package com.javatpoint.helloworld.client;
 import com.google.gwt.core.client.EntryPoint;
 /** * Entry point classes define onModuleLoad() */ 
public class SampleWebApplication implements EntryPoint {
     / * This is the entry point method. */ 
public void onModuleLoad() {
 // TODO
 } 
}

SampleWebApplication.html

package com.javatpoint.helloworld.client;

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.event.dom.client.ClickEvent;
import com.google.gwt.event.dom.client.ClickHandler;
import com.google.gwt.user.client.ui.Button;
import com.google.gwt.user.client.ui.Label;
import com.google.gwt.user.client.ui.RootPanel;
import com.google.gwt.user.client.ui.VerticalPanel;

/**
 * Entry point classes define onModuleLoad().
 */
public class SampleWebApplication implements EntryPoint {
    /** A vertical panel that hold other components over UI.*/
    VerticalPanel vPanel;
    /*
    * A label that gets updated on click of button.
    */
    Label lbl;    
    /**
     * This is the entry point method.
     */
    public void onModuleLoad() {
        vPanel= new VerticalPanel ();
        lbl= new Label ();
        
        /*
        * Button and its click handlar.
        */
        Button btn = new Button("GWT Button");
        btn.addClickHandler(new ClickHandler() {
        
            @Override
        public void onClick(ClickEvent event) {
        lbl.setText("You clicked GWT Button!");
        }
        });        
        /*
        * adding label and button into Vertical Panel.
        */
        vPanel.add(lbl);
        vPanel.add(btn);        
        /*
        * Adding vertical panel into HTML page.
        */
        RootPanel.get().add(vPanel);
        }
    }