6.4.2. Formatted Input Fields

In the previous example program, we wanted the program user to type numbers, not arbitrary strings. That is, the user is allowed to enter only digits 0 through 9 and a hyphen (-). The hyphen, if present at all, must be the first symbol of the input string.

On the surface, this input validation task sounds simple. We can install a key listener to the text field and consume all key events that aren’t digits or a hyphen. Unfortunately, this simple approach, although commonly recommended as a method for input validation, does not work well in practice. First, not every combination of the valid input characters is a valid number. For example, --3 and 3-3 aren’t valid, even though they are made up from valid input characters. ...

Get Core Java® Volume II—Advanced Features, Ninth Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.