继承换成组合
http://www.cnblogs.com/loulijun/archive/2012/03/04/2379082.html
http://blog.sina.com.cn/s/blog_628e5ca30100fng1.html
Java 继承与组合的区别
http://xifangyuhui.iteye.com/blog/819498
面向对象之继承和组合浅谈
http://www.cnblogs.com/whitewolf/archive/2010/05/03/1726519.html