I cleaned up more of the code. I went throught the space invaders game (SpcaceInvaders.java) and replaced all of the hard coded strings with constant variables. The new constant variables can be found in the class SpaceInvaderConstants.java. I then added the new string constants corresponding fonts in the file SpaceInvaderFonts. Now if any of the game messages need to be changed, they can be accessed all in the same file.

I then renamed the classes SpaceInvaderConstants and SpaceInvaderFonts to SIConstants and SIFonts respectively to reduce some of the verbosity in the code.
I also added a new file called SIDebugConstants, which contains all of the debug messages and fonts that are used to display the debug information when debug mode is enabled.