Dependency injection

Dependency injection (DI) is a design pattern in object-oriented computer programming whose purpose is to reduce the coupling between software components. It is similar to the factory method pattern. Frequently an object uses (depends on) work produced by another part of the system. With DI, the object does not need to know in advance about how the other part of the system works. Instead, the programmer provides (injects) the relevant system component in advance along with a contract that it will behave in a certain way.

http://code.google.com/p/roboguice/

Recommended by Alan Cooke.