {
$this- > _filled = true;
}

public function makeHollow() {
$this- > _filled = false;
}

abstract public function getArea();
}

You can now use the ShapeInfo class with any class that is derived from the Shape class, safe in the
knowledge that the child class implements getArea() .

So when the programmer attempts to add his Rectangle class without the getArea() method, it
generates an error:
Class Rectangle contains 1 abstract method and must therefore be declared
abstract or implement the remaining methods (Shape::getArea)

This should be enough to remind the programmer to add the required getArea() method to the 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;
}

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