PHP子类构造函数无法覆盖父类私有属性的问题咨询

问题描述 #

我有Java开发背景，想按照Java中子类构造函数的用法来操作PHP子类，但实际运行不符合预期：子类构造函数无法覆盖父类构造函数初始化的实例变量值。该父类仅作为多个子类复用公共方法的基类，不会被单独实例化。但实例化TestPackage子类时，得到的却是父类中初始化的错误值，请问这是什么原因？

相关代码 #

接口定义 #

interface Package{
    public function getCode();
    public function decode($packageCode);
    public function getSetName();
    public function getSetNameLang();
    public function getSubsetType();
    public function getSubsetTypeLang();
    public function setSubsetType($typeCode);
    public function getPackageOptionsArray();
    public function getChoicesArray();
    public function getOptionStatus($index);
    public function setOptionStatus($index, $booleanStatus);
}

父类 ParentPackage.php #

class ParentPackage implements Package{
    private int $packageCode;
    private int $subsetType;
    private String $setName;
    private String $setNameLang;
    private $packageOptionsArray;
    private $subsetTypeArray;

    public function __construct(){
        $this->packageCode = 0;
        $this->subsetType = 0;
        $this->setName = "Error: parentClass";
        $this->setNameLang = "Error: Parent Class";

        $this->packageOptionsArray = array("Error",   //too much stuff to pass
                                            "Error",  //to a constructor as a variable
                                            "Error",
                                            "Error",
                                            "Error",
                                            "Error",
                                            "Error",
                                            "Error",
                                            "Error",
                                            "Error",
                                            "Error",
                                            "Error",
                                            );
        
        $this->subsetTypeArray = array("Error: Initializer",   //16
                                        "Error: Initializer",  //32
                                        "Error: Initializer",  //48
                                        );

     //........A whole bunch more stuff that is too much to pass as a variable to a constructor
     //..... OR let's just say that I don't want to do it that way

    }

    // 省略Package接口中方法的实现，这些方法默认返回对应私有属性的值
}

子类 TestPackage.php #

class TestPackage extends ParentPackage{

    public function __construct(){
        parent::__construct();
        $this->packageCode = 0;
        $this->subsetType = 0;
        $this->setName = "layeredarch";
        $this->setNameLang = "Layered Arch";

        $this->packageOptionsArray = array("Customized welcome sign (choice of trellis half arch or smooth half arch insert up to 25 words text)",
                                            "3 piece seating chart half arch set (print service for cards is available for a small additional fee)",
                                            "Table numbers 1-30",
                                            "Gold Card option04 with choice of Gifts & Cards sign",
                                            "5 Reserved signs",
                                            "Up to 2 Double Half Arch Small signs (Gifts & Cards, Take One, Dont Mind if I Do, In Loving Memory)",
                                            "Up to 2 Sunset Small signs (Please Sign Our Guestbook, Gifts & Cards, In Loving Memory)",
                                            "1 Double Half Arch Medium sign (Cheers, The Bar, Guestbook, or Custom Acrylic Text)",
                                            "1 Double Full Arch Medium sign (Signature Drinks, or Custom Acrylic Text)",
                                            "Unplugged Ceremony sign",
                                            "Hairpin Record Player Prop",
                                            "%22Mr & Mrs%22 Custom Head Table Keepsake is a free gift in addition to the items above"
                                            );
        
        $this->subsetTypeArray = array("Full Set",   //16
                                        "Pick Six",  //32
                                        "Pick Four" //48
                                        );

    }
}

问题原因 #

核心问题在于PHP中父类的私有属性（private修饰）子类完全无法访问，这和Java的访问规则存在差异：

• Java中子类可以访问父类的protected属性或同包下的默认权限属性，但PHP的private是严格的类级访问控制——只有属性所在的类自身能读写该属性，子类即便直接继承，也无法直接操作父类的私有属性。
• 你在子类构造函数中写的$this->setName = "layeredarch"，并不是修改父类的$setName私有属性，而是在子类中新建了一个同名的公共属性。但父类中实现的getSetName()方法返回的是父类自己的私有属性值，所以调用该方法时得到的还是父类初始化的"Error: parentClass"。

解决方案 #

有两种常用的解决方式：

1. 将父类属性的访问修饰符改为protected #

把父类中所有需要子类修改的属性从private改成protected，这样子类就能直接读写这些属性，修改后的值会覆盖父类构造函数初始化的内容：

class ParentPackage implements Package{
    protected int $packageCode;
    protected int $subsetType;
    protected String $setName;
    protected String $setNameLang;
    protected $packageOptionsArray;
    protected $subsetTypeArray;

    // 构造函数和其他方法保持不变
}

2. 在父类中提供Setter方法 #

如果不想修改属性的访问权限，可以在父类中为每个需要修改的属性添加Setter方法，子类通过调用这些方法来更新属性值：

class ParentPackage implements Package{
    private int $packageCode;
    private int $subsetType;
    private String $setName;
    private String $setNameLang;
    private $packageOptionsArray;
    private $subsetTypeArray;

    // 构造函数不变

    public function setSetName(string $setName): void {
        $this->setName = $setName;
    }

    public function setSetNameLang(string $setNameLang): void {
        $this->setNameLang = $setNameLang;
    }

    // 为其他属性添加对应的Setter方法
}

然后在子类构造函数中调用这些Setter：

class TestPackage extends ParentPackage{
    public function __construct(){
        parent::__construct();
        $this->setSetName("layeredarch");
        $this->setSetNameLang("Layered Arch");
        // 调用其他Setter方法设置对应属性
    }
}

内容的提问来源于stack exchange，提问作者Adam Winter