BeeperPickingKarel 类的实例也将是一个机器人，它生活在同样类型世界里，并具有相同的状态属
性。
2. Any instance of the BeeperPickingKarel class will automatically respond to the same commands as an
instance of the Karel class. Because every robot in the Karel class knows how to respond to the
commands move(), turnLeft(), pickBeeper(), and putBeeper(), it follows that a instance of
BeeperPickingKarel will understand that same set of commands.
2.任何 BeeperPickingKarel 类的实例将会自动的响应与任何 karel 类的实例相同的命令，因为 每一
个 karel 类的机器人都知道如何响应 move(), turnLeft(), pickBeeper(), 和 putBeeper()这些命令，作为
它的跟随者，BeeperPickingKarel 类的实例也能明白这同一套命令。
In other words, the new BeeperPickingKarel class automatically acquires the state attributes and the
behavior of the Karel class from which it is derived. The process of taking on the structure and behavior
of the parent class is called inheritance.
在其它世界里，新的 BeeperPickingKarel 类自动获得 karel 类派生出的状态属性和行为。这个从它
的父类取得结构和行为的过程被称为继承。
When a class is defined by extension, the new class is said to be a subclass of the original. In this
example, BeeperPickingKarel is therefore a subclass of Karel. Symmetrically, Karel is said to be a
superclass of BeeperPickingKarel. Unfortunately, this terminology can be confusing for new
programmers, who are likely to make the intuitive inference that a subclass is somehow less powerful that
its superclass when in fact the opposite is true. A subclass inherits the behavior of its superclass and can
therefore respond to the entire set of commands available to that superclass. A subclass, however, usually
defines additional commands that are unavailable to the superclass. Thus, the typical subclass actually has
more functionality than the class from which it was derived. This idea is expressed much more clearly by
the notion of extension: a subclass extends its superclass and can therefore add new capabilities to it.
当一个类是由扩展而来的，这个新类被称为原始类的子类。在这个例子中，BeeperPickingKarel 因
此被称为 karel 类的一个子类。相对的，karel 类被称为 BeeperPickingKarel 类的超级类。
不幸的是，这个称呼可能会引起新程序员的混乱，谁都会直观的认为，一个子类的能力要比它的
超级类小，而事实恰恰相反。一个子类继承了超级类的行为，因此可以响应超级类的整套命令。
但是，一个子类通常会定义一些额外的命令，这些额外的命令超级类不能执行。因此，一个典型
的子类实际上拥有比派生它的原始类更多的功能。这个认识被扩展这个概念表达的更清楚：一个
子类扩展了它的超级类，子类因此获得新的能力。
Now that you have some idea about what class extension means, it now makes sense to look at the body
of the BeeperPickingKarel class. That body consists of the following lines:
现在你对什么是类扩展的意思有了一些概念，它使 BeeperPickingKarel 类的本体看起来产生了一
些意义。这个本体包含下面的这些行：
public void run() {
move();
pickBeeper();
move();
}
These lines represent the definition of a new method, which specifies the sequence of steps necessary to
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