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.

CodePudding user response：

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.

CodePudding user response：

This is one of those “I wonder if…” moments.

Personally, I’d try a tackle the problem more directly, at the source. This means “trapping” the Tab event some how and “replacing” it’s functionality.

So, I started by modifying