简单地创建一个计数器，该计数器在找到小写或大写字母时会递增，就像这样:

Simply create counters that increment when a lowercase or uppercase letter is found, like so:

for (int k = 0; k < input.length(); k++) {
    /**
     * The methods isUpperCase(char ch) and isLowerCase(char ch) of the Character
     * class are static so we use the Class.method() format; the charAt(int index)
     * method of the String class is an instance method, so the instance, which,
     * in this case, is the variable `input`, needs to be used to call the method.
     **/
    // Check for uppercase letters.
    if (Character.isUpperCase(input.charAt(k))) upperCase++;

    // Check for lowercase letters.
    if (Character.isLowerCase(input.charAt(k))) lowerCase++;
}

System.out.printf("There are %d uppercase letters and %d lowercase letters.",upperCase,lowerCase);