Encrypt String Using Substitution Cipher

I am trying to encrypt a string using substitution cipher, the program doesnt show any errors but the output is not what is expected. On encryption it shows random characters whereas i want it to be encrypted by another letter. This is what I have done:

/* Encryption using substitution */

import java.io.*;
public class SSExp1 {
public static void main(String args[]) {
char[] ip= new char[20];
char[] op= new char[20];
char key = '3';
String b;

[Code] .....