class LimitSizeTextField extends JTextField {
 
     public LimitSizeTextField(int cols) {
         super(cols);
     }
     public LimitSizeTextField() {
         super();
     }
 
     protected Document createDefaultModel() {
 	      return new LimitedDocument();
     }
 
     class LimitedDocument extends PlainDocument {
         private  int TEXT_MAXIMUM_SIZE = 4;
         public void insertString(int offs, String str, AttributeSet a) 
 	          throws BadLocationException  {
 
 	          if (str == null)  return;       
  	          if (getLength() >= TEXT_MAXIMUM_SIZE  ) return;
 	          super.insertString(offs, str, a);
 	      }
     }
 }