无法查看从父类继承的属性

Question

我创建了一个包含以下变量的银行帐户类，$Balance变量是私有的，因此我使用setter和getter函数访问余额的私有属性。

class BankAccount
{
    public $HoldersName;
    public $AccountNumber;
    public $SortCode;
    private $Balance = 1;
    public $APR = 0;
    public $Transactions = []; //array defined.

    public function __construct($HoldersName, $AccountNumber, $SortCode, $Balance = 1) //constructor where i.e. HoldersName is the same as the variable $HoldersName. 
    {                                                                                  //constructor automatically called when object is created.
        echo "Holders Name: " . $this->HoldersName  = $HoldersName . "</br>";
        echo "Account Number: " . $this->AccountNumber = $AccountNumber . "</br>";
        echo "Sort Code: " . $this->SortCode = $SortCode . "</br>";
        $this->Balance = $Balance >= 1 ? $Balance : 1;
    }


    public function set_Balance($Balance) //SET balance.
    {
        return $this->Balance=$Balance;
    }

    public function get_Balance() //GET balance.
    {
        return $this->Balance; //Allows us to access the prviate property of $Balance.
    }

}

SavingsAccount类扩展了BankAccount，因此它继承了BankAccount类中的所有内容。我创建了一个函数来计算利息，即余额6800 * (0.8+0.25) * 1。

class SavingsAccount extends BankAccount
{
    public $APR = 0.8;
    public $APRPaymentPreference;
    public $ExtraBonus = 0.25;

    public function CalculateInterest()
    {
        $this->Balance = $this->Balance * ($this->APR + $this->ExtraBonus) * 1; //this is where i get the error now

    }

    public function FinalSavingsReturn()
    {

    }
}

在这里，我创建了类SavingsAccount的一个实例，余额为6800，我试图调用函数SavingsAccount::CalculateInterest()，但出现了以下错误：

注意未定义的属性: SavingsAccount::$Balance在第42行

//define objects
$person2 = new SavingsAccount ('Peter Bond', 987654321, '11-11-11', 6800); //create instance of class SavingsAccount
echo "<b>Interest:</b>";
print_r ($person2->CalculateInterest());

Answer 1

正如您所说，$Balance属性是私有的。私有意味着它不能从定义类之外访问，甚至不能从继承该属性的子类访问。

<?php
class Foo {
    private $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->bar;}
}
new Foo;
new Bar;

如果我们运行这段代码，就会得到错误：

baz
PHP Notice:  Undefined property: Bar::$bar in php shell code on line 2

所以你有两个选择。您可以使用您的getter代替：

<?php
class Foo {
    private $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->getBar();}
}
new Foo;
new Bar;

或者，可以将属性声明为protected而不是private。

<?php
class Foo {
    protected $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->bar;}
}
new Foo;
new Bar;

这两种方法中的任何一种都提供了预期的产出：

baz
baz

有关属性和方法可见性的详细信息，请参阅这里的文件。