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public class HelloVaadin {

    public static class MyUI extends UI {
        @Override
        protected void init(VaadinRequest request) {

            //create the components you want to use
            // and set the main component with setContent(..)
            final Layout layout = new VerticalLayout();
            layout
                .addComponent(new Button("click me",
                                         event -> layout.addComponents(new Label("clicked again"))
                ));

            //set the main Component
            setContent(layout);
        }

        @WebServlet("/*")
        @VaadinServletConfiguration(productionMode = false, ui = MyUI.class)
        public static class MyProjectServlet extends VaadinServlet { }
    }
}