listFiles
返回了一个File数组，这时候遍历这个数组会出现NullPointerException

    public static void main(String[] args) {
        File file = new File("D:\\");
        int sum = getTxtSum(file);
        System.out.println(sum);
    }
    
    public static int getTxtSum(File f) {
        File[] subFile = f.listFiles();
        int count = 0;
        for (File file : subFile) {
            if (file.isDirectory()) {
                count += getTxtSum(file);
            }else if(file.isFile() && file.getName().endsWith(".txt")) {
                count += 1;
            }
        }
        
        return count;
    }