If You're Using Java’s Scanner Class for Keyboard Input, You're Doing it Wrong

Yesterday, I gave my third-year Java students a warm-up exercise. The students were free to use console, Swing, or JavaFX for user input. Most chose a console because they had minimal exposure to GUI coding and that is one of the main topics covered in my course. Within a few minutes, some hands went up in regards to using the Scanner class. Here is a simplified version of what they were trying to do:

public class App {

    public void perform() {
        firstScanner();
        secondScanner();
    }

    public void firstScanner() {
        String str = "";
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter a string: ");
        if (sc.hasNext()) {
            str = sc.next();
            sc.nextLine();
        }
        sc.close();
        System.out.println("str = " + str);
    }

    public void secondScanner() {
        int num = 0;
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter a number: ");
        if (sc.hasNextInt()) {
            num = sc.nextInt();
            sc.nextLine();
        }
        sc.close();
        System.out.println("num = " + num);
    }

    public static void main(String[] args) {
        App app = new App();
        app.perform();
        System.exit(0);
    }
}

Scanner Demo

When this program executes, it asks for the first String but never asks for the int . A run looks like:

Enter a string: 
bob
str = bob
Enter a number: 
num = 0

It never stops to ask for the number and immediately prints out the default value for num. I had never seen this problem before. When I teach an introductory course, I show my students that the Scanner for keyboard input should, in most cases, be a class variable initialized in the constructor. I also must admit that I never close the Scanner object when the keyboard is involved.

You can see that, in the code, the Scanner object is closed in each method. I didn’t think this was a problem, and so I stared at the student’s code for a few minutes and then went to Google. There was not much to be found. Most of the information showed that when using Scanner to access a file, you should close it. Most articles that used Scanner for keyboard input declared the Scanner in a method and closed it at the end of the method. There was one suggestion in a StackOverflow posting that suggested a potential problem with closing Scanner for keyboard input, but it was not definitive.

My next experiment was to remove the close() statements. The Scanner performed as originally expected:

Enter a string: 
bob
str = bob
Enter a number: 
23
num = 23

So, back to research on Google where I uncovered a fact not mentioned in any JavaDocs or in any article from any source when discussing the Scanner class. If  System.in, a BufferedInputStream from the keyboard, is closed, as happens when the Scanner object is closed, you are closing a System stream that cannot be re-opened. The program must be exited and then re-run to re-establish  System.in.

I thought this was strange and looked deeper into how the keyboard was connected to System.in. This stream is established by private methods in the JVM. I did see suggestions as to how this could be re-established, but I suspect that this code may have never been tested:

Suggestion #1

FileInputStream fdIn = new FileInputStream(FileDescriptor.in);
System.setIn(new BufferedInputStream(fdIn));

Suggestion #2

System.setIn(new FileInputStream(FileDescriptor.in));

Neither worked to re-establish the connection between System.in and the keyboard. If you know a way, then I’d dearly like to know. For now:

DO NOT CLOSE A SCANNER OBJECT INITIALIZED WITH  System.in!