Best way to remap input of certain characters/keys to a JTextComponent?

Question

I am working on a JTextPane that works (almost) exactly like the tags input field here on stackoverflow. For that I am converting text to components as soon as a user hits enter, tab or space. Naturally I do not want any of those characters to actually be input to the text pane. I found this solution, SSCCE:

import java.awt.Color;
import java.awt.event.ActionEvent;

import javax.swing.AbstractAction;
import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextPane;
import javax.swing.KeyStroke;
import javax.swing.text.BadLocationException;
import javax.swing.text.Element;
import javax.swing.text.StyledDocument;

@SuppressWarnings("serial")
public class TagTextPane extends JTextPane {

    public TagTextPane() {
        this.getInputMap().put(KeyStroke.getKeyStroke("SPACE"), "finalizeTag");
        this.getInputMap().put(KeyStroke.getKeyStroke("ENTER"), "finalizeTag");
        this.getInputMap().put(KeyStroke.getKeyStroke("TAB"), "focusNext");
        this.getActionMap().put("focusNext", new AbstractAction() {

            @Override
            public void actionPerformed(ActionEvent arg0) {
                finalizeTag();
                transferFocus();
            }
        });
        this.getActionMap().put("finalizeTag", new AbstractAction() {

            @Override
            public void actionPerformed(ActionEvent e) {
                finalizeTag();
            }
        });
    }

    private void finalizeTag() {
        StyledDocument doc = (StyledDocument) getDocument();
        Element element = doc.getCharacterElement(getCaretPosition() - 1);
        int start = element.getStartOffset();
        int len = element.getEndOffset() - start;
        String tag = "";
        try {
            tag = this.getDocument().getText(start, len);
        } catch (BadLocationException e) {
        }
        this.setSelectionStart(start);
        this.setSelectionEnd(start + len);
        JLabel label = new JLabel(tag);
        label.setBorder(BorderFactory.createLineBorder(Color.BLACK));
        this.insertComponent(label);        
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("TagTextPaneTest");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        TagTextPane tPane = new TagTextPane();
        frame.setSize(400, 100);
        frame.getContentPane().add(tPane);
        frame.setVisible(true);
    }
}

Hitting "tab" usually causes a tab character to be inserted and hitting "enter" usually causes a line break to be entered, as well as hitting "space" causes a space to be entered. The weird thing is that my code stops line breaks and tabs from being entered, but still allows spaces to be entered normally, while performing the intended action.

1. Why does this approach behave differently for those keys?

2. How can I stop the space from being entered? (Maybe I want to extend this behavior to commas and the like later on.)

3. How can I optimally control the behavior of certain characters and keystrokes in a JTextComponent? I have seen this, where the suggestion is to use a KeyListener, but I have also seen this, where Rob Camick points out, why a KeyListener should not be used. So should I rather use a DocumentFilter, or is that breaking a butterfly on a wheel?

4. Is this even the best way to build this kind of tag input field?

Answer 1

this.getInputMap().put(KeyStroke.getKeyStroke("SPACE"), "finalizeTag");

This is handling the keyPressed event for the space character.

You want to handle the keyTyped event of the space character:

this.getInputMap().put(KeyStroke.getKeyStroke(' '), "finalizeTag");