public class MyApplication implements EntryPoint {
  public void onModuleLoad() {
    final Button button = new Button("Click me");
    final Label label = new Label();
    button.addClickListener(new ClickListener() {
      public void onClick(Widget sender) {
        if (label.getText().equals(""))
          label.setText("Hello World!");
        else 
          label.setText("");
      }
    });
    RootPanel.get("slot1").add(button);
    RootPanel.get("slot2").add(label);
  }
}