Insert tab in JTextPane after linebreak

Question

It's a pretty small thing, but it still would be neat to have it.

Basically, when you program e.g. with Eclipse, it automatically inserts tabs in the class or in a for-loop, so you don't have to align your code after every linebreak again.

Is it possible to implement the "you-don't-have-to-press-tab-again-for-some-alignment-after-a-linebreak" feature somehow in a JTextPane or JEditorPane?

Answer 1

You can use a custom DocumentFilter to implement this behaviour:

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

public class EndOfLineFilter extends DocumentFilter
{
    @Override
    public void replace(FilterBypass fb, final int offset, int length, String text, AttributeSet a)
        throws BadLocationException
    {
        if (text.equals("\n"))
            text = addWhiteSpace(fb, offset, text);

        super.replace(fb, offset, length, text, a);
    }


    private String addWhiteSpace(FilterBypass fb, int offset, String text) throws BadLocationException
    {
        Document doc = fb.getDocument();
        Element rootElement = doc.getDefaultRootElement();
        int line = rootElement.getElementIndex( offset );
        int i = rootElement.getElement(line).getStartOffset();
        StringBuilder whiteSpace = new StringBuilder(text);

        while (true)
        {
            String temp = doc.getText(i, 1);

            if (temp.equals(" ") || temp.equals("\t"))
            {
                whiteSpace.append(temp);
                i++;
            }
            else
                break;
        }

        return whiteSpace.toString();
    }

    private static void createAndShowUI()
    {
        JTextArea textArea = new JTextArea(10, 30);
        AbstractDocument doc = (AbstractDocument)textArea.getDocument();
        doc.setDocumentFilter( new EndOfLineFilter() );

        JFrame frame = new JFrame("SSCCE");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add( new JScrollPane( textArea ) );
        frame.pack();
        frame.setLocationByPlatform( true );
        frame.setVisible( true );
    }

    public static void main(String[] args)
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                createAndShowUI();
            }
        });
    }
}

Read the section from the Swing tutorial on Implementing a DocumentFilter for more information.