PHP面向对象深入研究之【继承】，减少代码重复

继承

先看两个类

<?php
class CdProduct {
    public $playLength; // 播放时间
    public $title;
    public $producerMainName;
    public $producerFirstName;
    public $price;

    function __construct(   $title, $firstName,
                            $mainName, $price,
                            $playLength ) {
        $this->title             = $title;
        $this->producerFirstName = $firstName;
        $this->producerMainName  = $mainName;
        $this->price             = $price;
        $this->playLength        = $playLength;

    }

    function getPlayLength() {
        return $this->playLength;
    }

    function getSummaryLine() {
        $base  = "{$this->title} ( {$this->producerMainName}, ";
        $base .= "{$this->producerFirstName} )";
        $base .= ": playing time - {$this->playLength}";
        return $base;
    }

    function getProducer() {
        return "{$this->producerFirstName}".
               " {$this->producerMainName}";
    }
}

class BookProduct {
    public $numPages; // 看的页数
    public $title;
    public $producerMainName;
    public $producerFirstName;
    public $price;

    function __construct(   $title, $firstName,
                            $mainName, $price,
                            $numPages ) {
        $this->title             = $title;
        $this->producerFirstName = $firstName;
        $this->producerMainName  = $mainName;
        $this->price             = $price;
        $this->numPages          = $numPages;
    }

    function getNumberOfPages() {
        return $this->numPages;
    }

    function getSummaryLine() {
        $base  = "{$this->title} ( {$this->producerMainName}, ";
        $base .= "{$this->producerFirstName} )";
        $base .= ": page count - {$this->numPages}";
        return $base;
    }

    function getProducer() {
        return "{$this->producerFirstName}".
               " {$this->producerMainName}";
    }
}

$product1 = new CdProduct("cd1", "bob", "bobbleson", 4, 50 );
print $product1->getSummaryLine();
print "\n";

$product2 = new BookProduct("book1", "harry", "harrelson", 4, 30 );
print $product2->getSummaryLine();
print "\n";
?>
输出：
cd1 ( bobbleson, bob ): playing time - 50
book1 ( harrelson, harry ): page count - 30

　点评：这两个类，代码重复性太高，有相同性，也有差异性。不如用继承来简化处理。

采用继承来处理

<?php
class ShopProduct {
    public $numPages;
    public $playLength;
    public $title;
    public $producerMainName;
    public $producerFirstName;
    public $price;

    function __construct(   $title, $firstName,
                            $mainName, $price,
                            $numPages=0, $playLength=0 ) {
        $this->title             = $title;
        $this->producerFirstName = $firstName;
        $this->producerMainName  = $mainName;
        $this->price             = $price;
        $this->numPages          = $numPages;
        $this->playLength        = $playLength;
    }

    function getProducer() {
        return "{$this->producerFirstName}".
               " {$this->producerMainName}";
    }

    function getSummaryLine() {
        $base  = "$this->title ( {$this->producerMainName}, ";
        $base .= "{$this->producerFirstName} )";
        return $base;
    }
}

class CdProduct extends ShopProduct {

    function getPlayLength() { // 增加属于自己的方法
        return $this->playLength;
    }

    function getSummaryLine() { // 改造了父类的方法
        $base  = "{$this->title} ( {$this->producerMainName}, ";
        $base .= "{$this->producerFirstName} )";
        $base .= ": playing time - {$this->playLength}";
        return $base;
    }
}

class BookProduct extends ShopProduct {
    function getNumberOfPages() {
        return $this->numPages;
    }

    function getSummaryLine() {
        $base  = "{$this->title} ( {$this->producerMainName}, ";
        $base .= "{$this->producerFirstName} )";
        $base .= ": page count - {$this->numPages}";
        return $base;
    }
}

$product1 = new CdProduct("cd1", "bob", "bobbleson", 4, null, 50 );
print $product1->getSummaryLine();
print "\n";

$product2 = new BookProduct("book1", "harry", "harrelson", 4, 30 );
print $product2->getSummaryLine();
print "\n";

?>
输出：
cd1 ( bobbleson, bob ): playing time - 50
book1 ( harrelson, harry ): page count - 30

