Guideline 4. Think of objects as machines

Objects are invisible machines that programmers use as tools. When you design an object, you design a machine for programmers. I feel thinking of objects as machines is helpful, because it encourages you to focus on both functionality and usability when you design objects.

The two main qualities I look for in any machine are reliability and ease of use. If I buy a new cyclometer for my bicycle, for example, I want that cyclometer to work well -- to reliably keep accurate time, distance, and speed records. But I also want it to be easy to use. I want services I request often, such as switching the display between distance, time of day, and speed, to be quick and easy to access. I don't mind if less-used services, such as setting my tire size or the current time, are more difficult to access. But in no case do I want to invest more than a few minutes of my time pressing buttons or consulting the instruction booklet to access any functionality offered by my cyclometer. Similarly, if I instantiate a class in an API, I want the resulting object to work well. I want the object to do what it promises to do, in an efficient manner, every time I ask it. But I also want the object to be easy to use. I want its interface to allow me, with no more than a few minutes searching through the API documentation, to figure out how to make the object perform the desired service.