如何覆蓋PHP類的構造函數？

Question

這是我的代碼。我沒有經驗，所以請用簡單的語言告訴我答案。

<?php 
function println($message) { 
    echo "\n".$message; 
} 

println是essentialy回聲，但與消息之前的\ n。我只是習慣於Python 3，但不能使用print（）。

class Car { 
    public function __construct($name) { 
    //If the constructor is that of a car, it will be said that a car was made. 

     println("Car made!"); 
     $this->distance_drived = 0; 
     $this->name = $name; 
    } 
    function introductory() { 
     return "This car is called ".$this->name."."; 
    } 
} 

class Battery { 
    function __construct($max_capacity, $current_capacity) { 
     echo "Constructed!"; 
     $this->max_capacity = $max_capacity; 
     $this->current_capacity = $current_capacity; 
    } 
    function fill($amount) { 
     if ($amount + $this->current_capacity >= $this->max_capacity) { 
      $this->fill_full(); 
     } else { 
      $this->current_capacity += $amount; 
     } 
    } 
    function fill_full() { 
     $this->current_capacity = $this->max_capacity; 
    } 
    function use_power($amount) { 
     if ($amount + $this->current_capacity >= $this->max_capacity) { 
      return $this->current_capacity; 
      $this->current_capacity = 0; 
     } else { 
      $this->current_capacity -= $amount; 
      return $amount; 
     } 
    } 
    function check_percentage() { 
     return ($this->current_capacity/$this->max_capacity) * 100; 
    } 
} 

class ElectricCar extends Car { 
    public function __construct($name, $max_capacity, $current_capacity, $power_per_km) { 
     println("Electric car made!"); 

     //If the constructor is that of an electric car, it will be said that a car was made. 

     $this->distance_drived = 0; 
     $this->name = $name; 
     println($max_capacity); 
     $this->battery = new Battery($max_capacity, $current_capacity); 
    } 
    public function move($km) { 
     $power_required = $km * $this->power_per_km; 
     $used = $battery->use_power($power_required); 
     $this->distance_drived += $used/$this->power_per_km; 
    } 
} 

$toyota = new Car("Toyota 2017"); 
println($toyota->name); 
println($toyota->introductory()); 
$tesla = new Car("Tesla Model S", 1000, 750, 5); 
println($tesla->introductory()); 
println("Capacity is ".$tesla->battery->max_capacity); 

?> 

我的主要問題是，該消息仍然是汽車的在ElectricCar消息，所以__construct（）並沒有改變。

Answer 1

您的問題是在這條線：

$tesla = new Car("Tesla Model S", 1000, 750, 5); 

你從來沒有試圖創造一個ElectricCar。將該行更改爲

$tesla = new ElectricCar("Tesla Model S", 1000, 750, 5); 

並且所有內容都應按預期工作。

Answer 2

其實你已經覆蓋父構造函數，只是在子類中寫一個新的構造函數。當你不重寫構造函數時，則調用父函數。

從您的代碼中，我可以看到您將Tesla創建爲Car，而不是ElectricCar，這就是您獲取Car消息而不是ElectricCar消息的原因。

FYI當你希望你的孩子類的構造函數來擴展父類的構造函數，你所要做的是調用父類的構造這樣

parent::__construct();