Error after migrating vaadin 7 to 8

Question

I just used https://vaadin.com/docs/framework/v8/migration/migrating-to-vaadin8.html to migrate my application from vaadin 7 to vaadin 8.1.4. I was able to compile the widgetset file, but when I am trying to run the application on tomcat 8, I am getting following error

java.lang.NoSuchMethodError: com.vaadin.ui.Button.addClickListener(Lcom/vaadin/ui/Button$ClickListener;)Lcom/vaadin/shared/Registration;

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SEVERE: Servlet.service() for servlet [vaadin-servlet] in context with path [/activescreener] threw exception [com.vaadin.server.ServiceException: java.lang.NoSuchMethodError: com.vaadin.ui.Button.addClickListener(Lcom/vaadin/ui/Button$ClickListener;)Lcom/vaadin/shared/Registration;] with root cause java.lang.NoSuchMethodError: com.vaadin.ui.Button.addClickListener(Lcom/vaadin/ui/Button$ClickListener;)Lcom/vaadin/shared/Registration;

Is something wrong with my migration. I can see in my classes, not every import is migrated to *.v7

import com.vaadin.ui.Button;
import com.vaadin.ui.Button.ClickEvent;
import com.vaadin.ui.Button.ClickListener;
import com.vaadin.ui.CustomComponent;
import com.vaadin.v7.ui.HorizontalLayout;
import com.vaadin.ui.Layout;
import com.vaadin.ui.MenuBar;
import com.vaadin.ui.MenuBar.MenuItem;
import com.vaadin.v7.ui.OptionGroup;
import com.vaadin.ui.UI;
import com.vaadin.v7.ui.VerticalLayout;

Example, I still have com.vaadin.ui.Button;, Should it be com.vaadin.v7.ui.Button?

The button is correct. `NoSuchMethodError` is often a problem with the classpath. Maybe you have multiple Vaadin version jars on the classpath? — Steffen Harbich, Sep 23 '17 at 09:20
How can search for it? I mean how to check for the classpath? — user1631306, Sep 23 '17 at 14:12
What is your dependency management tool? Are you using maven or gradle? — Steffen Harbich, Sep 23 '17 at 16:50
Have a look [here](https://stackoverflow.com/a/2834352/1063673) to get your dependencies. Also, it would be helpful to see your pom.xml. — Steffen Harbich, Sep 24 '17 at 14:10
Have a look at the dependency tree. Is vaadin occuring in different versions? Maybe old add-ons are pulling it into the classpath. Also I recommend that you read about the concept of classpath because it is important in Java. — Steffen Harbich, Sep 24 '17 at 14:53

Error after migrating vaadin 7 to 8

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