JTextAreaの最終行だけ編集可能になるよう設定する

class NonEditableLineDocumentFilter extends DocumentFilter {
  public static final String PROMPT = "> ";
  @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);
  }

  @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();
    if (index == count - 1 && offset - promptPosition >= 0) {
      if (text.equals("\n")) {
        String line = doc.getText(promptPosition, offset - promptPosition);
        String[] args = line.split(" ");
        String cmd = args[0];
        if (cmd.isEmpty()) {
          text = "";
        } else {
          text = String.format("%n%s: command not found", cmd);
        }
        text += "\n" + PROMPT;
      }
      fb.replace(offset, length, text, attrs);
    }
  }
}