http://java.sun.com/j2se/1.4.1/docs/api/
edit:
oooh I READ the assignment. so you're gonna read in text, and convert it to digits. then you're gonna read in digits and un-convert it, based on the numbers it is. sounds like fun! Some things off hand to be careful of: which stream you use. some bring in raw data (that's what you'll want, i think) and some convert straight to characters. be careful.
same w/ your output streams! hrm. other than that, it's gonna be some down'n'dirty bit reading, converting and writing. good luck.