ClientModule.java

// add these bindings
install(new DefaultModule.Builder().tokenFormatter(RouteTokenFormatter.class).build());
bind(UserService.class).to(UserServiceImpl.class).asEagerSingleton();

UserService.java

public interface UserService {
    String getUsername(int userId);

    void saveUsername(int userId, String username);

    Map<Integer, String> getUsers();
}

UserServiceImpl.java

// Dummy implementation of a UserService
public class UserServiceImpl implements UserService {
    private Map<Integer, String> usernames;

    public UserServiceImpl() {
        usernames = new HashMap<>();

        usernames.put(1, "Arcbees");
        usernames.put(2, "Joel");
        usernames.put(3, "Olivier");
    }

    @Override
    public String getUsername(int userId) {
        return usernames.get(userId);
    }

    @Override
    public void saveUsername(int userId, String username) {
        usernames.put(userId, username);
    }

    @Override
    public Map<Integer, String> getUsers() {
        return usernames;
    }
}