问题
I am trying to write a method to handle all my input from the user in a console application. I call this method a total of 5 times. The first time, the condition is
a) the number must be positive and real-valued (double)
the next 4 times the condition is
b) the number must be greater than 1
This is my method:
private static double numChk(int errNum) {
final String[] ERROR_MESSAGE = {
"\nPlease enter only positive, real-valued numbers",
"\nPlease enter only positive numbers greater than 1" };
Scanner in = new Scanner(System.in);
double tempData;
while (!in.hasNextDouble()) {
System.out.println(ERROR_MESSAGE[errNum]);
System.out.print(" Please try again: ");
in.next();
}
tempData = in.nextDouble();
// in.close();
return tempData;
}
this is an example call to this method:
do {
System.out
.println("Please enter only positive, real-valued numbers");
System.out.print(" Enter a constant: ");
mu = numChk(0);
} while (mu <= 0);
note the "// in.close();" in the method I wrote. Without closing the Scanner in, this method works fine. However, my assignment requires me to make sure I close all open input streams. if I close the input stream in the method and re-open it, I get a NoSuchElementException. I know I could just put all of this into my main method and close the input at the end of it however, I would like to know if there is a way to do this (input validation, multiple times) and be able to close the input stream.
回答1:
When you call in.close() you are also closing the System.in. The next time you try to use a new Scanner, it will try to use a closed System.in, and that is why you get an exception when you try to read from it
See: Using Scanner.next() to get text input
回答2:
If you call in.close() in the method, you need some way of determining if it is the last input or not.
I would recommend putting the scanner in your main method, passing the scanner in to the method, and then closing it when you're done with all your input.
What I suspect is happening is you're sending the input piped as if from a file. The first scanner buffers all the input, and when you close and re-open it, you end up losing the data that was buffered to the first scanner. I can't guarantee that, but that's what it looks like.
回答3:
After while loop it tries to call, by this line there may not be any inputs.
tempData = in.nextDouble();
来源：https://stackoverflow.com/questions/12410173/trying-to-write-a-method-that-checks-user-input-in-java-getting-nosuchelementex