Solution Review: Message Encryption

class RunEncryption 
{
    public static void main(String args[]) 
    {
        // Encrypting a String
        System.out.println(encrypt("hefOP830nskdd0qSW uIm", '0', '.', "tuy83AB9"));    
    }
    // Method to encrypt the string
    public static String encrypt(String stringToBeEncrypted, char oldChar, char newChar, String extraString)
    {
        String str = stringToBeEncrypted.toLowerCase();
        String update = str.substring(0, str.length()/2) + extraString + str.substring(str.length()/2);
        return update.replace(oldChar, newChar);
    }
}