Character counter

The problem

Write a program that asks the user to enter a string and then asks the user to enter a character. The program should count and display the number of times that the specified character appears in the string.

Breaking it down

Create variables

String stringToSearch; // The string to search
String letter = null; // The letter to count

Get user's input

// Create a Scanner object for keyboard input.
Scanner keyboard = new Scanner(System.in);

// Get a string from the user.
System.out.print("Enter a string:");
stringToSearch = keyboard.nextLine();

// Retrieve the letter to count.
System.out.print("Enter a letter contained in the string:");
letter = keyboard.nextLine();

Count the number of times a letters in string

// Get the letter to count.
double numberOfLettersInString = countLetters(stringToSearch, letter);

static double countLetters (String stringToSearch, String letter) {

    checkNotNull(stringToSearch);
    checkNotNull(letter);

    return StringUtils.countMatches(stringToSearch, letter);
}

Display output

System.out.println("The letter " + letter + " appears " + numberOfLettersInString + " times in the string:\n" + stringToSearch);

Output

Enter a string:running down the road
Enter a letter contained in the string:r
The letter r appears 2.0 times in the string:
running down the road

Unit tests

@Test(expected=NullPointerException.class)
public void count_letters_null_to_search () {
    CharacterCounter.countLetters(null, "");
}

@Test(expected=NullPointerException.class)
public void count_letters_null_letter () {
    CharacterCounter.countLetters("", null);
}

@Test
public void count_letters () {

    double count = CharacterCounter.countLetters("aaaa", "a");
    assertTrue(count == 4);

}

Level Up

if apache common's StringUtils wasn't available, what the algroithm look like?
validate users input
determine a better way to output the information