Java do-while loop simple issue

Question

I haven't coded in a few years and have been writing simple Java programs to re-familiarize myself with basic principles. However, I'm having trouble getting my do-while loop to act how I want it to. I guess I'm not understanding exactly how do-while loops work, but what I want is to gather user input from System.in as an int, and continue to ask for a valid int input if they enter some other data form. What I have is:

do {
    System.out.print("Input: ");

    userOption = userInput.nextInt();

} while (!userInput.hasNextInt());

System.out.println("You chose to: " + menuItems.get((userOption - 1)));

This doesn't work for two reasons. First, if I enter a non-int value, it immediately crashes throwing an input mismatch exception. Second, if I do enter a valid int, I always have to enter it twice. I.E., the console will ask for "Input: ", I'll enter say "2", the console will advance to the next line and wait for another input (but without printing anything), and then when I enter a second int it outputs the "You chose to [...]".

I have tried over a dozen different variations that just keep getting more complex, while loops inside do-while, inside if-else, but I'm sure I'm over complicating matters and am missing a simple concept. Any help is very much appreciated!

1) If you catch the exception it won't crash (exceptions are there to help, not to be a nuisance), and there you can put your logic for a wrong input, and 2) Call hasNextInt before reading the int and store the value in a boolean variable. — m0skit0
– m0skit0, Commented Feb 22, 2015 at 23:32

Community · Accepted Answer · 2017-05-23 12:06:30Z

1

Here is a stand alone example:

import java.util.*;

public class Foo {
  public static void main(String args[]){
        int userOption;
        Scanner userInput = new Scanner(System.in);

        System.out.print("Input: "); 
        while(!userInput.hasNextInt()){
            userInput.next();
            System.out.print("Error: Please enter an integer\nInput: "); 
        } 
        userOption = userInput.nextInt();

        System.out.println("You chose to: " +userOption);
    }
}

We ask for input the first time and while we can't find an integer we're going to display an error message and discard whatever was entered by user (userInput.next(), see also: https://stackoverflow.com/a/15973109/1063730).

edited May 23, 2017 at 12:06

CommunityBot

11 silver badge

answered Feb 22, 2015 at 23:55

nuala

2,6894 gold badges33 silver badges54 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Damon Getsman · Accepted Answer · 2015-02-22 23:35:42Z

0

Well, in order to avoid crashing when it's not an integer, load it into a String first, then let your integer value = parseInt(yourInputAsString);.

As for the looping structure and double input, you may want to look into the exact behavior of userInput.hasNextInt();. I would suggest docs.oracle.sun on whatever kind of object that userInput is.

Or are you required to use userInput object's method? I'm not really sure what that is, can you clarify?

Hope this helps a bit at least.

answered Feb 22, 2015 at 23:35

Damon Getsman

428 bronze badges

Comments

Zielu · Accepted Answer · 2015-02-22 23:39:21Z

It is not crashing, you get exception when the entered value was not int (you asked for int, there was no int, so java could only throw exception). You need to enter int value twice, becauce first int that you enter you read in the userOption statement, so do while conditions needs a new int value to be entered to get the condition true and quit.

Now, knowing it, you should be able to fix your program arround.

In the loop, check if there is a nextInt, if it is consume it (set your variable), if it is not int, read the input and discard it (I guess you use scanner, so nextLine will do the job), you can print out prompt to the user saying that int is required.

I am not writing the exact code as obviously you want to practise. Hint, I would start with while(!input.hasNextInt) {...} Good luck.

P.S. The only time I see do while in Java code is when someone just starts.

Collectives™ on Stack Overflow

Java do-while loop simple issue

3 Answers 3

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related