Swing - Tabs to Spaces

How do you make a swing text component (JTextEditor, JEditorPane, etc.) to insert spaces whenever the user hits the "Tab" key?

The answer is below. It is based on setting a document filter on the component's document object. The (static) install() method can do all the setup for you.

package com.blogspot.javadots;

import javax.swing.text.AbstractDocument;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.DocumentFilter;
import javax.swing.text.JTextComponent;

public class TabToSpaceFilter extends DocumentFilter {
   
   private String spaces = "";
   private JTextComponent textComponent;
   
   public TabToSpaceFilter(int tabSize, JTextComponent tc) {
      textComponent = tc;
      for(int i = 0; i < tabSize; ++i)
         spaces += " ";         
   }
   
   @Override
   public void insertString(FilterBypass fb, int offset, String text,
      AttributeSet attr) throws BadLocationException {
      super.insertString(fb, offset, translate(offset, text), attr);
   }

   @Override
   public void replace(FilterBypass fb, int offset, int length,
      String text, AttributeSet attr) throws BadLocationException {
      super.replace(fb, offset, length, translate(offset, text), attr);
   }

   private String translate(int offset, String s) {
      int col = columnOf(offset);
      
      StringBuilder sb = new StringBuilder();
      int top = s.length();
      for(int i = 0; i < top; ++i, ++col) {
         char c = s.charAt(i);
         if(c == '\t') 
            sb.append(spaces.substring(col % spaces.length()));
         else
            sb.append(c);
      }   
      
      return sb.toString();
   }

   private int columnOf(int i) {
      String s = textComponent.getText();
      if(i == 0)
         return 0;
      int prev = s.lastIndexOf("\n", i-1);
      if(prev < 0)
         return i;
      
      return (i-prev)-1;
   }
   
   public static void install(int tabSize, JTextComponent area) {
      AbstractDocument ad = (AbstractDocument) area.getDocument();
      ad.setDocumentFilter(new TabToSpaceFilter(tabSize, area));
   }
}