java - How to change the color of specific words in a JTextPane?

Question

How do I change the color of specific words in a JTextPane just while the user is typing? Should I override JTextPane paintComponent method?

Answer 1

No. You are not supposed to override the paintComponent() method. Instead, you should use StyledDocument. You should also delimit the words by your self.

Here is the demo, which turns "public", "protected" and "private" to red when typing, just like a simple code editor:

import javax.swing.*;
import java.awt.*;
import javax.swing.text.*;

public class Test extends JFrame {
    private int findLastNonWordChar (String text, int index) {
        while (--index >= 0) {
            if (String.valueOf(text.charAt(index)).matches("\W")) {
                break;
            }
        }
        return index;
    }

    private int findFirstNonWordChar (String text, int index) {
        while (index < text.length()) {
            if (String.valueOf(text.charAt(index)).matches("\W")) {
                break;
            }
            index++;
        }
        return index;
    }

    public Test () {
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(400, 400);
        setLocationRelativeTo(null);

        final StyleContext cont = StyleContext.getDefaultStyleContext();
        final AttributeSet attr = cont.addAttribute(cont.getEmptySet(), StyleConstants.Foreground, Color.RED);
        final AttributeSet attrBlack = cont.addAttribute(cont.getEmptySet(), StyleConstants.Foreground, Color.BLACK);
        DefaultStyledDocument doc = new DefaultStyledDocument() {
            public void insertString (int offset, String str, AttributeSet a) throws BadLocationException {
                super.insertString(offset, str, a);

                String text = getText(0, getLength());
                int before = findLastNonWordChar(text, offset);
                if (before < 0) before = 0;
                int after = findFirstNonWordChar(text, offset + str.length());
                int wordL = before;
                int wordR = before;

                while (wordR <= after) {
                    if (wordR == after || String.valueOf(text.charAt(wordR)).matches("\W")) {
                        if (text.substring(wordL, wordR).matches("(\W)*(private|public|protected)"))
                            setCharacterAttributes(wordL, wordR - wordL, attr, false);
                        else
                            setCharacterAttributes(wordL, wordR - wordL, attrBlack, false);
                        wordL = wordR;
                    }
                    wordR++;
                }
            }

            public void remove (int offs, int len) throws BadLocationException {
                super.remove(offs, len);

                String text = getText(0, getLength());
                int before = findLastNonWordChar(text, offs);
                if (before < 0) before = 0;
                int after = findFirstNonWordChar(text, offs);

                if (text.substring(before, after).matches("(\W)*(private|public|protected)")) {
                    setCharacterAttributes(before, after - before, attr, false);
                } else {
                    setCharacterAttributes(before, after - before, attrBlack, false);
                }
            }
        };
        JTextPane txt = new JTextPane(doc);
        txt.setText("public class Hi {}");
        add(new JScrollPane(txt));
        setVisible(true);
    }

    public static void main (String args[]) {
        new Test();
    }
}

The code is not so beautiful since I typed it quickly but it works. And I hope it will give you some hint.