数字键盘转化

import java.util.Scanner;
 
public class Main2 {
    public static void main(String [] args) {
        Scanner in = new Scanner(System.in );
        System.out.println("Enter a letter:");
        String input = in.nextLine(); 
        if (input.length() == 1 && Character.isLetter(input.charAt(0))) {
            
            char a = input.toUpperCase().charAt(0);
 
         
            if (a >= 'A' && a <= 'C') {
                System.out.println("The corresponding number is 2");
            } else if (a >= 'D' && a <= 'F') {
                System.out.println("The corresponding number is  3");
            } else if (a >= 'G' && a <= 'I') {
                System.out.println("The corresponding number is 4");
            } else if (a >= 'J' && a <= 'L') {
                System.out.println("The corresponding number is 5");
            } else if (a >= 'M' && a <= 'O') {
                System.out.println("The corresponding number is  6");
            } else if (a >= 'P' && a <= 'S') {
                System.out.println("The corresponding number is  7");
            } else if (a >= 'T' && a <= 'V') {
                System.out.println("The corresponding number is 8");
            } else if (a >= 'W' && a <= 'Z') {
                System.out.println("The corresponding number is 9");
            }
        } else {
           
            System.out.println(" error");
        }
       
        in.close();
    }
}