Make parts of a JTextArea non editable (not the whole JTextArea!)

Question

I\'m currently working on a console window in Swing. It\'s based on a JTextArea and works like a common command line. You type a command in one line and press enter. In the

Answer 1

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.text.*;
public class OnlyEditCurrentLineTest {
  public JComponent makeUI() {
    JTextArea textArea = new JTextArea(8,0);
    textArea.setText("> aaa\n> ");
    ((AbstractDocument)textArea.getDocument()).setDocumentFilter(
        new NonEditableLineDocumentFilter());
    JPanel p = new JPanel(new BorderLayout());
    p.add(new JScrollPane(textArea), BorderLayout.NORTH);
    return p;
  }
  public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
      @Override public void run() { createAndShowGUI(); }
    });
  }
  public static void createAndShowGUI() {
    JFrame f = new JFrame();
    f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    f.getContentPane().add(new OnlyEditCurrentLineTest().makeUI());
    f.setSize(320,240);
    f.setLocationRelativeTo(null);
    f.setVisible(true);
  }
}
class NonEditableLineDocumentFilter extends DocumentFilter {
  @Override public void insertString(
      DocumentFilter.FilterBypass fb, int offset, String string,
      AttributeSet attr) throws BadLocationException {
    if(string == null) {
      return;
    }else{
      replace(fb, offset, 0, string, attr);
    }
  }
  @Override public void remove(
      DocumentFilter.FilterBypass fb, int offset,
      int length) throws BadLocationException {
    replace(fb, offset, length, "", null);
  }
  private static final String PROMPT = "> ";
  @Override public void replace(
      DocumentFilter.FilterBypass fb, int offset, int length,
      String text, AttributeSet attrs) throws BadLocationException {
     Document doc = fb.getDocument();
     Element root = doc.getDefaultRootElement();
     int count = root.getElementCount();
     int index = root.getElementIndex(offset);
     Element cur = root.getElement(index);
     int promptPosition = cur.getStartOffset()+PROMPT.length();
     //As Reverend Gonzo says:
     if(index==count-1 && offset-promptPosition>=0) {
       if(text.equals("\n")) {
         String cmd = doc.getText(promptPosition, offset-promptPosition);
         if(cmd.isEmpty()) {
           text = "\n"+PROMPT;
         }else{
           text = "\n"+cmd+"\n    xxxxxxxxxx\n" + PROMPT;
         }
       }
       fb.replace(offset, length, text, attrs);
     }
  }
}