textArea.getDocument().addUndoableEditListener疑问

class Test extends  JFrame
{        final UndoManager undo =new UndoManager();
UndoableEditListener undoHandler = new UndoHandler();
JTextArea textArea=new JTextArea();
.....
          public Test()
          {    ....
              textArea.getDocument().addUndoableEditListener(undoHandler);
              .....
          }

class UndoHandler implements UndoableEditListener
{
  public void undoableEditHappened(UndoableEditEvent e)
  {
   undo.addEdit(e.getEdit());
  }
}
}
textArea.getDocument().addUndoableEditListener(undoHandler);这句放的位置有没有限制
我把它放到构造函数Test()外  怎么就编译不通过