Swing Hacks

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Chapter 7, Text

#49 Force Text Input into Specific Formats

HACK

Constraining a Document

Hopefully, you won’t be surprised to know that you don’t need to touch the

view classes—

JTextField, JTextArea, etc.—to add text constraint function-

ality. Text entry is happening in the model—in other words the

Document

—

so that’s where you tie in your regex code. This hack, listed in Example 7-2,

subclasses

PlainDocument

to run the regex check on every call to

insertString( )

.

Example 7-2. A document allowing input that matches only a regex

import javax.swing.text.*;

import java.util.regex.*;

public class RegexConstrainedDocument extends PlainDocument {

Pattern pattern;

Matcher matcher;

public RegexConstrainedDocument () { super( ); }

public RegexConstrainedDocument (AbstractDocument.Content c) { super(c); }

public RegexConstrainedDocument (AbstractDocument.Content c, String p) {

super (c);

setPatternByString (p);

}

public RegexConstrainedDocument (String p) {

super( );

setPatternByString (p);

}

public void setPatternByString (String p) {

Pattern pattern = Pattern.compile (p);

// checks the document against the new pattern

// and removes the content if it no longer matches

try {

matcher = pattern.matcher (getText(0, getLength( )));

System.out.println ("matcher reset to " +

getText (0, getLength( )));

if (! matcher.matches( )) {

System.out.println ("does not match");

remove (0, getLength( ));

}

} catch (BadLocationException ble) {

ble.printStackTrace( ); // impossible?

}

}

public Pattern getPattern( ) { return pattern; }

public void insertString (int offs, String s, AttributeSet a)

throws BadLocationException {