Decrypting A File With Array

My assignment is to encrypt and then decrypt a file in Java. I used an array to add 100 to each character per the assignment but I need figuring out a way to subtract the 100 from each character so the file is decrypted again. I tried to create another array to decrement the 100 from each character but cant seem to figure it out/get it working. Here is my assignment and my code so far:

"Although there are complex encryption techniques, you should come up with a simple one of your own. For example, you can read the first file one character at a time, and add 100 to the character code of each character before it is written to the second file.

Then write a program to decrypt the second file. The program should read the content of the second file, restore the data to its original state, and write to a third file."

import java.io.*;
public class FileEncryption {
public static void main(String [] args)
throws IOException {
int[] encrypt = { 100 };

[Code] ....