Mock web service created from retrofit RestAdapter

I am using Retrofit and injecting RestAdapter and WebService class/interface through Dagger2

In the tests, I am mocking my WebService class with MockWebService implements WebService and I am getting this exception from Retrofit

java.lang.IllegalArgumentException: Only interface endpoint definitions are supported.
        at retrofit.Utils.validateServiceClass(Utils.java:102)
        at retrofit.RestAdapter.create(RestAdapter.java:193)

Here is the interface class for WebService

public interface WebService {

@FormUrlEncoded
@POST("/loginByEmail")
void loginByEmail (@Field("emailaddress") String email,
                   @Field("password") String password,
                   @Field("checksum") String checksum,
                   Callback<ServiceModel<User>> callback);
}

And, here is my MockWebService class

public class MockWebService200 implements WebService {
    @Override
    public void loginByEmail (@Field("emailaddress") String email,
                          @Field("password") String password,
                          @Field("checksum") String checksum,
                          Callback<ServiceModel<User>> callback) {
    ServiceModel<User> userModel = getServiceModel( getUser() );
    callback.success( userModel, null );
    }

    @Override public <T> ServiceModel<T> getServiceModel (T model) {
        // Skipped
    }

    @Override public User getUser () {
    // Skipped
    }
    }

getUser() and getServiceModel() are skipped here.

Questions:

1- How to fix this?

2- Is my approach correct to mock web services through polymorphism and what alternatives I can use here?