设计精良的模块和设计糟糕的模块的唯一最大区别，就是对其他模块隐藏本模块内部数据和其他实现细节的程度。——Joshua Bloch
The single most important factor that distinguishes a well-designed module from a poorly designed one is the degree to which the module hides its internal data and otherimplementationdetails from other modules. —Joshua Bloch