Section 10 - Abstract Methods and Abstract Classes - Classes and Objects in PHP5 [10]_PHP Tutorial

Section 10--Abstract methods and abstract classes

Object-oriented programs are built through the hierarchical structure of classes. In single inheritance languages ​​such as PHP, class inheritance is tree-like . A root class has one or more subclasses, and each subclass inherits one or more lower-level subclasses. Of course, there may be multiple root classes to implement different functions. In a good design In the system, 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, then it can also work with any one that inherits from the root class. Cooperation with subclasses.

Abstract methods are just like placeholders for general methods in subclasses (take up a place but have no effect). It is different from general methods - there is no code. If there is a or more abstract methods, then this class becomes an abstract class. You cannot instantiate abstract classes. You must inherit 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 you do not override all methods, the subclass is still abstract. If a class contains an abstract method (even if there is only one ), you must declare that this class is abstract, add abstract.

before the class keyword } and end it with a semicolon;.

In Example 6.13, we defined a class Shape that contains the getArea method. However, it is impossible to determine the area of ​​the figure because we do not know the shape. We declared the getArea method as abstract. 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 Shape object that only For a class with an abstract method, 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 illustrate your class definition. Or use an interface. For example, you can write a myClass implements myIterface. These two methods can be chosen according to personal preference.

/*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{} (overwrite the abstract methods in aaa in bbb)
2. interface aaa{}
class bbb implements aaa{} (override abstract methods in aaa in bbb)
*/

Listing 6.13 Abstract classes

<?php 

   //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() . 

               "<br>
"); 

       } 



       if($s instanceof Polygon) 

       { 

           print("Sides: " . 

               $s->getNumberOfSides() . 

               "<br>
"); 

       } 



       if($s instanceof Color) 

       { 

           print("Color: $s->name<br>
"); 

       } 



       print("<br>
"); 

   } 



?> 

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