Make Interfaces Easy to Use Correctly and Hard to Use Incorrectly

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Revision as of 23:59, 10 November 2008 by ScottMeyers (Talk | contribs)
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One of the most common tasks in software development is interface specification. Interfaces occur at the highest level of abstraction (user interfaces), at the lowest (function interfaces), and at numerous levels in between (class interfaces, library interfaces, package interfaces, etc.). Regardless of whether you work with end users to specify how they'll interact with a system under design, work with other developers to specify an API, or declare functions private to a class you're working on, interface design is an important part of your job. If you do it well, the resulting interfaces will be a pleasure to use and will actually boost productivity. If you do it poorly, your interfaces will be a source of frustration and errors.

Good interfaces have two crucial characteristics:

  • They're easy to use correctly. People using a well-designed interface almost always use the interface correctly, because that's the path of least resistance. In a GUI, they almost always click on the right icon, button, or menu entry, because it's the obvious and easy thing to do. In an API, they almost always pass the correct parameters with the correct types and values, because, again, it's obvious and easy. With interfaces that are easy to use correctly, things just work.
  • They're hard to use incorrectly. Good interfaces anticipate mistakes people might make, and they make them difficult -- ideally impossible -- to commit. A GUI might disable or remove commands that make no sense in the current context, for example, and an API might allow parameters to be passed in any order to eliminate "parameters passed out of order" problems.

A good way to design interfaces that are easy to use correctly is to exercise them before they exist. Mock up a GUI -- possibly on a whiteboard or using index cards on a table -- and play with it before any of the underlying code has been created. Write calls to an API before any of the functions have been declared. Walk through various common use cases and specify how you want the interface to behave. What do you want to be able to click on? What do you want to be able to pass? Easy to use interfaces seem natural, because they let you do what you want to do. You're more likely to come up with such interfaces if you develop them from the user's point of view. (Such a perspective is one of the strengths of test-first programming.)

Making interfaces hard to use incorrectly requires two things. First, you must anticipate errors that users might inadvertently make, and you must find ways to prevent such errors. Second, you must observe how the interface is misused during early release, and you must be willing to modify the interface -- yes, modify the interface! -- to prevent such errors before deploying the interface further. The best way to prevent incorrect use is to make it impossible. If users keep wanting to undo an irrevocable action, make it revocable! If they keep passing the wrong value to an API, modify the API to take the kind of value users want to pass! Interfaces exist for the convenience of their users, not their implementers.

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