PHP OOP - 继承

PHP - 什么是继承？

OOP 中的继承 = 当一个类派生于另一个类时。

子类将从父类继承所有公共和受保护的属性和方法。此外，它还可以有自己的属性和方法。

继承类是通过使用extends关键字。

让我们看一个例子：

示例

<?php
class Fruit {
public $name;
public $color;
public function __construct($name, $color) {
    $this->name = $name;
    $this->color = $color;
}
public function intro() {
    echo "The fruit is {$this->name} and the color is {$this->color}.";
}
}

// Strawberry is inherited from Fruit
class Strawberry extends Fruit {
public function message() {
    echo "Am I a fruit or a berry? ";
}
}
$strawberry = new Strawberry("Strawberry", "red");
$strawberry->message();
$strawberry->intro();
?>

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示例解释

Strawberry 类继承自 Fruit 类。

这意味着由于继承，Strawberry 类可以使用 Fruit 类中的 public $name 和 $color 属性以及 public __construct() 和 intro() 方法。

Strawberry 类也有自己的方法：message()。

PHP - 继承和受保护的访问修饰符

在上一章中我们了解到protected属性或方法可以在类中访问，也可以由从该类派生的类访问。这意味着什么？

让我们看一个例子：

示例

<?php
class Fruit {
public $name;
public $color;
public function __construct($name, $color) {
    $this->name = $name;
    $this->color = $color;
}
protected function intro() {
    echo "The fruit is {$this->name} and the color is {$this->color}.";
}
}

class Strawberry extends Fruit {
public function message() {
    echo "Am I a fruit or a berry? ";
}
}

// Try to call all three methods from outside class
$strawberry = new Strawberry("Strawberry", "red"); // OK. __construct() is public
$strawberry->message(); // OK. message() is public
$strawberry->intro(); // ERROR. intro() is protected
?>

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在上面的例子中我们看到如果我们尝试调用protected如果从类外部调用方法 (intro())，我们将收到错误。public方法会很好用！

让我们看另一个例子：

示例

<?php
class Fruit {
public $name;
public $color;
public function __construct($name, $color) {
    $this->name = $name;
    $this->color = $color;
}
protected function intro() {
    echo "The fruit is {$this->name} and the color is {$this->color}.";
}
}

class Strawberry extends Fruit {
public function message() {
    echo "Am I a fruit or a berry? ";
    // Call protected method from within derived class - OK
    $this -> intro();
}
}

$strawberry = new Strawberry("Strawberry", "red"); // OK. __construct() is public
$strawberry->message(); // OK. message() is public and it calls intro() (which is protected) from within the derived class
?>

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在上面的示例中，我们看到一切正常！这是因为我们称之为protected派生类内部的方法 (intro())。

PHP - 重写继承方法

可以通过在子类中重新定义方法（使用相同的名称）来覆盖继承的方法。

看下面的例子。子类（Strawberry）中的 __construct() 和 intro() 方法将覆盖父类（Fruit）中的 __construct() 和 intro() 方法：

示例

<?php
class Fruit {
public $name;
public $color;
public function __construct($name, $color) {
    $this->name = $name;
    $this->color = $color;
}
public function intro() {
    echo "The fruit is {$this->name} and the color is {$this->color}.";
}
}

class Strawberry extends Fruit {
public $weight;
public function __construct($name, $color, $weight) {
    $this->name = $name;
    $this->color = $color;
    $this->weight = $weight;
}
public function intro() {
    echo "The fruit is {$this->name}, the color is {$this->color}, and the weight is {$this->weight} gram.";
}
}

$strawberry = new Strawberry("Strawberry", "red", 50);
$strawberry->intro();
?>

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PHP - 最终关键字

这个final关键字可用于防止类继承或防止方法重写。

以下示例显示如何防止类继承：

示例

<?php
final class Fruit {
// some code
}

// will result in error
class Strawberry extends Fruit {
// some code
}
?>

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以下示例展示了如何防止方法重写：

示例

<?php
class Fruit {
final public function intro() {
// some code
}
}

class Strawberry extends Fruit {
// will result in error
public function intro() {
// some code
}
}
?>

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