Don't Ask, Do Tell

There's a great piece up at The Pragmatic Programmer about some very basic, but very important OOP concepts. Read the article for a complete explanation, but here's a very short summary...

1. Don't ask an object for information about it's state and then make decisions based on that data. You shouldn't be making decisions about the state of an object and then changing that state yourself. This logic should be inside that particular object. Just tell it what you want done and then let it do that to itself.
2. Don't tightly couple objects together by passing data around from one to another. Once you've returned some data from an object, any semantic meaning associated with it is lost. Basic OOP states that data should be stored along with it's state/methods; i.e. in its own object. Related to point 1.
3. Don't use multiple methods to access/work upon a single invariant state variable. Multithreading will ensure that invariants can no longer be guaranteed invariable (unless you lock properly). Better to use something like a map() call to apply actions to data.

In other words, follow basic OOP fundamentals rigorously. Keep data and the methods/logic that act upon it together in one place. Don't scatter it all about.