一直疑惑 ArrayList<int> 怎么搞，但懶得去查，今天網上搜到了：
The generics facility in Java can be used only for
object (reference) types. It cannot be used primitive
types like 'int' or 'float'.
So, if you wanted an ArrayList that contained only
integer data, then you'd need to use the object type
which corresponds to int: java.lang.Integer.
ArrayList<Integer> numbers;
numbers = new ArrayList<Integer>(40);
Fortunately, the new Autobox/unbox features of Java can
make this work just like an ArrayList of int.
numbers.add(27);
The value 27 will be automatically boxed as an Integer
object, then added to the ArrayList<Integer>.
For more information, consult
http://java.sun.com/developer/technicalArticles/J2SE/generics/index.html
http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf