Principle of Least Surprise
As simple as it can be and no simpler!
Use Java idioms where they fit (and only where they fit)
There's exactly one way to do it
Start small and grow as necessary:
It's easier to put something in than take something out.
if I may make one point that highly influenced the end-game when we were finishing up XML 1.0 in 1998: if you leave something out, you can always put it in later. The reverse is not true.
--Tim Bray on the public-qt-comments mailing list
During the design I added methods that were necessary to produce certain sample programs.