Letter Count - Ignore All Numbers Or Punctuation Marks

I'm trying to make a program that will count each letter like if I put the word

e-1
h-1
l-2
o-1

The problem I'm running into is that if I put something like How now, brown cow? It should give me something like this

b - 1
c - 1
h - 1
n - 2
o - 4
r - 1
w - 4

but it doesn't instead the whole program crashes because of the punctuation marks and the spacing. Is there a way for me to fix that? All I know is that it has something to do with the alphabet[pos(letter)]++;

import java.util.Scanner;

public class CharCount {
static Scanner keyboard = new Scanner(System.in);
//this method should allow the user to enter a line of text (that should all be lower case) and then go to the printResults method
public static void main(String[] args) {
String text, character;
int count;
char letter;
int[] alphabet;
System.out.println("Enter a line of text:");
text = keyboard.nextLine().toLowerCase();// any text entered will be lower case
alphabet=countLetters(text);

[code]....