Swing Hacks

Auto-Completing Text Fields #50

Chapter 7, Text

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265

HACK

When you type a comma, a question mark, or lowercase letters, they are

ignored.

You can do smart things with this tool, but also really stupid things. Let’s

say you want to restrict input to the characters that will be in a North Amer-

ican phone number of the form 123-456-7890. You can set the pattern to

[0-9\-]*, which will allow the user to type in only numbers and the hyphen.

Now you’re feeling clever, but you don’t like the fact that this still allows

users to type patterns that aren’t phone numbers, like

1111111 or -1-1-1-1.

So, you set the pattern to an exact description of the phone number pattern:

[0-9]{3}-[0-9]{3}-[0-9]{4}

But now your users can’t enter anything! Why? Because although that regu-

lar expression does specify the phone number pattern, no substring of it will

ever match. This expression specifies exactly 12 characters, so when you

type the first one, there’s only one character—it doesn’t match the pattern,

so it’s rejected.

The moral of the story here is to be thoughtful. You might decide to wire up

a

FocusListener so you impose the regex pattern only when the user moves

off the field, and change the “delete everything” behavior to something a lit-

tle less forceful; for example, you could pop up a dialog telling the user that

her input isn’t in the right format. Just don’t be surprised when regular

expressions give you results ...