Alright, I know the basics of complex ideas in Java. Right now, I am on 2D arrays. I'm kinda stuck on Matrix Multiplication. Anyways, that doesn't matter. I'm programming in a contest tomorrow for $40, and I want to know the main things I should know and understand for it. Complex ideas, mind you. I am in Computer Science II currently, on my way to AP Computer Science. I know writing to a file, reading from a file, getting input, 1D arrays, all the loops, methods, classes, random numbers, string tokenizer, replacing letters in a string, etc. I just need to get the ideas that are more complex. Thanks for the help guys.