Dear Jaber,

thank you!

I downloaded Think in Java 2nd edition(from the address you told someone here). I had the book Think in Java Chinese edition(1st edition). I have read both of them several times before polymorphism. i think most of the phenomenon before polymorphism i had some sense.

I will try to finish all the excercises and examples before polymorphism in two days in the 2nd edition. Maybe it's helpful to understand the concepts.

thank you!