Java Scanner Issues: Troubleshooting Read Input from Keyboard

[jjc]

I am having some trouble with the Java Scanner to read input from the keyboard.

I am trying to keep my input cleaned up and secure, so I am checking for the right type and value, and clearing the input buffer if it isn't. I also restart the prompt loop if it isn't right, so they can try again. I am using the .nextLine() method to clear the input buffer.

The problem that I am seeing is that I sometimes don't get a repeated prompt until I enter in a second input. Things seem to work fine without the nextLine() calls, but then I always feel like I am leaving a dangling possible problem.

I was also trying to use combinations of while(!talkToMe.hasNextInt()) loops to try and get away from my Flag system shown below, but it didn't quite seem to work so I stuck with the flags.

I have read through the Java API documentation but that didn't seem to help, and I have done other searches for explanations of the Scanner object. It just feels like I am missing some key element of how the Scanner class is working. Is it an interrupt driven thing? A sit-and-wait-for-the-input thing? These questions might be the root of it; perhaps by executing .nextLine() it is actually waiting instead of going to the next statement.

Thanks for the help,
JJCHere is a code chunk at the moment:

private int createPin1 (String prompt) {
	int pin1=-1;
	boolean flag = false;
	while (!flag){
		talkToMe.nextLine();   //not sure if this line is going to mess things up.
		do {
			System.out.print(""+ prompt);   //Asking for pin number.
		}while (!talkToMe.hasNextInt());

		pin1 = talkToMe.nextInt();
		talkToMe.nextLine();  //not sure if this might mess things up again.
		if (pin1 <= 0){
			System.out.println("Try again; please enter an integer > 0.");
			// System.out.println(""+ prompt);
		}else flag = true;
	}
	return pin1;
}

 

[KataKoniK]

Dear JJC,

Thank you for reaching out for help with your Java Scanner issue. From your code chunk, it seems like you have a good understanding of how the Scanner class works. However, there are a few things that could be causing the problem you are experiencing.

Firstly, the .nextLine() method does indeed wait for user input. This means that when you call it twice in a row, it will wait for two separate inputs. This could be why you are not getting a repeated prompt until the second input.

Secondly, the .hasNextInt() method only checks for the next input in the Scanner's buffer. If there is no input, it will wait for the user to enter something. This could also be why your while loop is not working as expected.

To address these issues, I would suggest restructuring your code to only use the .nextLine() method once, and then use the .hasNextInt() method to check if the input is an integer. If it is not, you can use the .nextLine() method again to clear the input buffer and prompt the user to enter an integer.

Additionally, you could also consider using a try-catch block to handle any exceptions that may occur when trying to convert the input to an integer. This can help prevent your code from crashing if the user enters a non-integer input.

I hope this helps and please let me know if you have any further questions.

 

[Mene]

Hi JJC,

Thank you for reaching out with your issue with the Java Scanner. It seems like you are on the right track with your troubleshooting methods, such as checking for the right type and value and using the .nextLine() method to clear the input buffer. However, it is possible that the use of .nextLine() is causing the repeated prompt issue you are experiencing.

The Scanner class is designed to be a blocking operation, meaning that it will wait for input before moving on to the next statement. So when you execute .nextLine(), it is possible that it is waiting for the user to enter a second input before moving on to the next statement. This could explain why you are not seeing the repeated prompt until a second input is entered.

One suggestion I have is to use a combination of .next() and .nextInt() instead of .nextLine() to read input from the keyboard. This can help to avoid any issues with the input buffer and ensure that the program is moving on to the next statement after each input.

Additionally, instead of using a flag system, you could use a try-catch block to handle any potential exceptions that may occur when reading input. This can help to simplify your code and make it more efficient.

I hope this helps to clarify some things about how the Scanner class works. If you continue to experience issues, I recommend reaching out to the Java community or seeking further guidance from online resources. Keep up the good work with your troubleshooting efforts!