在子类里,我们可以通过重载父类方法来改变实体的行为.
ruby> class Human
| def identify
| print "I'm a person.\n"
| end
| def train_toll(age)
| if age < 12
| print "Reduced fare.\n";
| else
| print "Normal fare.\n";
| end
| end
| end
nil
ruby> Human.new.identify
I'm a person.
nil
ruby> class Student1<Human
| def identify
| print "I'm a student.\n"
| end
| end
nil
ruby> Student1.new.identify
I'm a student.
nil

如果我们只是想增强父类的 identify 方法而不是完全地替代它,就可以用 super.
ruby> class Student2<Human
| def identify
| super
| print "I'm a student too.\n"
| end
| end
nil
ruby> Student2.new.identify
I'm a human.
I'm a student too.
nil

super 也可以让我们向原有的方法传递参数.这里有时会有两种类型的人...
ruby> class Dishonest<Human
| def train_toll(age)
| super(11) # we want a cheap fare.
| end
| end
nil
ruby> Dishonest.new.train_toll(25)
Reduced fare.
nil
ruby> class Honest<Human
| def train_toll(age)
| super(age) # pass the argument we were given
| end
| end
nil
ruby> Honest.new.train_toll(25)
Normal fare.
nil

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