点评：继承处理很好的解决了差异性，相通性问题。

进一步优化处理

<?php
class ShopProduct {
    // 抽离出共有属性
    public $title;
    public $producerMainName;
    public $producerFirstName;
    public $price;

    function __construct(   $title, $firstName,
                            $mainName, $price ) {
        $this->title             = $title;
        $this->producerFirstName = $firstName;
        $this->producerMainName  = $mainName;
        $this->price             = $price;
    }

    function getProducer() {
        return "{$this->producerFirstName}".
               " {$this->producerMainName}";
    }

    function getSummaryLine() {
        $base  = "{$this->title} ( {$this->producerMainName}, ";
        $base .= "{$this->producerFirstName} )";
        return $base;
    }
}

class CdProduct extends ShopProduct {
    // 抽离出属于自己特有的属性
    public $playLength;

    function __construct(   $title, $firstName,
                            $mainName, $price, $playLength ) {
        parent::__construct(    $title, $firstName,
                                $mainName, $price ); // 继承父类的构造函数
        $this->playLength = $playLength; // 处理自己专有的属性
    }

    function getPlayLength() {
        return $this->playLength;
    }

    function getSummaryLine() {
        $base  = "{$this->title} ( {$this->producerMainName}, ";
        $base .= "{$this->producerFirstName} )";
        $base .= ": playing time - {$this->playLength}";
        return $base;
    }
}

class BookProduct extends ShopProduct {
    public $numPages;

    function __construct(   $title, $firstName,
                            $mainName, $price, $numPages ) {
        parent::__construct(    $title, $firstName,
                                $mainName, $price );
        $this->numPages = $numPages;
    }

    function getNumberOfPages() {
        return $this->numPages;
    }

    function getSummaryLine() {
        $base  = "$this->title ( $this->producerMainName, ";
        $base .= "$this->producerFirstName )";
        $base .= ": page count - $this->numPages";
        return $base;
    }
}

$product1 = new CdProduct("cd1", "bob", "bobbleson", 4, 50 );
print $product1->getSummaryLine();
print "\n";

$product2 = new BookProduct("book1", "harry", "harrelson", 4, 30 );
print $product2->getSummaryLine();
print "\n";

?>
输出：
cd1 ( bobbleson, bob ): playing time - 50
book1 ( harrelson, harry ): page count - 30

点评：这里把共有属性在父类中，其他个性属性放在自己的类中处理。并设置自己的构造方法，继承父类的构造方法。

进一步继承父类的方法

<?php
class ShopProduct {
    public $title;
    public $producerMainName;
    public $producerFirstName;
    public $price;

    function __construct(   $title, $firstName,
                            $mainName, $price ) {
        $this->title             = $title;
        $this->producerFirstName = $firstName;
        $this->producerMainName  = $mainName;
        $this->price             = $price;
    }

    function getProducer() {
        return "{$this->producerFirstName}".
               " {$this->producerMainName}";
    }

    function getSummaryLine() {
        $base  = "{$this->title} ( {$this->producerMainName}, ";
        $base .= "{$this->producerFirstName} )";
        return $base;
    }
}

class CdProduct extends ShopProduct {
    public $playLength;

    function __construct(   $title, $firstName,
                            $mainName, $price, $playLength ) {
        parent::__construct(    $title, $firstName,
                                $mainName, $price );
        $this->playLength = $playLength;
    }

    function getPlayLength() {
        return $this->playLength;
    }

    function getSummaryLine() {
        $base = parent::getSummaryLine();
        $base .= ": playing time - {$this->playLength}";
        return $base;
    }
}

class BookProduct extends ShopProduct {
    public $numPages;

    function __construct(   $title, $firstName,
                            $mainName, $price, $numPages ) {
        parent::__construct(    $title, $firstName,
                                $mainName, $price );
        $this->numPages = $numPages;
    }

    function getNumberOfPages() {
        return $this->numPages;
    }

    function getSummaryLine() {
        $base = parent::getSummaryLine();
        $base .= ": page count - {$this->numPages}";
        return $base;
    }
}

$product1 = new CdProduct("cd1", "bob", "bobbleson", 4, 50 );
print $product1->getSummaryLine();
print "\n";

$product2 = new BookProduct("book1", "harry", "harrelson", 4, 30 );
print $product2->getSummaryLine();
print "\n";

?>
输出：
cd1 ( bobbleson, bob ): playing time - 50
book1 ( harrelson, harry ): page count - 30

