6. 类变量和类方法
--------------------
　　6.1 Ruby类也允许定义类变量和方法。
======
一个类变量允许在一个类的所有实例间共享单个变量。在Ruby中，两个@@号用于指示类变量。例如，如果你想要使一个BankAccount类的所有实例共享相同的利息率，那么该类可能被如下定义：

class BankAccount
@@interestRate = 6.5
def BankAccount.getInterestRate()
@@interestRate
end
attr_accessor :balance
def initialize (bal)
@balance = bal
end
end

　　6.2 继承
======
例子：Square类继承Rectangle

Square类可以使用下列定义创建：

class Square < Rectangle
end

重载现有的Square的initialize方法：

class Square < Rectangle
def initialize (size)
@height = size
@width = size
end
end

　　6.3 所有类的基类 Object
======
Ruby中的一切几乎都派生自Object类

位于任何类之外的方法：

def feel?
return "I feel fine."
end

　　为了执行这个方法，只要输入该方法名，而不需要类或实例：

irb(main):042:0> feel?
=> "I feel fine."

　　此方法看似另外一种语言（如C）中的函数或过程。事实上，尽管这些方法看上去好象不属于任何类，但是这些方法却都是你已经添加到Object类上的方法，它（因为Object是所有类的超类）反过来也把这一方法添加到你的继承类上。因此，现在你可以在任何对象（如Square和Rectangle的实例）甚至一个类（如Rectangle类）上调用这个方法。

irb(main):043:0> sq1=Square.new(4)
=> #<Square:0x5a18b50 @width=4， @height=4>
irb(main):044:0> rect1=Rectangle.new(5，7)
=> #<Rectangle:0x5a139a8 @width=7， @height=5>
irb(main):045:0> sq1.feel?
=> "I feel fine."
irb(main):046:0> rect1.feel?
=> "I feel fine."
irb(main):047:0> Rectangle.feel?
=> "I feel fine."
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