logical error( if loops not working as i expected to)

Even after multiple attempts, i can't seem to alter my code so both the conditions may apply. I have only been able to satisfy one at a time.

public static String caeserEncrypt(String originalMessage, int shift){
       String finalMessage = " ";  
       int length = originalMessage.length();
       for(int i= 0;i<length;i++){
         char c = originalMessage.charAt(i);
         char shiftedletter = ' ';
         if((int)'a'<=(int)c &&(int)c<='z'||(int)'A'<=(int)c &&(int)c<='Z'){


          if(shiftedletter>'z' || shiftedletter>'Z'){
           shiftedletter = (char)(originalMessage.charAt(i)-(26-shift));
          }
          else{
           shiftedletter = (char)(originalMessage.charAt(i)+shift);
          }
         }
         else{
            shiftedletter = originalMessage.charAt(i);
         }

          finalMessage = finalMessage + shiftedletter;