Encrypting Caesar Cipher

import edu.duke.*;

public class CaesarCipher 
{
    public String encrypt(String input, int key) 
    {
        //Make a StringBuilder with message (encrypted)
        StringBuilder encrypted = new StringBuilder(input);
        //Write down the alphabet
        String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        //Compute the shifted alphabet
        String shiftedAlphabet = alphabet.substring(key)+
        alphabet.substring(0,key);
        String alphabet2=alphabet.toLowerCase();
        String shiftedAlphabet2=alphabet2.substring(key)+
        alphabet2.substring(0,key);
        char currChar;
        //Count from 0 to < length of encrypted, (call it i)
        for(int i = 0; i < encrypted.length(); i++) {
            //Look at the ith character of encrypted (call it currChar)
            currChar = encrypted.charAt(i);
            if(Character.isUpperCase(currChar)){
            //Find the index of currChar in the alphabet (call it idx)
            int idx = alphabet.indexOf(currChar);
        
      
            
            //If currChar is in the alphabet
            
            if(idx != -1){
                //Get the idxth character of shiftedAlphabet (newChar)
                char newChar = shiftedAlphabet.charAt(idx);
                //Replace the ith character of encrypted with newChar
                encrypted.setCharAt(i, newChar);
            }
            //Otherwise: do nothing
        }
          if(Character.isLowerCase(currChar)){
              int idx = alphabet2.indexOf(currChar);
        
      
            
            //If currChar is in the alphabet
            
            if(idx != -1){
                //Get the idxth character of shiftedAlphabet (newChar)
                char newChar = shiftedAlphabet2.charAt(idx);
                //Replace the ith character of encrypted with newChar
                encrypted.setCharAt(i, newChar);
            }
              
            }
        }
            
        //Your answer is the String inside of encrypted
        return encrypted.toString();
    
}
    public void testCaesar() {
        int key = 3;
        FileResource fr = new FileResource();
        String message = fr.asString();
        String encrypted = encrypt(message, key);
        System.out.println(encrypted);
        String decrypted = encrypt(encrypted, 26-key);
        System.out.println(decrypted);
    }
}