Let us consider the problem of finding the *maximum of a set of integers*
in input. Assume that:

- we know the interval over which the integers range, in the sense that in the program we can denote the extremes of the interval (e.g., we know that all integers are greater than or equal to 0).

String s; // current string in input int n; // current integer int max; // current maximum max = -1; s = JOptionPane.showInputDialog("Input an integer"); while (s != null) { n = Integer.parseInt(s); if (n > max) max = n; s = JOptionPane.showInputDialog("Input an integer"); } if (max == -1) System.out.println("empty set of values"); else System.out.println("maximum = " + max);

*Note:* If the user does not input any integer, then the computed maximum
value is -1. Since -1 is not in the interval of allowed values (we have
assumed that all integers are greater than or equal to 0), it can never be
returned in the case where the user inputs at least one value. Therefore, in
this case, we can use the comparison of the result with -1 as a means to detect
whether the user has input at least a value or not.