Question

I am writing a text editor(in java) where I am using a JTextArea for the main text editing and I am putting the text into a stringbuilder.

When I highlight the text and do something like delete that block of text, I want it to update in the stringbuilder.

So my question is, is there a way to get the first and last index of the highlighted text in the stringbuilder?

Thanks.

Answer 1

You can keep them in sync using a DocumentListener as below. However, I heavily recommend rethinking whatever approach you’re trying to take here. It seems like you’re trying to use an external StringBuilder as the “model” to your JTextArea, but the Document is the model. So I fail to see when this would be a good idea.

public static void main(String[] args) {
    JTextArea area = new JTextArea();
    final StringBuilder builder = new StringBuilder();
    area.getDocument().addDocumentListener(new DocumentListener() {
        @Override
        public void removeUpdate(DocumentEvent e) {
            builder.delete(e.getOffset(), e.getOffset() + e.getLength());
            System.out.println("Removed " + e.getLength() + " chars from SB");
        }

        @Override
        public void insertUpdate(DocumentEvent e) {
            try {
                builder.insert(e.getOffset(), 
                        e.getDocument().getText(e.getOffset(), e.getLength()));
                System.out.println("Inserted " + e.getLength() + " chars into SB.");
            } catch ( BadLocationException ble ) {
                ble.printStackTrace();
            }
        }

        @Override public void changedUpdate(DocumentEvent e) { /* no-op */ }
    });

    final JFrame frame = new JFrame("DocumentListener Test");
    frame.add(area);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.addWindowListener(new WindowAdapter() {
        @Override
        public void windowClosing(WindowEvent e) {
            System.out.println("Final SB contents:");
            System.out.println(builder.toString());
        }
    });
    frame.pack(); frame.setVisible(true);
}