点评：同样的结果，可以优化优化再优化。这里继承父类的方法。parent::getSummaryLine()。不过这个用的比较少。

继续添加一些有意思的内容

<?php
class ShopProduct {
    private $title;
    private $discount = 0;
    private $producerMainName;
    private $producerFirstName;
    protected $price;

    function __construct(   $title, $firstName,
                            $mainName, $price ) {
        $this->title             = $title;
        $this->producerFirstName = $firstName;
        $this->producerMainName  = $mainName;
        $this->price             = $price;
    }

    function setDiscount( $num ) {
        $this->discount=$num;
    }

    function getPrice() {
        return ($this->price - $this->discount);
    }

    function getProducer() {
        return "{$this->producerFirstName}".
               " {$this->producerMainName}";
    }

    function getSummaryLine() {
        $base  = "{$this->title} ( {$this->producerMainName}, ";
        $base .= "{$this->producerFirstName} )";
        return $base;
    }
}

class CdProduct extends ShopProduct {
    public $playLength;

    function __construct(   $title, $firstName,
                            $mainName, $price, $playLength ) {
        parent::__construct(    $title, $firstName,
                                $mainName, $price );
        $this->playLength = $playLength;
    }

    function getPlayLength() {
        return $this->playLength;
    }

    function getSummaryLine() {
        $base = parent::getSummaryLine();
        $base .= ": playing time - {$this->playLength}";
        return $base;
    }
}

class BookProduct extends ShopProduct {
    public $numPages;

    function __construct(   $title, $firstName,
                            $mainName, $price, $numPages ) {
        parent::__construct(    $title, $firstName,
                                $mainName, $price );
        $this->numPages = $numPages;
    }

    function getPrice() {
        return $this->price;
    }

    function getNumberOfPages() {
        return $this->numPages;
    }

    function getSummaryLine() {
        $base = parent::getSummaryLine();
        $base .= ": page count - {$this->numPages}";
        return $base;
    }
}

$product1 = new CdProduct("cd1", "bob", "bobbleson", 4, 50 );
$product1->setDiscount( 3 );
print $product1->getSummaryLine();
print "\n";
print "price: {$product1->getPrice()}\n";

$product2 = new BookProduct("book1", "harry", "harrelson", 4, 30 );
$product2->setDiscount( 3 ); // 折扣对book无效
print $product2->getSummaryLine();
print "\n";
print "price: {$product2->getPrice()}\n";

?>
输出：
cd1 ( bobbleson, bob ): playing time - 50
price: 1
book1 ( harrelson, harry ): page count - 30
price: 4

点评:父类添加了折扣，book继承之后，修改了getPrice方法，所以折扣对book无效。

私有化属性，通过方法来设置与获取

<?php
class ShopProduct {
    // 私有化属性，通过方法来设置与获取
    private $title;
    private $producerMainName;
    private $producerFirstName;
    protected $price;
    private $discount = 0;

    public function __construct(   $title, $firstName,
                            $mainName, $price ) {
        $this->title             = $title;
        $this->producerFirstName = $firstName;
        $this->producerMainName  = $mainName;
        $this->price             = $price;
    }

    public function getProducerFirstName() {
        return $this->producerFirstName;
    }

    public function getProducerMainName() {
        return $this->producerMainName;
    }

    public function setDiscount( $num ) {
        $this->discount=$num;
    }

    public function getDiscount() {
        return $this->discount;
    }

    public function getTitle() {
        return $this->title;
    }

    public function getPrice() {
        return ($this->price - $this->discount);
    }

    public function getProducer() {
        return "{$this->producerFirstName}".
               " {$this->producerMainName}";
    }

    public function getSummaryLine() {
        $base  = "{$this->title} ( {$this->producerMainName}, ";
        $base .= "{$this->producerFirstName} )";
        return $base;
    }
}

class CdProduct extends ShopProduct {
    private $playLength = 0;

    public function __construct(   $title, $firstName,
                            $mainName, $price, $playLength ) {
        parent::__construct(    $title, $firstName,
                                $mainName, $price );
        $this->playLength = $playLength;
    }

    public function getPlayLength() {
        return $this->playLength;
    }

    public function getSummaryLine() {
        $base = parent::getSummaryLine();
        $base .= ": playing time - {$this->playLength}";
        return $base;
    }

}

class BookProduct extends ShopProduct {
    private $numPages = 0;

    public function __construct(   $title, $firstName,
                            $mainName, $price, $numPages ) {
        parent::__construct(    $title, $firstName,
                                $mainName, $price );
        $this->numPages = $numPages;
    }

    public function getNumberOfPages() {
        return $this->numPages;
    }

    public function getSummaryLine() {
        $base = parent::getSummaryLine();
        $base .= ": page count - {$this->numPages}";
        return $base;
    }

    public function getPrice() {
        return $this->price;
    }
}

$product1 = new CdProduct("cd1", "bob", "bobbleson", 4, 50 );
print $product1->getSummaryLine()."\n";
$product2 = new BookProduct("book1", "harry", "harrelson", 4, 30 );
print $product2->getSummaryLine()."\n";
?>

输出：
cd1 ( bobbleson, bob ): playing time - 50
book1 ( harrelson, harry ): page count - 30

点评：这里进一步私有化了属性，要想获取只能通过方法。这样就确保了安全性。