protected void processRequest(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {
     String username = request.getParameter("username");
     String password = request.getParameter("password");

     switch(username) {
     case "client1":
         if(PASSWORD.equals(password)) {
             request.getSession(true).setAttribute("username", username);
             response.sendRedirect("app1/apk-1-index.html");
         }
         break;

     case "client2":
         if(PASSWORD.equals(password)) {
             request.getSession(true).setAttribute("username", username);
             response.sendRedirect("app2/apk-2-index.html");
         }
         break;

     case "client3":
         if(PASSWORD.equals(password)) {
             request.getSession(true).setAttribute("username", username);
             response.sendRedirect("app3/apk-3-index.html");
         }
         break;

     case "client4":
         if(PASSWORD.equals(password)) {
             request.getSession(true).setAttribute("username", username);
             response.sendRedirect("app4/apk-4-index.html");
         }
         break;
     case "client5":
         if(PASSWORD.equals(password)) {
             request.getSession(true).setAttribute("username", username);
             response.sendRedirect("app5/apk-5-index.html");
         }
         break;
     case "client6":
         if(PASSWORD.equals(password)) {
             request.getSession(true).setAttribute("username", username);
             response.sendRedirect("app5/apk-5-index.html");
         }
         break;
    default:
        response.sendRedirect("/Test2/index.jsp");
     }
}

case <THIS PART IS DIFFERENT>:
if(PASSWORD.equals(password)) {
    request.getSession(true).setAttribute("username", username);
    response.sendRedirect(<THIS PART IS DIFFERENT>);
}

Map<String, String> redirects = new HashMap<>();
redirects.put("client1", "app1/apk-1-index.html");
redirects.put("client2", "app2/apk-2-index.html");
redirects.put("client3", "app3/apk-3-index.html");
....

String redirect = redirects.get(username);
if (redirect == null) {
    response.sendRedirect("/Test2/index.jsp");
    return;
}

if (PASSWORD.equals(password)) {
    request.getSession(true).setAttribute("username", username);
    response.sendRedirect(redirect);
}