Six Simple Rules

1. Clarity and simplicity are of paramount importance.
2. Modules should be as small as possible but no smaller.
3. Code should be reused rather than copied.
4. The dependencies between modules should be kept to a minimum.
5. Errors should be detected as soon as possible after they are made, ideally at compile time.
6. You should not slavishly follow these rules, but you should violate them only occasionally and with good reason.

Jashua Bloch in Effective java.