dialog.addComponentListener(
     new  ComponentAdapter()
    {
         public   void  componentResized(ComponentEvent e)
        {
             boolean  widthLess = getSize().width < minimumSize.width;
             boolean  heightLess = getSize().height < minimumSize.height;
            
             if  (!widthLess && !heightLess)
            {
                 return ;
            }
            
            Dimension newSize =  new  Dimension(getSize());
             if  (widthLess)
            {
                newSize.width = minimumSize.width;
            }
             if  (heightLess)
                newSize.height = minimumSize.height;
            
            setSize(newSize);
        }
    }
);