Creating Encryption / Decryption Program?

For this week's assignment, I am supposed to create a java program that encrypt's a user's sentence, outputs the encrypted message, then decrypts the message, outputting the user's original message.

This is what I have so far. It doesn't compile the last few lines since that is not correct Java syntax.

Java Code:

import java.util.Scanner;
import java.io.*;
public class Decrypter {
public static void main(String[] args) {
String sentence;
final char REAL_ALPHA[] = { 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h',

[Code] .....

So you can see I have "realAlpha" which are regular letters of the alphabet, and "codeAlpha" which are the letters I want to encrypt with. I want 'a' equal to 'z,' 'b' equal to 'y', 'c' equal to 'x' and so forth in that sort of reverse order, you know? So the word "apple" would encrypt into "zkkov."

I am stuck as to how to replace the letters in this way. You can see I tried fumbling with sentence.replace to switch the letters out between the two arrays. Also, I only barely understand arrays.