Chapter 4 Exercise 15, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

*4.15 (Phone key pads) The international standard letter/number mapping found on the telephone is shown below:



Write a program that prompts the user to enter a letter and displays its corresponding number.

Enter a letter: A
The corresponding number is 2
Enter a letter: a
The corresponding number is 2
Enter a letter: +
+ is an invalid input 

import java.util.Scanner;
 
public class ProgrammingEx4_15 {
 
 public static void main(String[] args) {
 
  System.out.print("Enter a letter:");
  Scanner input = new Scanner(System.in);
  String s = input.next();
 
  char c = s.toLowerCase().charAt(0);
  int n;
 
  switch (c) {
  case 'a':
  case 'b':
  case 'c':
   n = 2;
   break;
 
  case 'd':
  case 'e':
  case 'f':
   n = 3;
   break;
 
  case 'g':
  case 'h':
  case 'i':
   n = 4;
   break;
 
  case 'j':
  case 'k':
  case 'l':
   n = 5;
   break;
 
  case 'm':
  case 'n':
  case 'o':
   n = 6;
   break;
 
  case 'p':
  case 'q':
  case 'r':
  case 's':
   n = 7;
   break;
 
  case 't':
  case 'u':
  case 'v':
   n = 8;
   break;
 
  case 'w':
  case 'x':
  case 'y':
  case 'z':
   n = 9; //change from n=8; from Akrious7's comment below
   break;
 
  default:
   n = 0;
   break;
  }
 
  if (n != 0) {
   System.out.print("The corresponding number is " + n);
   System.exit(0);
  }
 
  System.out.print(s + " is an invalid input");
 
 }
 
}