Java 泛型

reference

public static  List backwards (List input) {  
       List output = new ArrayList();  
       return output;  
}

I"m declaring an arbitrary type T for this method,
which can be anything (non-primitive) for each call of the method.

List

This method will return a List containing elements of that type T.

List input

This method will take a parameter called input, 
which is a List containing elements of type T, or any super-type of T. For example, 
if T is Integer, input could be a List, List, or List