How to restrict user input to the integer 1 or 2 and not allow user to enter anything other than a integer in java?

Question

I'm having so much trouble with this. I have tried multiple things but it still gives me an error message and crashes. I want to loop it until the user enters 1 or 2.

System.out.println("Please choose if you want a singly-linked list or a doubly-linked list.");
System.out.println("1. Singly-Linked List");
System.out.println("2. Doubly-Linked List");
int listChoice = scan.nextInt();
//Restrict user input to an integer only; this is a test of the do while loop but it is not working :(
do {
    System.out.println( "Enter either 1 or 2:" );
    listChoice = scan.nextInt();
} while ( !scan.hasNextInt() );

It gives me this error:

Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Unknown Source)
at java.util.Scanner.next(Unknown Source)
at java.util.Scanner.nextInt(Unknown Source)
at java.util.Scanner.nextInt(Unknown Source)
at Main.main(Main.java:35)

Use next instead of nextInt and add try/catch to handle NumberFormatException ... (Obviously use if to check if it's 1 or 2) — Idos
– Idos, Commented Sep 25, 2017 at 22:50
@Idos thanks! why would I use an if statement? it doesn't loop like a while loop. I don't want it to stop asking the user to enter in 1 or 2. — sukiyo
– sukiyo, Commented Sep 25, 2017 at 22:52

Makoto · Accepted Answer · 2017-09-25 22:52:58Z

2

Use nextLine instead of nextInt, and deal with the exception afterwards.

boolean accepted = false;
do {
    try {
        listChoice = Integer.parseInt(scan.nextLine());
        if(listChoice > 3 || listChoice < 1) {
            System.out.println("Choice must be between 1 and 2!");
        } else {
            accepted = false;
        }
    } catch (NumberFormatException e) {
        System.out.println("Please enter a valid number!");
    }
} while(!accepted);

answered Sep 25, 2017 at 22:52

Makoto

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2 Comments

sukiyo Over a year ago

Thank u so much!!! I think that is a really great thought process. I want to think logically too, but I am only a beginner and I feel stupid. I believe accepted = true in the else statement. If I put it as false, it keeps looping over and over again for me. And I also believe that it is if(listChoice >=3 || listChoice < 1)

sukiyo Over a year ago

It also still gives me trouble when I enter in a letter. I changed it to catch (InputMismatchException e) but it still says I have an InputMismatchException error.

Manan Adhvaryu · Accepted Answer · 2017-09-26 01:39:19Z

2

You could try this with the Switch Case statement as well.

switch(listchoice)
{
    case 1:
    //Statment
    break;

    case 2:
    //statement
    break;

    default:
    System.out.println("Your error message");
    break;
}

edited Sep 26, 2017 at 1:39

answered Sep 26, 2017 at 1:15

Manan Adhvaryu

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Comments

Bohemian · Accepted Answer · 2017-09-26 02:30:27Z

1

Because the user can enter anything, you have to read in a line as a String:

String input = scan.nextLine();

Once you've done that, the test is easy:

input.matches("[12]")

All together:

String input = null;
do {
    System.out.println("Enter either 1 or 2:");
    input = scan.nextLine();
} while (!input.matches("[12]")); 

int listChoice = Integer.parseInt(input); // input is only either "1" or "2"

answered Sep 26, 2017 at 2:30

Bohemian♦

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1 Comment

sukiyo Over a year ago

that's an interesting approach! i'll try it as well! thank u!

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