问Java字符串输出将字符串的范围限制为字母表

EN

Write a program which will input a String from the keyboard, output the number of
seperate words, where a word is one or more characters seperated by spaces. Your
program should only count words as groups of characters in the rang A..Z and a..z

public class Exercise10 {

public static void main(String[] args) {
    String input;
    int counter = 0;
    Scanner keyboard = new Scanner(System.in);

    System.out.println("Please enter your text: ");
    input = keyboard.nextLine();

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

        if(input.charAt(i) == ' '){
            counter++;
            }   
    }

    System.out.println(counter + 1);
    keyboard.close();

    }
 }

Your program should only count words as groups of characters in the rang A..Z and 
a..z