general principles, to four points:
• Separate out the things that change from those that stay the same.
• Program to an interface, not an implementation.
• Prefer composition over inheritance.
• Delegate, delegate, delegate.
following sections, we will look at each of these principles in turn, to see
what they can tell us about building software.
Separate Out the Things That Change from Those
That Stay the Same
Software engineering would be a lot easier if only things would stay the same. We
could build our classes serene in the knowledge that, once finished, they would continue to do exactly what we built them to do. Of course, things never stay the same,
not in the wider world and certainly not in software engineering. Changes in com-
puting hardware, operating systems, and compilers, combined with ongoing bug fixes and ever-migrating requirements, all take their toll.
A key goal of software engineering is to build systems that allow us to contain the
damage. In an ideal system, all changes are local: You should never have to comb
through all of the code because A changed, which required you to change B, which
triggered a change in C, which rippled all the way down to Z. So how do you achieve— or at least get closer to—that ideal system, the one where all changes are local?
You get there by separating the things that are likely to change from the things
that are likely to stay the same. If you can identify which aspects of your system design
are likely to change, you can isolate those bits from the more stable parts. When
requirements change or a bug fix comes along, you will still have to modify your code,
but perhaps, just perhaps, the changes can be confined to those walled-off, change-prone areas and the rest of your code can live on in stable peace.
But how do you effect this quarantine? How do you keep the changing parts from
infecting the stable parts?
Program to an Interface, Not an Implementation
A good start is to write code that is less tightly coupled to itself in the first place. If our
classes are to do anything significant, they need to know about each other. But what

创新互联www.cdcxhl.cn，专业提供香港、美国云服务器，动态BGP最优骨干路由自动选择，持续稳定高效的网络助力业务部署。公司持有工信部办法的idc、isp许可证， 机房独有T级流量清洗系统配攻击溯源，准确进行流量调度，确保服务器高可用性。佳节活动现已开启，新人活动云服务器买多久送多久。

你所需要的网站建设服务，我们均能行业靠前的水平为你提供.标准是产品质量的保证，主要从事成都网站设计、做网站、企业网站建设、手机网站制作设计、网页设计、品牌网站设计、网页制作、做网站、建网站。成都创新互联拥有实力坚强的技术研发团队及素养的视觉设计专才。
分享名称：designpatterninruby-创新互联
文章分享：http://kswjz.com/article/ipcpe.html