尝试对null对象引用调用接口方法“”Retrofit2.Call com.example.matagaruda.Api.Api.userLogin(String，String)“”

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public class Example {
    public static void main(String[] args) {
        // 假设Api接口已经被正确初始化
        Api api = RetrofitClient.getClient().create(Api.class);

        // 检查api对象是否为null
        if (api != null) {
            Call<User> call = api.userLogin("username", "password");
            call.enqueue(new Callback<User>() {
                @Override
                public void onResponse(Call<User> call, Response<User> response) {
                    if (response.isSuccessful()) {
                        User user = response.body();
                        System.out.println("Login successful: " + user);
                    } else {
                        System.out.println("Login failed: " + response.message());
                    }
                }

                @Override
                public void onFailure(Call<User> call, Throwable t) {
                    t.printStackTrace();
                }
            });
        } else {
            System.out.println("Api object is null");
        }
    }
}