How about a readln for numbers?

[Ethan McCue]

Currently, "don't mix nextLine with next/next int/etc" is an extremely
common footgun.

In one of the coding help discords, this is the auto message we send when
people run into trouble with that.

Mixing any nextXXX method with nextLine from the Scanner class for user
input, will not ask you for input again but instead result in an empty line
read by nextLine. To prevent this, when reading user input, always only use
nextLine. If you need an int, do

int value = Integer.parseInt(scanner.nextLine());

instead of using nextInt. Assume the following:

Scanner scanner = new Scanner(System.in);

System.out.println("Enter your age:");int age = scanner.nextInt();
System.out.println("Enter your name:");String name = scanner.nextLine();

System.out.println("Hello " + name + ", you are " + age + " years old");

When executing this code, you will be asked to enter an age, suppose you
enter 20. However, the code will not ask you to actually input a name and
the output will be:

Hello , you are 20 years old.

The reason why is that when you hit the enter button, your actual input is

20\n

and not just 20. A call to nextInt will now consume the 20 and leave the
newline symbol \n in the internal input buffer of System.in. The call to
nextLine will now not lead to a new input, since there is still unread
input left in System.in. So it will read the \n, leading to an empty input.
So every user input is not only a number, but a full line. As such, it
makes much more sense to also use nextLine(), even if reading just an age.
The corrected code which works as intended is:

Scanner scanner = new Scanner(System.in);

System.out.println("Enter your age:");// Now nextLine, not nextInt
anymoreint age = Integer.parseInt(scanner.nextLine());
System.out.println("Enter your name:");String name = scanner.nextLine();

System.out.println("Hello " + name + ", you are " + age + " years old");

The nextXXX methods, such as nextInt can be useful when reading multi-input
from a single line. For example when you enter 20 John in a single line.


On Thu, Nov 7, 2024, 11:36 AM Kenneth Fogel <kfogel at dawsoncollege.qc.ca>
wrote:

> The readln method introduced as part java.base is great because it
> includes the prompt for the input. The only shortcoming is that if the
> input must be a number then you still need to employ a static class member
> such as Double.parseDouble() to make it a number:
>
>
>
> // Currently
>
> var loan = Double.parseDouble(readln("Loan: "));
>
>
>
> // My delusional idea
>
> var loan = readDbl("Loan: ");
>
>
>
> I can think of reasons why this could be a bad idea. I taught my students
> to use a construct using Scanner to create a console input that passed my
> Beethoven test (while humming Beethoven’s 5th symphony randomly strike
> keys on your keyboard as if you were playing the symphony and your code
> should just report invalid input and not an exception). If we had a readInt
> and/or readDbl they would not pass this test. That Python would happily
> accept anything and then fail when the value was used in a calculation is
> no better. But, for learning Java could we have a readInt or readDbl
> alongside readln? I know that DataInputStream has such methods, but
> without a prompt and it must be attached to an input stream such as a file.
>
>
>
> As always, just thinking out loud. Feel free to use my name in vain.
>
>
>
> Ken
>
>
>
>
>
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