title: I create new Java web project with Vaadin framework.When I extends my class to com.vaadin.Application class the program says "cannot find symbol..."
body: <p>Please help !
  I create new Java web project with Vaadin framework.When I extends my class to  com.vaadin.Application class the program says "cannot find symbol..."  </p>
  
  <pre><code>package com;
  import com.vaadin.ui.*;
  public class Main extends com.vaadin.Application {
      public void init() {
        Window main=new Window("aa");
          setMainWindow(main);
            }
  }
  </code></pre>