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What is important is to be consistent in the API and to know that it must be stabilized asap, if people start using it - it will be hard to change it. Here are a good tips about it:
How to Design a Good API and Why it Matters by Joshua Bloch
See original including video presentation here: http://www.infoq.com/articles/API-Design-Joshua-Bloch
All programmers are API designers. Good programs are modular, and intermodular boundaries define APIs. Good modules get reused.
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