Substitution Cipher

 import java.io.*;
import java.util.*;
import java.util.HashMap;
import java.util.Map;
public class CaesarCipher
{
public static void main(String[] args) 
{
 String allLetters = "abcdefghijklmnopqrstuvwxyzABCDEFGHI" +"JKLMNOPQRSTUVWXYZ";
 Map<Character, Character> dict1 = new HashMap<>();
 int key = 4;
 for (int i = 0; i < allLetters.length(); i++)
 {
 dict1.put(allLetters.charAt(i),allLetters.charAt((i + key) %allLetters.length()));
 }
 String plainText = "I am studying Data Encryption";
 StringBuilder cipherText = new StringBuilder();
 for (char c : plainText.toCharArray()) 
{
 if (allLetters.indexOf(c) != -1)
{
 cipherText.append(dict1.get(c));
 } 
else {
cipherText.append(c);
 }
 }
System.out.println("Cipher Text is: " + cipherText);
Map<Character, Character> dict2 = new HashMap<>();
for (int i = 0; i < allLetters.length(); i++) {
dict2.put(allLetters.charAt(i),
 allLetters.charAt((i - key + allLetters.length()) % allLetters.length()));
 }
StringBuilder decryptedText = new StringBuilder();
for (char c : cipherText.toString().toCharArray())
{
 if (allLetters.indexOf(c) != -1)
 {
decryptedText.append(dict2.get(c));
 } else {
 decryptedText.append(c);
 }
}
System.out.println("Recovered plain text: " + decryptedText);
}
}