public class MaxPQ >什么写法？求教，谢谢！
Key extends Comparable
之前没见过 也没学过 特来求教
http://stackoverflow.com/questions/10971888/implements-vs-extends-in-generics-in-javahttp://docs.oracle.com/javase/7/docs/api/java/lang/Comparable.html?is-external=true
