Java语言程序设计 例题6.20（计算字符串中的字母数）

*6.20 (Count the letters in a string) Write a method that counts the number of letters in

a string using the following header:

public static int countLetters(String s)

Write a test program that prompts the user to enter a string and displays the number

of letters in the string.

*6.20（计算字符串中的字母数）编写一种方法，使用以下标题计算字符串中的字母数：