public interface SomeScreenView {
    void startLoading(); 
    void stopLoading(); 
    void mapDataItems(final Collection<DataItem> items);
}

public class SomeScreenPresenter extends Presenter {
    private SomeScreenView mView;

    public void setView(SomeScreenView view) {
        mView = view;
    }

    @Override
        public void initialize() {
        mView.startLoading();

        mView.mapDataItems(...);
        mView.stopLoading();
    }
}

@EActivity(R.layout.activity_some_screen) 
public class SomeScreenActivity extends Activity implements SomeScreenView { 
    private SomeScreenPresenter mPresenter; 

    @ViewById(R.id.drawer_layout) 
    protected ProgressBar mProgressBar;

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
        super.onCreate(savedInstanceState); 

        mPresenter = new SomeScreenPresenter(this);
        mPresenter.gradletialize(); 
    } 
}

public class Product {
    private String id;
    private String name;
    private double price;

    public Product(String id, String name, double price) {
        this.id = id;
        this.name = name;
        this.price = price;
    }

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public double getPrice() {
        return price;
    }
}

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

public interface ApiService {
    @GET("products")
    Call<List<Product>> getProducts();
}

public class Purchase {
    private String userId;
    private List<Product> products;

    public Purchase(String userId, List<Product> products) {
        this.userId = userId;
        this.products = products;
    }

    public String getUserId() {
        return userId;
    }

    public List<Product> getProducts() {
        return products;
    }
}

import android.util.Log;
import java.util.List;

public class ProductPresenter {

    private ProductView view;
    private ApiService apiService;
    private List<Product> products;

    public ProductPresenter(ProductView view, ApiService apiService) {
        this.view = view;
        this.apiService = apiService;
    }

    public void loadProducts() {
        apiService.getProducts().enqueue(new retrofit2.Callback<List<Product>>() {
            @Override
            public void onResponse(Call<List<Product>> call, retrofit2.Response<List<Product>> response) {
                if (response.isSuccessful()) {
                    products = response.body();
                    view.showProducts(products);
                } else {
                    view.showError("Failed to load products");
                }
            }

            @Override
            public void onFailure(Call<List<Product>> call, Throwable t) {
                view.showError("Network error");
            }
        });
    }

    public void addToCart(Product product) {
        // Логіка додавання товару в кошик
        view.showProductAddedToCart(product);
    }

    public void makePurchase(String userId) {
        if (products == null || products.isEmpty()) {
            view.showError("Your cart is empty");
            return;
        }
        Purchase purchase = new Purchase(userId, products);
        // Можна реалізувати мережевий запит для оформлення покупки
        view.showPurchaseSuccess(purchase);
    }
}

public interface ProductView {
    void showProducts(List<Product> products);
    void showError(String error);
    void showProductAddedToCart(Product product);
    void showPurchaseSuccess(Purchase purchase);
}

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ListView;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;

import java.util.List;

public class MainActivity extends AppCompatActivity implements ProductView {

    private ProductPresenter presenter;
    private ListView productsListView;
    private Button buyButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        productsListView = findViewById(R.id.productsListView);
        buyButton = findViewById(R.id.buyButton);

        // Ініціалізація Retrofit
        ApiService apiService = RetrofitClient.getRetrofitInstance().create(ApiService.class);
        presenter = new ProductPresenter(this, apiService);

        // Завантаження продуктів
        presenter.loadProducts();

        buyButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // Оформлення покупки (потрібно передати userId)
                presenter.makePurchase("user123");
            }
        });
    }

    @Override
    public void showProducts(List<Product> products) {
        // Налаштовуємо адаптер для відображення товарів
        ProductAdapter adapter = new ProductAdapter(this, products);
        productsListView.setAdapter(adapter);
    }

    @Override
    public void showError(String error) {
        Toast.makeText(this, error, Toast.LENGTH_SHORT).show();
    }

