没有这种元素异常扫描器

Question

Scanner s = new Scanner(new File("src/mail_list"));    
while (s.hasNextLine()){
        String line1 = s.nextLine();
        if (line1.startsWith("Users")){
            line1 = s.nextLine();
            while (!(line1 = s.nextLine()).isEmpty()) {
                String arr[] = line1.split(" ", 4);
                users.add(new User(arr[0],arr[1],arr[2],arr[3]));
            }
        }
        if (line1.startsWith("Lists")){
            line1 = s.nextLine();
            while (!(line1 = s.nextLine()).isEmpty()) { //exception here
                String arr1[] = line1.split(" ", 2);
                ArrayList<String> tmp = new ArrayList<String>();
                StringTokenizer st = new StringTokenizer(arr1[1]);
                while (st.hasMoreTokens()) {
                    tmp.add(st.nextToken());
                }
                list.add(new List((arr1[0]), tmp));
            }
        }
    }

/*-testfile-start*/
Keyword: Users
username1 Name1 Surname1 email1
username2 Name2 Surname2 email2

Keyword: Lists
list_name username1 username2 ...
/*-testfile-end*/

I'm using the above code in order to sort things from the above testfile pattern. Basically it means that if I encounter the keyword "Users" I have to add the said info about the user.

I marked in the code where the exception rises. Any ideas on how to counter it?

Answer 1

You are calling nextLine() twice but only checking hasNextLine() once.

String line1 = s.nextLine();
    if (line1.startsWith("Users")){
        line1 = s.nextLine();

Meaning you are getting the next line without knowing if there is one, which throws the exception if there isn't one.

Answer 2

I found a stupid solution to it. I just added a 'dummy' char 2 lines after the last line. and it works. It's not a perfect solution but since the testfile is not meant to be seen by any1 i'll take it for now... Thank's everybody who brainstormed with me for this 45 mins.

Answer 3

Do！（line1 = s.nextLine（））！= null，不为空，因为它无法读取空行。

Answer 4

From Scanner#nextLine :

Throws:
NoSuchElementException - if no line was found.

And you have this code:

while (s.hasNextLine()) {
    //checked if there's line to read
    String line1 = s.nextLine();
    if (line1.startsWith("Users")) {
        //not checked if there's line to read
        line1 = s.nextLine();
        //not checked here either
        while (!(line1 = s.nextLine()).isEmpty()) {
            String arr[] = line1.split(" ", 4);
            users.add(new User(arr[0],arr[1],arr[2],arr[3]));
        }
    }
    //similar in code below...
}

Make sure to validate there's a line to be read before using Scanner#nextLine . Handle the exceptions accordingly.

Answer 5

As you can see, this method throws NoSuchElementException when no line was found.

if (line1.startsWith("Lists")){
        line1 = s.nextLine(); // <=============== ?
        while (!(line1 = s.nextLine()).isEmpty()) { //exception here

How do you know that you have more lines there and in your comment line?