hasNext() - when does it block and why?

问题:

I'm trying to read commands via a Scanner Object. For checking the Input Syntax I use sc.hasNext() (for the case of missing commands). It did work fine for many cases already, but now I have the case that's described in the JavaAPI as "MAY block and wait for Input".

When does the hasNext() method block and how can I control it? The funny Thing is that it work's perfectly fine with 3 cases before the block. Also the JavaAPI describes hasNext() as the proper method for checking wether there is another Input or not so that the Method next() doesn't produce an Exception.

Here is the code I did produce till now:

if (sc.hasNext() && sc.next().equals("create")) {
    if (sc.hasNextInt()) {
        width = sc.nextInt();
        if (width > 0) {
            if (sc.hasNextInt()) {
                heigth = sc.nextInt();
                if (heigth > 0) {
                    if (sc.hasNext()) {  //At this point the hasNext Statement blocks for //no reason till an Input is made.
                        charset = sc.next();
                        Image = new AsciiImage(width, heigth,charset);
                    } else {
                        ret = false;
                        System.out.println("INPUT MISMATCH");
                    }
                } ...//and so on

Thanks in advance, I couldn't find anything on this Topic an my own. Edit: The Scanner is defined as a System.in, but that shouldn't be a Problem - at least it hasn't been one till now.

回答1:

There is a difference between testing via Console or via TextFile. If I read from Console the program expects a Stream and it will wait for further Input.

When testing via Input from Textfile (still with System.in for the Scanner, using Java Program ) the hasNext() will return false at the end of the file as no further Input can be done.

I can't really find documentation (in https://docs.oracle.com/javase/9/docs/api/java/util/Scanner.html#hasNext--) on this Topic. So if anyone finds a proper and technical correct answer I would be very greatfull.

回答2:

I'm not sure , but the following is my own experience :
when the Scanner object is fed with a file , it will not be blocking !
By the term "fed with a file " I mean that the scanner is constructed like this :
Scanner scanner = new Scanner("myFile.txt");

But if the scanner is constructed using the getInputStream()method of a Socket object , like this :

input = socket.getInputStream();
Scanner scanner = new Scanner(input);

the scanner will be blocking !

回答3:

If you have nothing else to do while waiting for user input, then it's fine to be blocked at that call until the next input arrives.

If you do want to run other code while waiting for input, spawn a new thread and call hasNext and other blocking scanner methods from there.