    @Override
    public void showProductAddedToCart(Product product) {
        Toast.makeText(this, "Product added to cart: " + product.getName(), Toast.LENGTH_SHORT).show();
    }

    @Override
    public void showPurchaseSuccess(Purchase purchase) {
        Toast.makeText(this, "Purchase successful! " + purchase.getProducts().size() + " items bought.", Toast.LENGTH_SHORT).show();
    }
}

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.TextView;
import java.util.List;

public class ProductAdapter extends BaseAdapter {

    private Context context;
    private List<Product> products;

    public ProductAdapter(Context context, List<Product> products) {
        this.context = context;
        this.products = products;
    }

    @Override
    public int getCount() {
        return products.size();
    }

    @Override
    public Object getItem(int position) {
        return products.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        if (convertView == null) {
            convertView = LayoutInflater.from(context).inflate(R.layout.product_item, parent, false);
        }

        Product product = products.get(position);

        TextView nameTextView = convertView.findViewById(R.id.productName);
        TextView priceTextView = convertView.findViewById(R.id.productPrice);

        nameTextView.setText(product.getName());
        priceTextView.setText("$" + product.getPrice());

        return convertView;
    }
}

public class RetrofitClient {

    private static Retrofit retrofit;
    private static final String BASE_URL = "https://api.example.com/";

    public static Retrofit getRetrofitInstance() {
        if (retrofit == null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl(BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create())
                    .build();
        }
        return retrofit;
    }
}

import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;

public class GreetingViewModel extends ViewModel {

    private MutableLiveData<String> name = new MutableLiveData<>("");
    private MutableLiveData<String> greetingMessage = new MutableLiveData<>("");

    public LiveData<String> getName() {
        return name;
    }

    public void setName(String newName) {
        name.setValue(newName);
        updateGreetingMessage();
    }

    public LiveData<String> getGreetingMessage() {
        return greetingMessage;
    }

    private void updateGreetingMessage() {
        String currentName = name.getValue();
        if (currentName == null || currentName.isEmpty()) {
            greetingMessage.setValue("Привіт, незнайомцю!");
        } else {
            greetingMessage.setValue("Привіт, " + currentName + "!");
        }
    }
}

import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.databinding.DataBindingUtil;
import androidx.lifecycle.ViewModelProvider;

import com.example.mvvmexample.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Використовуємо Data Binding
        ActivityMainBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_main);

        // Ініціалізуємо ViewModel
        GreetingViewModel viewModel = new ViewModelProvider(this).get(GreetingViewModel.class);

        // Прив'язуємо ViewModel до Binding
        binding.setViewModel(viewModel);

        // Дозволяємо LiveData автоматично оновлювати UI
        binding.setLifecycleOwner(this);
    }
}

private MutableLiveData<List<String>> items = new MutableLiveData<>(new ArrayList<>());

public LiveData<List<String>> getItems() {
    return items;
}

public void addItem(String item) {
    List<String> currentItems = items.getValue();
    if (currentItems != null) {
        currentItems.add(item);
        items.setValue(currentItems);
    }
}

import androidx.databinding.ObservableArrayList;

public class CalculatorViewModel extends ViewModel {
    // Список операцій у калькуляторі
    public ObservableArrayList<String> operations = new ObservableArrayList<>();

    // Додати операцію
    public void addOperation(String operation) {
        operations.add(operation);
    }

    // Видалити операцію
    public void removeOperation(String operation) {
        operations.remove(operation);
    }
}

public class MyViewModel extends ViewModel {
    public MutableLiveData<String> liveUserName = new MutableLiveData<>();

    public void updateName(String name) {
        liveUserName.setValue(name);
    }
}

public class MyViewModel extends ViewModel {
    public MutableLiveData<List<String>> liveItems = new MutableLiveData<>();

    public void addItem(String item) {
        List<String> currentItems = liveItems.getValue();
        if (currentItems == null) {
            currentItems = new ArrayList<>();
        }
        currentItems.add(item);
        liveItems.setValue(currentItems);
    }
}