YAGNI : Java Glossary

Extreme programming speak for YAGNI (You Aren’t Going to Need It). Extreme Programming promotes the idea that you do the simplest thing that works. You don’t add additional capability to handle things that you might need in the future until you know that you do need it because we are lousy at predicting the future and if we never need it, we have wasted resources.

You can obviously take this notion to extreme. Some things you should routinely build in for the future especially when they would be awkward to add later. If you come back to complex code to make a minor modification later, there is a more chance you will introduce an error than if you provide the functionality at the start. Further, you have to come up to speed again on how it all works before you can safely make your modification.