SparkJava Web get(string, Route) in the type spark is not applicable

Question

I am building a simple REST API with Spark Java framework. I encountered this issue and not sure why this is happening. If it worked for

get("/".(req,res) -> "Hello World" 

But did not work when I use

(request, response) -> {}

Am I missing any libraries?

My dependency looks like this:

<dependency>
    <groupId>com.sparkjava</groupId>
    <artifactId>spark-core</artifactId>
    <version>2.5</version>
</dependency>

import static spark.Spark.*;
public class HelloWorld {
    public static void main(String[] args) {
        get("/",(request, response) -> {        
        });
        post("/transaction/add", (request, response) -> {
        });
    }
 }

Error message for the method:

Answer 1

It's Route source.

@FunctionalInterface
public interface Route {
    Object handle(Request arg0, Response arg1) throws Exception;
}

You have to return Object in lambda expression!