Basic tutorial on abstract methods and abstract classes of PHP object model

　Object-orientedPrograms are built through a hierarchical structure of classes. In single inheritance languages ​​such as PHP, class inheritance is tree-like. A root class has one or more subclasses. Each subclass then inherits one or more subclasses. Of course, there may be multiple root classes used to implement different functions. In a well-designed architecture, each root class should have a useful interface that can be used by application code. If our application code is designed to work with the root class, it can also work with any subclass that inherits from the root class.

An abstract method is a placeholder like a normal method in a subclass (it takes up a place but does not work). It is different from a normal method - it does not have any code. If there are one or more abstract methods in a class, then the class becomes Abstract Class. You cannot instantiate abstract classes. You must inherit from them and then instantiate subclasses. You can also think of abstract classes as a template for subclasses.

If you override all abstract methods, the subclass becomes a normal class. If all methods are not overridden, the subclass is still abstract. If a class contains an abstract method (even if there is only one), you must declare the class to be abstract by adding abstract before the class keyword.

The syntax of declaring an abstract method is different from that of declaring a general method. The abstract method does not have the main body contained in braces {} like the general method, and ends with a semicolon;.

In Example 6.13, we defined a class Shape that contains a getArea method. But since it is impossible to determine the area of ​​the figure without knowing the shape, we declared the getArea method as an abstract method. You cannot instantiate a Shape object, but you can inherit from it or use it in an expression, as in Example 6.13.

If you create a class with only abstract methods, you define an interface. To illustrate this situation, PHP has the interface and implements keywords. You can use interface instead of abstract class and implements instead of extends to describe your class definition or use an interface. For example, you can write a myClass implements myIterface. These two methods can be used according to personal preference. choose.

/*Note:
The two methods refer to:
1. abstract class aaa{} (note that there are only abstract methods in aaa, no general methods)
class bbb extends aaa{} (Override the abstract method in aaa in bbb)
2. interface aaa{}
class bbb implements aaa{} (Override the abstract method in aaa in bbb)
*/

Listing 6.13 Abstract classes

//abstract root class 抽象根类 
abstract class Shape 
{ 
　abstract function getArea(); //定义一个抽象方法 
}
//abstract child class 抽象子类 
abstract class Polygon extends Shape //多边形 
{ 
　abstract function getNumberOfSides(); 
}
//concrete class 实体类 三角形类 
class Triangle extends Polygon 
{ 
　public $base; 
　public $height;
　public function getArea() //覆写计算面积方法 
　{ 
　　return(($this-＞base * $this-＞height)/2); 
　}
　public function getNumberOfSides() //覆写边数统计方法 
　{ 
　　return(3); 
　} 
}
//concrete class 实体类四边形 
class Rectangle extends Polygon 
{ 
　public $width; 
　public $height;
　public function getArea() 
　{ 
　　return($this-＞width * $this-＞height); 
　}
　public function getNumberOfSides() 
　{ 
　　return(4); 
　} 
}
//concrete class 实体类 圆形 
class Circle extends Shape 
{ 
　public $radius;
　public function getArea() 
　{ 
　　return(pi() * $this-＞radius * $this-＞radius); 
　} 
}
//concrete root class 定义一个颜色类 
class Color 
{ 
　public $name; 
}
$myCollection = array(); //建立形状的集合，放入数组
//make a rectangle 
$r = new Rectangle; 
$r-＞width = 5; 
$r-＞height = 7; 
$myCollection[] = $r; 
unset($r);
//make a triangle 
$t = new Triangle; 
$t-＞base = 4; 
$t-＞height = 5; 
$myCollection[] = $t; 
unset($t);
//make a circle 
$c = new Circle; 
$c-＞radius = 3; 
$myCollection[] = $c; 
unset($c);
//make a color 
$c = new Color; 
$c-＞name = "blue"; 
$myCollection[] = $c; 
unset($c);
foreach($myCollection as $s) 
{ 
　if($s instanceof Shape) //如果$s是Shape类的实例 
　{ 
　　print("Area: " . $s-＞getArea() . "n"); 
　}
　if($s instanceof Polygon) 
　{ 
　　print("Sides: " .$s-＞getNumberOfSides()."n"); 
　}
　if($s instanceof Color) 
　{ 
　　print("Color: $s-＞name　n"); 
　}
　print("n"); 
}

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