Vaadin is a Java library which is used to develop web-based user interfaces. Java developer can use SWING SWT or AWT to create the web page as he/she is designing a software with a UI. Knowledge of HTML or CSS is not necessary. Therefor, the developer need only to think a bout the logic of the web application and develop it in the way he/she develop a desktop software.
To use Vaadin we need to download Vaadin library from its website. Then we can create a Java web based project and import thar library file as a dependent to the project. For some IDEs there are some plugins too. There is a maven archetype also. Vaadin runs on any Applet Server like Apache-tomcat, Jetty etc. So in order to use Vaadin you must install some applet server. All these things are based on Java and therefor it needs Java Development Kit or JDK.
Now we can try a simple code. If you use Eclipse you will get a system generated sample code. It will be like this.
public class MyVaadinApplication extends Application
private Window mainWindow;
public void init()
mainWindow = new Window("Myproject Application");
Label label = new Label("Hello Vaadin user");