目标1包，导入，内类与接口。

Objective 1, Packages, import, inner classes, interfaces.

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SAAJ 定义了 SOAP 消息中各个部分的类与接口。

SAAJ defines classes and interfaces for all of the pieces of a SOAP message.

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这本书里有一些关于类与接口设计的建议。你能否简单的讲述一下你对于良好设计的指导准则？

The book includes hints for class and interface design. In one paragraph, can you describe a guideline for a good design?

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