PHP 类扩展不起作用，为什么这是正确扩展类的方法？

Question

您好，我想了解 inherteince 如何使用面向对象编程在 PHP 中工作。主类是Computer，继承类是Mouse。我正在用鼠标类扩展计算机类。我在每个类中都使用 __construct，当我启动类时，我首先使用 pc 类型，如果它之后有鼠标。由于某种原因，计算机返回空值？这是为什么？

class Computer {
    protected $type = 'null';

    public function __construct($type) {
        $this->type = $type;
    }
    public function computertype() {
        $this->type = strtoupper($this->type);
        return $this->type;
    }

}

class Mouse extends Computer {
    protected $hasmouse = 'null';
    public function __construct($hasmouse){
         $this->hasmouse = $hasmouse;

    }
    public function computermouse() {
         if($this->hasmouse == 'Y') {
             return 'This Computer has a mouse';
         }
    }

}

$pc = new Computer('PC', 'Y');
echo $pc->computertype;
echo $pc->computermouse;

Answer 1

这不是继承的工作方式。

继承(又名 extend)允许您根据父类的属性和方法扩展子类。

考虑以下示例:

class Peripheral {
  private $connectionType;
  private $isConnected = false;

  function __construct($connectionType) {
    $this->connectionType = $connectionType;
  }

  function getConnectionType() {
    return $this->connectionType;
  }

  function connect() {
    $this->isConnected = true;
  }

  function disconnect() {
    $this->isConnected = false;
  } 

  function isConnected() {
    return $this->isConnected;
  }
}

您现在可以扩展 Peripheral 类，因为 Mouse 是一个 Peripheral。

class Mouse extends Peripheral {
  private $numOfButtons;

  function __construct($connectionType, $numberOfButtons) {
    parent::__construct($connectionType);
    $this->numOfButtons = $numOfButtons;
  }

  function getNumOfButtons() {
    return $this->numOfButtons;
  }

  function leftClick() {
    if($this->isConnected())
      echo "Click!";
  }

  function rightClick() {
    if($this->isConnected())
      echo "RightClick!";
  }
}

现在，因为 Mouse 是 Peripheral 的子类，所以 Peripheral 中定义的所有方法都可以在 Mouse 中访问>，但反之则不然……

所以虽然我可以这样做:

$mouse = new Mouse('USB', 2);
echo $mouse->getConnectionType(); // USB
$mouse->connect();
$mouse->leftClick();

我不能做以下事情:

$perip = new Peripheral('USB');
echo $perip->getConnectionType(); // USB
$perip->connect();
$perip->leftClick(); // ERROR: leftClick not defined.