Why is else statement a infinite loop?

Question

I am fairly new to Java and I am trying to work on my data validation. The code runs fine when I use valid data, but when I put in a string instead of an integer the code just loops forever. It just loops the, "Bad input. Please enter a number." Thanks in advance!

    //Get input from user
    System.out.print("What is your name (Last, First)? ");
    String name = scan.nextLine();
    System.out.print("enter a date:");
    String datein = scan.nextLine();

    boolean valid = false;
    while (valid != true)
    {   
        System.out.print("Electricity used (KW):");
        if (scan.hasNextDouble())
        {
            electricityUsed = scan.nextDouble();
            valid = true;
        }
        else
            System.out.println("Bad input. Please enter a number.");
    }

score 8 · Answer 1 · answered Dec 31 '12 at 10:45

8

because hasNextDouble always returns false.

here is the doc. You answer your own question :

but when I put in a string instead of an integer

answered Dec 31 '12 at 10:45

NimChimpsky

46,453
60
198
311

2

+1 If you type anything but a double, it will keep finding that it is not a double, and never consume it. Add `scan.nextLine();` to the `else` block. – Peter Lawrey Dec 31 '12 at 10:47
Thank you very much guys. the scan.nextLine(); fixed the problem. Thank you for the link above about hasNextDouble. – user1939469 Dec 31 '12 at 23:55

score 3 · Accepted Answer · answered Dec 31 '12 at 10:46

3

To fix it add a scan.nextLine() to your else branch.

answered Dec 31 '12 at 10:46

Henry

42,982
7
68
84

score 2 · Answer 3 · answered Dec 31 '12 at 10:49

A simpler approach is to use the following.

System.out.print("Electricity used (KW):");
while(!scan.hasNextDouble()) {
    scan.nextLine();
    System.out.println("Bad input. Please enter a number.");
    System.out.print("Electricity used (KW):");
}
double electricityUsed = scan.nextDouble();

Why is else statement a infinite loop?

3 Answers3