5.7 Example: defaults in HTTP responses 示例：HTTP 响应中的默认值

The HTTP projects also had to provide support for generating HTTP responses. The most common mistake students made in this area was inadequate defaults. Each HTTP response must specify an HTTP protocol version; one team required callers to specify this version explicitly when creating a response object. However, the response version must correspond to that in the request object, and the request must already be passed as an argument when sending the response (it indicates where to send the response). Thus, it makes more sense for the HTTP classes to provide the response version automatically. The caller is unlikely to know what version to specify, and if the caller does specify a value, it probably results in information leakage between the HTTP library and the caller. HTTP responses also include a Date header specifying the time when the response was sent; the HTTP library should provide a sensible default for this as well.

HTTP 项目还必须提供对生成 HTTP 响应的支持。学生在该领域中最常见的错误是默认值不足。每个 HTTP 响应必须指定一个 HTTP 协议版本。一个团队要求呼叫者在创建响应对象时明确指定此版本。但是，响应版本必须与请求对象中的版本相对应，并且在发送响应时必须已将请求作为参数传递（它指示将响应发送到何处）。因此，HTTP 类自动提供响应版本更为有意义。调用者不太可能知道要指定哪个版本，并且如果调用者确实指定了一个值，则可能导致 HTTP 库和调用者之间的信息泄漏。HTTP 响应还包括一个 Date 标头，用于指定发送响应的时间；HTTP 库也应该为此提供一个合理的默认值。

Defaults illustrate the principle that interfaces should be designed to make the common case as simple as possible. They are also an example of partial information hiding: in the normal case, the caller need not be aware of the existence of the defaulted item. In the rare cases where a caller needs to override a default, it will have to know about the value, and it can invoke a special method to modify it.

默认值说明了应该设计接口以使常见情况尽可能简单的原则。它们还是隐藏部分信息的一个示例：在正常情况下，调用者无需知道默认项的存在。在极少数情况下，调用方需要覆盖默认值，它必须知道该值，并且可以调用特殊方法来对其进行修改。

Whenever possible, classes should “do the right thing” without being explicitly asked. Defaults are an example of this. The Java I/O example on page 26 illustrates this point in a negative way. Buffering in file I/O is so universally desirable that noone should ever have to ask explicitly for it, or even be aware of its existence; the I/O classes should do the right thing and provide it automatically. The best features are the ones you get without even knowing they exist.

只要有可能，类就应该“做正确的事”，而无需明确要求。默认值就是一个例子。第 26 页上的 Java I/O 示例以负面方式说明了这一点。普遍希望在文件 I/O 中缓冲，以至于没有人需要明确要求它，甚至不知道它的存在。I/O 类应该做正确的事情并自动提供它。最好的功能是您甚至不知道它们存在的功能。

img Red Flag: Overexposure img

If the API for a commonly used feature forces users to learn about other features that are rarely used, this increases the cognitive load on users who don’t need the rarely used features.

如果常用功能的 API 迫使用户了解很少使用的其他功能，则这会增加不需要很少使用功能的用户的认知负担。