下面代码是书上例子，在php下运行也没错误，但有几点不解
1.abstract 抽象类 里面竟然没有抽象方法？
2. function __construct( $duration, CostStrategy $strategy ) {
还有这种传递方式？我仔细看了一下，应该是把下面声明的抽象类CostStrategy以别名方式传进来，是这样吗，如果是真的，抽象类CostStrategy都不需要被继承吗？
这段代码把我搞糊涂了

 <?php
abstract class Lesson {
    private   $duration;
    private   $costStrategy;

    function __construct( $duration, CostStrategy $strategy ) {
        $this->duration = $duration;
        $this->costStrategy = $strategy;
    }

    function cost() {
        return $this->costStrategy->cost( $this );
    }

    function chargeType() {
        return $this->costStrategy->chargeType( );
    }

    function getDuration() {
        return $this->duration;
    }

    // more lesson methods...
}


abstract class CostStrategy {
    abstract function cost( Lesson $lesson );
    abstract function chargeType();
}

class TimedCostStrategy extends CostStrategy {
    function cost( Lesson $lesson ) {
        return ( $lesson->getDuration() * 5 );
    }
    function chargeType() {
        return "hourly rate";
    }
}

class FixedCostStrategy extends CostStrategy {
    function cost( Lesson $lesson ) {
        return 30;
    }

    function chargeType() {
        return "fixed rate";
    }
}

class Lecture extends Lesson {
    // Lecture-specific implementations ...
}

class Seminar extends Lesson {
    // Seminar-specific implementations ...
}

$lessons[] = new Seminar( 4, new TimedCostStrategy() );
$lessons[] = new Lecture( 4, new FixedCostStrategy() );

foreach ( $lessons as $lesson ) {
    print "lesson charge {$lesson->cost()}. ";
    print "Charge type: {$lesson->chargeType()}\n";
}
?>