chat App(Vaadin)

package com.example.ChatAPP;

import java.util.ArrayList;
import java.util.Date;
import java.util.List;

import com.vaadin.data.provider.ListDataProvider;
import com.vaadin.event.ShortcutAction.KeyCode;
import com.vaadin.navigator.View;
import com.vaadin.ui.Button;
import com.vaadin.ui.CustomComponent;
import com.vaadin.ui.Grid;
import com.vaadin.ui.HorizontalLayout;
import com.vaadin.ui.Label;
import com.vaadin.ui.TextField;
import com.vaadin.ui.VerticalLayout;

public class Chatapp extends CustomComponent implements View {
    /**
                 * 
                 */
    private static final long serialVersionUID = 1L;

    private static final List<Msg> MSGS = new ArrayList<>();

    private static final ListDataProvider<Msg> DP = new ListDataProvider<>(MSGS);

    private static int UCOUNT = 1;

    private VerticalLayout vlytbaseLayout;

    int counter;
    Button send;

    public Chatapp() {
        send = new Button("send");
        send.setClickShortcut(KeyCode.ENTER);
        vlytbaseLayout = new VerticalLayout();

        setCompositionRoot(vlytbaseLayout);

        Grid<Msg> grid = new Grid<>();

        grid.addColumn(Msg::getMsg).setCaption("Chat");

        grid.setDataProvider(DP);

        Label lbl = new Label();
        lbl.setCaption("SecretUser" + UCOUNT++);
        TextField tf = new TextField();
        tf.setWidth(200, Unit.PERCENTAGE);

        Button send = new Button("send");
        send.setClickShortcut(KeyCode.ENTER);

        grid.addStyleName("chatgrid");

        HorizontalLayout hlyt = new HorizontalLayout();
        hlyt.addComponents(lbl, tf, send);
        hlyt.setExpandRatio(tf, 1);

        vlytbaseLayout.addComponents(grid, hlyt);
        vlytbaseLayout.setSizeFull();
        grid.setWidth(100, Unit.PERCENTAGE);

        send.addClickListener(e -> {
            grid.scrollToEnd();
            String value = tf.getValue();
            if (!"".equals(value.trim())) {
                MSGS.add(new Msg(new Date() + " " + lbl.getCaption() + " " + value));
                tf.clear();
                MyUI.ACTIVE_UIS.stream().forEach(ui -> {
                    ui.access(() -> {
                        DP.refreshAll();
                    });
                });
            }
                    });
    }
}

class Msg {
    String msg;

    Msg(String msg) {
        this.msg = msg;
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }

            }

package com.example.ChatAPP;

import java.util.ArrayList;
import java.util.List;

import javax.servlet.annotation.WebServlet;

import com.vaadin.annotations.Theme;
import com.vaadin.annotations.VaadinServletConfiguration;
import com.vaadin.navigator.Navigator;
import com.vaadin.server.VaadinRequest;
import com.vaadin.server.VaadinServlet;
import com.vaadin.ui.UI;
import com.vaadin.ui.VerticalLayout;

/**
 * This UI is the application entry point. A UI may either represent a browser window 
 * (or tab) or some part of a html page where a Vaadin application is embedded.
 * <p>
 * The UI is initialized using {@link #init(VaadinRequest)}. This method is intended to be 
 * overridden to add component to the user interface and initialize non-component functionality.
 */
@Theme("mytheme")
public class MyUI extends UI {
    public static final List<UI> ACTIVE_UIS = new ArrayList<>();
    @Override
    protected void init(VaadinRequest vaadinRequest) {
        final VerticalLayout layout = new VerticalLayout();
        layout.setSizeFull();
        Navigator n = new Navigator(this, layout);
        n.addView("haha", Chatapp.class);
        ACTIVE_UIS.add(this);
        UI.getCurrent().setPollInterval(1000);
        setContent(layout);
    }

    @Override
    public void detach() {
        ACTIVE_UIS.remove(this);
        super.detach();
    }
    @WebServlet(urlPatterns = "/*", name = "MyUIServlet", asyncSupported = true)
    @VaadinServletConfiguration(ui = MyUI.class, productionMode = false)
    public static class MyUIServlet extends VaadinServlet {
    }
}