模块的接口包含两种信息：正式信息和非正式信息。接口的形式部分在代码中明确指定，并且其中一些可以通过编程语言检查其正确性。例如，方法的形式接口是其签名，其中包括其参数的名称和类型，其返回值的类型以及有关该方法引发的异常的信息。大多数编程语言都确保对方法的每次调用都提供正确数量和类型的参数以匹配其签名。类的形式接口包括其所有公共方法的签名以及任何公共变量的名称和类型。
每个接口还包括非正式元素。这些没有以编程语言可以理解或执行的方式指定。接口的非正式部分包括其高级行为，例如，函数删除由其参数之一命名的文件的事实。如果对类的使用存在限制（也许必须先调用一种方法），则这些约束也是类接口的一部分。通常，如果开发人员需要了解特定信息才能使用模块，则该信息是模块接口的一部分。接口的非正式方面只能使用注释来描述，而编程语言不能确保描述是完整或准确的。对于大多数接口，非正式方面比正式方面更大，更复杂。
明确指定接口的好处之一是，它可以准确指示开发人员使用关联模块所需要知道的内容。这有助于消除 2.2. 复杂性的症状 中描述的“未知的未知”问题。
The interface to a module contains two kinds of information: formal and informal. The formal parts of an interface are specified explicitly in the code, and some of these can be checked for correctness by the programming language. For example, the formal interface for a method is its signature, which includes the names and types of its parameters, the type of its return value, and information about exceptions thrown by the method. Most programming languages ensure that each invocation of a method provides the right number and types of arguments to match its signature. The formal interface for a class consists of the signatures for all of its public methods, plus the names and types of any public variables.
Each interface also includes informal elements. These are not specified in a way that can be understood or enforced by the programming language. The informal parts of an interface include its high-level behavior, such as the fact that a function deletes the file named by one of its arguments. If there are constraints on the usage of a class (perhaps one method must be called before another), these are also part of the class’s interface. In general, if a developer needs to know a particular piece of information in order to use a module, then that information is part of the module’s interface. The informal aspects of an interface can only be described using comments, and the programming language cannot ensure that the description is complete or accurate. For most interfaces the informal aspects are larger and more complex than the formal aspects.
One of the benefits of a clearly specified interface is that it indicates exactly what developers need to know in order to use the associated module. This helps to eliminate the “unknown unknowns” problem described in Section 2.2.