新类可从现有的类中产生并保留现有类的成员变量和方法，并可根据需要对 它们加以修改。新类还可添加新的变量和方法，这种现象就称为类的继承。 -New classes from existing classes and retain existing members of the class variables and methods can be modified according to need them. The new class can add new variables and