上传一本书（Agile_Java英文版）

英文版得书,另外添加一本java核心技术的英文版.(如需要中文版，留言)
最近在看这本书,书中一些好的编程习惯也列出来，供以后参考.

1.Design your methods to either change object state or return information, not both.
2.Single-Responsibility Principle.
One of the most basic design principles in object-oriented programming is that a class should do one thing and do it well. By virtue of doing this one thing, the class should have only one reason to change. This is known as the Single-Responsibility Principle
3.By declaring the constructor as private, only code in the DateUtil class can construct new DateUtil instances. No other code will be able to do so. While it wouldn't be harmful to allow creation of DateUtil objects, keeping clients from doing something nonsensical and useless is a good idea
4.It is possible to access a class method or variable from the instance side of a class without specifying the class name.Even though it will work, avoid doing this. Accessing class methods without using the class name introduces unnecessary confusion in your code and is considered bad form.

5. The more you depend upon concrete types, the more difficult your system is to change. The more you depend upon abstract types (interfaces), the easier your system is to change. By introducing interfaces, you build an abstraction barrier between the client and a concrete server class.