Looping Until Valid Input

The user can then type anything into the input field and when they click OK, the text entered is stored in a String that input points to. Often times we may want to ensure that what the user enters is consistent with what we expect. For example, we may want to make sure they don't just click OK without typing anything.

If the user doesn't enter the correct information, we could ask them again to enter the appropriate information. One technique would be to have a loop that continually asks the user to enter the input until we are sure that they have entered something that we can use. This might looks something like this:

String input;
do {
  input = JOptionPane.showInputDialog("Please enter something");
} while(!input.isEmpty());

This works fine for situations where we have very simple criteria for valid input (in this case, the user just needs to type at least one character before clicking OK). If our conditions on what makes up valid input is more complicated, we can create a boolean flag that will keep track of whether or not we received valid input.

boolean isValid = false;
String input;
do {
  input = JOptionPane.showInputDialog("Please enter something");
  if(!input.isEmpty()) {
    isValid = true;
  }
} while(!isValid);

Whenever the user clicks OK (or hits the enter key), the text typed in the text field is converted to a string and returned. If the user clicks Cancel or the X in the upper right corner of the window (or hits the esc key), the method returns null. A null is the value for a reference that isn't actually pointing to an object. In this case, it's a way of telling your program that "nothing" was returned from the showInputDialog() method.

Below I've introduced another flag (canceled) which will be true if the user cancels the dialog box.

boolean isValid = false;
boolean canceled = false;
String input;
do {
  input = JOptionPane.showInputDialog("Please enter something");
  if(input==null) {
    canceled = true;
  } else {
    isValid = !input.isEmpty();
  }
} while(!canceled && !isValid);

Here we only want to loop again if the user has not canceled and has not entered valid input.

We can use this same structure to check for much more complicated input conditions. For example, ensuring the user entered a single digit integer, might look something like this:

boolean isValid = false;
boolean canceled = false;
int singleDigit = -30;       // Set it to some invalid value
do {
  String input = JOptionPane.showInputDialog("Please enter something");
  if(input==null) {
    canceled = true;
  } else {
    Scanner converter = new Scanner(input);
    if(converter.hasNextInt()) {
      singleDigit = convereter.nextInt();
      isValid = singleDigit<10 && singleDigit>-10);
    }
  }
} while(!canceled && !isValid);
if(canceled) {
  JOptionPane.showMessageDialog(null, "You canceled the dialog box!");
} else {
  JOptionPane.showMessageDialog(null, "Great job!  You entered " + singleDigit);
}