Swing Hacks

Force Text Input into Specific Formats #49

Chapter 7, Text

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263

HACK

This class holds onto a Pattern and a Matcher to perform the regex match-

ing. The pattern can be set in the constructor, or later with a call to

setPatternByString( ). In either case, the method compiles the pattern and

creates a

Matcher with the Document’s text. Changing the pattern could, of

course, create a mismatch with any existing text in the

Document, so the

Matcher immediately calls matches( ) and if the text does not match, it

deletes all the text from the

Document.

Perhaps the more typical case is when the

Document.insertString() method is

called. This will happen on every keystroke in a

JTextComponent. Assuming the

Matcher is not null, meaning that the Pattern has been set at some point, you

simply need to call

Matcher.matches( ) against the Document’s new contents to

see if they comply with the regex constraints. If not, return early, never call-

ing the superclass’s

insert( ) method, and thus disallowing the input.

Adding Constrained Text Fields

Since the Document isn’t actually visible, you need to put it in something in

order to test it. The

TestRegexConstrainedDocument class creates a JTextField

for you to type in a regular expression to enforce, and a longer JTextField

for text to test. You create this latter JTextField with the seldom-seen con-

structor that takes a

Document, an initial-value String, and a width (specified ...