How do I safely scan in an integer in a while loop?

Question

I want to scan in an integer and put it in a while loop so that if an invalid entry is entered the program will ask to enter another integer but when I run this it won't allow me to enter and new integer and just prints invalid entry multiple times.

do {
    System.out.println("Maximum number of Students:");
    if(input.hasNextInt()) {
        maxstudents = input.nextInt();
        if (maxstudents > 0) {
            break;
        }
    } else {
        System.out.println("Invalid entry");
    }
} while(true);

score 3 · Answer 1 · answered Feb 17 '19 at 05:58

3

Change the else to :

  else {
    System.out.println("Invalid entry");
    input.nextLine();
  }

So on an invalid input you need to move the cursor to the next line so that you can accept the input again.

answered Feb 17 '19 at 05:58

Nicholas K

14,118
7
25
49

score 0 · Answer 2 · answered Feb 17 '19 at 05:58

@Nicholas K is right. But I offer you to add another warning message for situation with negative integer and move this code into separate method.

public static int getMaxStudents(Scanner scan) {
    do {
        System.out.print("Maximum number of Students: ");

        if (scan.hasNextInt()) {
            int maxStudents = scan.nextInt();

            if (maxStudents > 0)
                return maxStudents;
        }

        System.out.println("Enter a valid positive integer number");
        scan.nextLine();    // <-- this is mandatory
    } while (true);
}

score 0 · Answer 3 · answered Feb 17 '19 at 06:04

do {
        System.out.println("Maximum number of Students:");
            if(input.hasNextInt()) {
                maxstudents = input.nextInt();
                    if (maxstudents > 0) {
                        break;
                    }
            }
            else {
                System.out.println("Invalid entry:"+scanner.next());
            }
    }while(true);

How do I safely scan in an integer in a while loop?

3 Answers3