File CountLetters.java contains a program that reads a word from the user and prints the number of occurrences of each letter in the word. Save it to your directory and study it, then compile and run it to see how it works. In reading the code, note that the word is converted to all upper case first, then each letter is translated to a number in the range 0..25 (by subtracting 'A') for use as an index. No test is done to ensure that the characters are in fact letters.

Now modify the body of the catch so that it prints a useful message (e.g., "Not a letter") followed by the exception. Compile and run the program. Although it's useful to print the exception for debugging, when you're trying to smoothly handle a condition that you don't consider erroneous you often don't want to. In your print statement, show the character that created the out of bounds index. Run the program again; much nicer!

// ****************************************************************

// CountLetters.java

//

// Reads a words from the standard input and prints the number of

// occurrences of each letter in that word.

//

// ****************************************************************

import java.util.Scanner;

public class CountLetters

{

public static void main(String[] args)

{

int[] counts = new int[26];

Scanner scan = new Scanner(System.in);

//get word from user

System.out.print("Enter a single word (letters only, please): ");

String word = scan.nextLine();

//convert to all upper case

word = word.toUpperCase();

//count frequency of each letter in string

for (int i=0; i < word.length(); i++)

counts[word.charAt(i)-'A']++;

//print frequencies

System.out.println();

for (int i=0; i < counts.length; i++)

if (counts [i] != 0)

System.out.println((char)(i +'A') + ": " + counts[i]);

}

}