System.in
is the "standard" input stream which supplies user input data. Once closed, this stream can not be re-opened. One such example is in the case of using a scanner to read the user input as follows:
public class Test {
public static void main(String[] args) {
boolean finished;
do {
Scanner inputScanner = new Scanner(System.in);
finished = inputScanner.hasNext("exit");
boolean validNumber = inputScanner.hasNextDouble();
if (validNumber) {
double number = inputScanner.nextDouble();
System.out.print(number);
} else if (!finished) {
System.out.println("Please try again.");
}
inputScanner.close();
} while (!finished);
}
}
In this example, an instance of type Scanner
is created and used to read a series of numbers from the user (please ignore other details with this code which go beyond the scope of this example, I know the scanner should be created and closed outside the loop). After a number is retrieved from user input, the instance of this Scanner
(i.e., the input stream) is closed. However, when another number is requested from user, and new instance is created, the input stream cannot be opened again. In case of this example, it creates a infinite loop.
The question is: why is not possible to reopen a closed stream?