the managerLauncherApi file:

package com.phehossof.launcher;

import com.google.gson.GsonBuilder;

import java.util.ArrayList;
import java.util.List;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

class ManagerLauncherApi {

    private static InterfaceLauncherApi service;
    private static ManagerLauncherApi managerLauncherApi;

    ManagerLauncherApi() {
        String BASE_URL = "http://phoenixhospman.com/api/launcher/";
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create(new GsonBuilder().setLenient().create()))
                .build();
        service = retrofit.create(InterfaceLauncherApi.class);
    }

    static synchronized ManagerLauncherApi getInstance() {
        if (managerLauncherApi == null) {
            managerLauncherApi = new ManagerLauncherApi();
        }
        return managerLauncherApi;
    }

    InterfaceLauncherApi getApi() {
        return service;
    }

    void logon(String username, String password, Callback<List<ObjectUser>> callback) {
        Call<List<ObjectUser>> logonCall = service.logon(username, password);
        logonCall.enqueue(callback);
    }

    void getGradeName(int grade, Callback<List<ObjectGrade>> callback) {
        Call<List<ObjectGrade>> gradeNameCall = service.getGradeName(grade);
        gradeNameCall.enqueue(callback);
    }

    void getDepartmentName(int department, Callback<List<ObjectDepartment>> callback) {
        Call<List<ObjectDepartment>> departmentCall = service.getDepartmentName(department);
        departmentCall.enqueue(callback);
    }
}

and in the interfaceLauncherApi file

package com.phehossof.launcher;

import java.util.ArrayList;
import java.util.List;

import retrofit2.Call;
import retrofit2.http.GET;
import retrofit2.http.Path;

public interface InterfaceLauncherApi {

    @GET("logon/{username}/{password}")
    Call<List<ObjectUser>> logon(@Path("username") String username,
                                 @Path("password") String password);

    @GET("getGradeName/{grade}")
    Call<List<ObjectGrade>> getGradeName(@Path("grade") int grade);

    @GET("getDepartmentName/{department}")
    Call<List<ObjectDepartment>> getDepartmentName(@Path("department") int department);
}