How can I replace a tab with four spaces in JTextArea?

Question

I'm using a JTextArea in a JFrame. I would like the tab key to insert four spaces instead of a tab.

The method setTabSize does not work, as it puts a tab ('\t') in the contents of the text area.

How can I have JTextArea insert four spaces instead of a tab whenever I press the tab key? That way the getText() method will return indentations of four spaces for every tab.

Answer 1

I would avoid using KeyListeners (as a general rule with JTextComponents) and even Key Bindings, since while Key Bindings would work for keyboard input, it wouldn't work for copy-and-paste.

In my mind, the best way is to use a DocumentFilter set on the JTextArea's Document (which is a PlainDocument, by the way). This way, even if you copy and paste text into the JTextAreas, one with tabs, then all the tabs will automatically be converted to 4 spaces on insertion.

For example:

import javax.swing.*;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.DocumentFilter;
import javax.swing.text.PlainDocument;

public class TestTextArea {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            JTextArea textArea = new JTextArea(20, 50);
            JScrollPane scrollPane = new JScrollPane(textArea);

            int spaceCount = 4;
            ((PlainDocument) textArea.getDocument()).setDocumentFilter(new ChangeTabToSpacesFilter(spaceCount));

            JFrame frame = new JFrame("GUI");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(scrollPane);
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        });
    }
    
    private static class ChangeTabToSpacesFilter extends DocumentFilter {
        private int spaceCount;
        private String spaces = "";
        
        public ChangeTabToSpacesFilter(int spaceCount) {
            this.spaceCount = spaceCount;
            for (int i = 0; i < spaceCount; i++) {
                spaces += " ";
            }
        }

        @Override
        public void insertString(FilterBypass fb, int offset, String string, AttributeSet attr)
                throws BadLocationException {
            string = string.replace("\t", spaces);
            super.insertString(fb, offset, string, attr);
        }
        
        @Override
        public void remove(FilterBypass fb, int offset, int length) throws BadLocationException {
            super.remove(fb, offset, length);
        }
        
        @Override
        public void replace(FilterBypass fb, int offset, int length, String text, AttributeSet attrs)
                throws BadLocationException {
            text = text.replace("\t", spaces);
            super.replace(fb, offset, length, text, attrs);
        }
        
    }

}

So now, even if I copy and paste a document with tabs within it, into the JTextArea, all tabs will be automatically replaced with spaceCount spaces.