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Part I. Objects
People-Oriented API Design
Guideline 1. Design objects for people, not for computers
by Bill Venners

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Monkeys on Your Back

In the real world, as you work to design and implement software, you have several concerns to keep in mind -- several "monkeys on your back." Each monkey competes with the others for your attention, trying to convince you to take its particular concern to heart as you work. One large, heavy monkey hangs on your back with its arms around your neck and repeatedly yells, "You must meet the schedule!" Another monkey, this one perched on top of your head (as there is no more room on your back), beats its chest and cries, "You must accurately implement the specification!" Still another monkey jumps up and down on top of your monitor yelling, "Robustness, robustness, robustness!" Another keeps trying to scramble up your leg crying, "Don't forget about performance!" And every now and then, a small monkey peeks timidly at you from beneath the keyboard. When this happens, the other monkeys become silent. The little monkey slowly emerges from under the keyboard, stands up, looks you in the eye, and says, "You must make the code easy to read and easy to change." With this, all the other monkeys scream and jump onto the little monkey, forcing it back under the keyboard. As you sit there in your cubicle and work on your software, to which monkey should you listen? Alas, in most cases you must listen to all of them. To do a "good" job, you will need to find a way to keep all these monkeys happy -- to strike a proper balance between these often conflicting concerns.

An object design is good to the extent it achieves the optimum balance among the concerns pressing on the design. And that proper balance depends on the situation. There are times when you should design, times when you should hack, and times when you should do something in-between.

In my experience, however, the most important monkey in object design is usually the monkey under the keyboard. This monkey gently reminds you to make the software easy to understand and change -- easy for people to understand, easy for people to change. When you hack, you are concerned about telling a computer what to do. When you design, you should be primarily concerned about communicating with other programmers.

Objects are for people. The reason objects exist is to help human programmers do their jobs. This is important to keep in mind when designing APIs as well as objects, because if objects are for people, then so are object interfaces. (After all, API means application programmer interface.) When you design an object or API, you are primarily designing for the benefit of human programmers.

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