import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.*;
import java.nio.charset.Charset;
import javax.servlet.RequestDispatcher

public class Main extends HttpServlet {
  @Override
  protected void doGet(HttpServletRequest req, HttpServletResponse resp)
      throws ServletException, IOException {
    //resp.getWriter().print("Hello from Java! Charset: " + Charset.defaultCharset());
    //resp.sendRedirect("/html/index.html");
     RequestDispatcher view = req.getRequestDispatcher("/html/index.html");
        // don't add your web-app name to the path

        view.forward(req, resp);  
  }
}

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.*;
import java.nio.charset.Charset;
import javax.servlet.RequestDispatcher;

public class Main extends HttpServlet {
  @Override
  protected void doGet(HttpServletRequest req, HttpServletResponse resp)
      throws ServletException, IOException {
    //resp.getWriter().print("Hello from Java! Charset: " + Charset.defaultCharset());
    //resp.sendRedirect("/html/index.html");
     RequestDispatcher view = req.getRequestDispatcher("/html/index.html");
        // don't add your web-app name to the path

        view.forward(req, resp);  
  }
}

Update Main.java