Question

I don’t understand why this code works as I expect:

package myPackage;

import java.util.InputMismatchException;
import java.util.Scanner;

public class ScanTest {

    public static void main(String[] args) 
    {
        Scanner scan1, scan2;
        double weight = 0.0;

        scan1 = new Scanner(System.in);
        System.out.print("Enter a rational: ");
        try {
            weight = scan1.nextDouble();
            System.out.println("You entered " + weight);
        }
        catch(InputMismatchException ime) {
            System.out.println("Invalid data.");
        }
        scan1.close();

        /*scan2 = new Scanner(System.in);
        System.out.print("Enter a rational: ");
        try {
            weight = scan2.nextDouble();
            System.out.println("You entered " + weight);
        }
        catch(InputMismatchException ime) {
            System.out.println("Invalid data.");
        }
        scan2.close();*/
    }
}

and this code fails:

package myPackage;

import java.util.InputMismatchException;
import java.util.Scanner;

public class ScanTest {

    public static void main(String[] args) 
    {
        Scanner scan1, scan2;
        double weight = 0.0;

        scan1 = new Scanner(System.in);
        System.out.print("Enter a rational: ");
        try {
            weight = scan1.nextDouble();
            System.out.println("You entered " + weight);
        }
        catch(InputMismatchException ime) {
            System.out.println("Invalid data.");
        }
        scan1.close();

        scan2 = new Scanner(System.in);
        System.out.print("Enter a rational: ");
        try {
            weight = scan2.nextDouble();
            System.out.println("You entered " + weight);
        }
        catch(InputMismatchException ime) {
            System.out.println("Invalid data.");
        }
        scan2.close();
    }
}

(Note that the two versions are nearly identical. There is merely an uncommented block in the second.)

The output I get from the second version is:

Enter a rational: .97
You entered 0.97
Enter a rational: Exception in thread "main" java.util.NoSuchElementException
    at java.util.Scanner.throwFor(Unknown Source)
    at java.util.Scanner.next(Unknown Source)
    at java.util.Scanner.nextDouble(Unknown Source)
    at myPackage.ScanTest.main(ScanTest.java:28)

I don’t understand why the Scanner object waits for input in the first block, but throws an Exception in the second. The behavior I would expect is:

Enter a rational: .97
You entered 0.97
Enter a rational: .43
You entered 0.43

The offending line 28 is:

weight = scan2.nextDouble();

I would appreciate anyone’s insight into this behavior!

Answer 1

I believe the call to scan1.close() also closes System.in so that no more data can be read from the stream. Check out the close() method description here.

So… by the time you start reading from scan2, System.in will no longer return any data.