Caesar Cipher in Java

Credit goes to Musa Mohammed for his help.

import java.util.Scanner;

public class CaesarCipher {

public static final String ALPHABET = "abcdefghijklmnopqrstuvwxyz";
     public static String encrypt(String plain_Text,int shiftKey)
     {
           plain_Text = plain_Text.toLowerCase();
           String cipherText="";
           for(int i=0;i<plain_Text.length();)
           {
           if(plain_Text.charAt(i) == ' ') {
           
           cipherText += plain_Text.charAt(i);
           i++;
           }
           
           else {
           
           int charPosition = ALPHABET.indexOf(plain_Text.charAt(i));
           int keyVal = (shiftKey+charPosition)%26;
           char replaceVal = ALPHABET.charAt(keyVal);
           cipherText += replaceVal;
           i++;
           }
           }
         
           return cipherText;
     }
     public static String decrypt(String cipherText, int shiftKey)
     {
           cipherText = cipherText.toLowerCase();
           String plain_Text="";
           for(int i=0;i<cipherText.length();)
           {
               
                
                if(cipherText.charAt(i) == ' ') {
                plain_Text += cipherText.charAt(i);
                i++;
                }
                
                
                
                else {
                int charPosition = ALPHABET.indexOf(cipherText.charAt(i));
                int keyVal = (charPosition-shiftKey)%26;
                if(keyVal<0)
                {
                      keyVal = ALPHABET.length() + keyVal;
                      
                }
                char replaceVal = ALPHABET.charAt(keyVal);
                plain_Text += replaceVal;
                i++;
                }
           }
           return plain_Text;
     }
public static void main(String[] args) {
 Scanner input = new Scanner(System.in);
 System.out.println("This program encypts and decrypts Caesar Cipher with n shift: ");
 System.out.println("Please enter plain text: ");
 String plaintext = input.nextLine();
 System.out.println("Enter n shift integer: ");
 int n = input.nextInt();
 String encryption = encrypt(plaintext, n);
 System.out.println("Here is the cipher text: " + encryption);
 System.out.println("Decrypted message: " + decrypt(encryption, n));
 }
}