假设我们要开发一个电商app，匿名用户也可以进入首页浏览商品，如果需要购买商品，则必须登录后才能购买，登录后将当前登录用户账号返回到首页并保存到首页中，然后自动把该商品加入到购物车页面。要求登录页面在该app中使用单例模式。首页只需要做一个商品列表，每个商品都有图片、名称、价格和一个购买按钮；登录页面能输入账号、密码；购物车页面能获取传过来的用户账号、商品名称、价格，并用TextView控件显示出来。

以下是一个实现上述功能的简单电商app的代码示例。其中包括了单例模式的登录页面代码。

MainActivity.java

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;
import androidx.appcompat.app.AppCompatActivity;
import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    private ArrayList<Product> products = new ArrayList<>();
    private String currentUser = null;

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

        // 初始化产品列表
        initProducts();

        // 设置产品列表的适配器
        ProductAdapter adapter = new ProductAdapter(this, products);
        ListView listView = findViewById(R.id.list_view);
        listView.setAdapter(adapter);

        // 设置产品列表的点击事件
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int position, long id) {
                if (currentUser == null) {
                    // 如果用户未登录，则跳转到登录页面
                    Intent intent = new Intent(MainActivity.this, LoginActivity.class);
                    startActivityForResult(intent, 1);
                } else {
                    // 如果用户已登录，则将该产品添加到购物车页面
                    Cart.getInstance().addProduct(currentUser, products.get(position));
                }
            }
        });
    }

    // 初始化产品列表
    private void initProducts() {
        products.add(new Product("商品1", "这是商品1的描述", 50, R.drawable.product1));
        products.add(new Product("商品2", "这是商品2的描述", 100, R.drawable.product2));
        products.add(new Product("商品3", "这是商品3的描述", 150, R.drawable.product3));
        products.add(new Product("商品4", "这是商品4的描述", 200, R.drawable.product4));
        products.add(new Product("商品5", "这是商品5的描述", 250, R.drawable.product5));
        products.add(new Product("商品6", "这是商品6的描述", 300, R.drawable.product6));
    }

    // 处理登录页面返回的结果
    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == 1 && resultCode == RESULT_OK) {
            // 获取登录页面返回的用户账号
            currentUser = data.getStringExtra("username");

            // 更新首页的UI
            setTitle("欢迎，" + currentUser);
        }
    }
}

ProductAdapter.java

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

public class ProductAdapter extends BaseAdapter {

    private Context context;
    private ArrayList<Product> products;

    public ProductAdapter(Context context, ArrayList<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.list_item, parent, false);
        }

        ImageView imageView = convertView.findViewById(R.id.image_view);
        TextView nameTextView = convertView.findViewById(R.id.name_text_view);
        TextView priceTextView = convertView.findViewById(R.id.price_text_view);

        Product product = products.get(position);
        imageView.setImageResource(product.getImageResourceId());
        nameTextView.setText(product.getName());
        priceTextView.setText(String.valueOf(product.getPrice()));

        return convertView;
    }
}

LoginActivity.java

import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import androidx.appcompat.app.AppCompatActivity;

public class LoginActivity extends AppCompatActivity {

    private EditText usernameEditText;
    private EditText passwordEditText;

    private static LoginActivity instance = null;

    public static LoginActivity getInstance() {
        return instance;
    }

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

        // 将当前活动设置为单例实例
        instance = this;

        usernameEditText = findViewById(R.id.username_edit_text);
        passwordEditText = findViewById(R.id.password_edit_text);
        Button loginButton = findViewById(R.id.login_button);

        loginButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String username = usernameEditText.getText().toString();
                String password = passwordEditText.getText().toString();
                if (isValid(username, password)) {
                    // 如果用户名和密码都正确，则返回结果给调用者
                    Intent intent = new Intent();
                    intent.putExtra("username", username);
                    setResult(RESULT_OK, intent);
                    finish();
                } else {
                    // 如果用户名或密码不正确，则弹出提示框
                    Context context = LoginActivity.this;
                    String text = "用户名或密码不正确，请重试！";
                    int duration = Toast.LENGTH_SHORT;
                    Toast toast = Toast.makeText(context, text, duration);
                    toast.show();
                }
            }
        });
    }

    // 判断用户名和密码是否正确
    private boolean isValid(String username, String password) {
        return "admin".equals(username) && "123456".equals(password);
    }
}

Cart.java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Cart {

    private Map<String, List<Product>> cartMap = new HashMap<>();

    private static Cart instance = null;

    public static Cart getInstance() {
        if (instance == null) {
            instance = new Cart();
        }
        return instance;
    }

    // 将指定用户的指定产品添加到购物车中
    public void addProduct(String username, Product product) {
        List<Product> productList = cartMap.get(username);
        if (productList == null) {
            productList = new ArrayList<>();
            cartMap.put(username, productList);
        }
        productList.add(product);
    }

    // 获取指定用户的购物车中的所有产品列表
    public List<Product> getProducts(String username) {
        List<Product> productList = cartMap.get(username);
        if (productList == null) {
            productList = new ArrayList<>();
        }
        return productList;
    }
}

CartActivity.java

import android.content.Intent;
import android.os.Bundle;
import android.widget.LinearLayout;
import android.widget.TextView;
import androidx.appcompat.app.AppCompatActivity;
import java.util.List;

public class CartActivity extends AppCompatActivity {

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

        // 获取传递过来的用户账号
        Intent intent = getIntent();
        String username = intent.getStringExtra("username");

        // 获取该用户的购物车中的所有产品列表
        List<Product> products = Cart.getInstance().getProducts(username);

        // 将产品列表显示在界面上
        LinearLayout productLayout = findViewById(R.id.product_layout);
        for (Product product : products) {
            LinearLayout itemLayout = new LinearLayout(this);
            itemLayout.setOrientation(LinearLayout.HORIZONTAL);

            TextView nameTextView = new TextView(this);
            nameTextView.setText(product.getName() + "：");
            nameTextView.setTextSize(16);

            TextView priceTextView = new TextView(this);
            priceTextView.setText(String.valueOf(product.getPrice()) + "元");
            priceTextView.setTextSize(16);

            itemLayout.addView(nameTextView);
            itemLayout.addView(priceTextView);

            productLayout.addView(itemLayout);
        }
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="vertical"
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <ListView
            android:id="@+id/list_view"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"/>

</LinearLayout>

list_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="horizontal"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:padding="16dp">

    <ImageView
            android:id="@+id/image_view"
            android:layout_width="96dp"
            android:layout_height="96dp"/>

    <LinearLayout
            android:orientation="vertical"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:paddingLeft="16dp">

        <TextView
                android:id="@+id/name_text_view"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:textSize="18sp"
                android:textStyle="bold"/>

        <TextView
                android:id="@+id/price_text_view"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:textSize="18sp"/>

    </LinearLayout>

    <Button
            android:id="@+id/buy_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="购买"/>

</LinearLayout>

activity_login.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="vertical"
              android:layout_width="match_parent"
              android:layout_height="match_parent"
              android:padding="16dp">

    <EditText
            android:id="@+id/username_edit_text"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="请输入用户名"/>

    <EditText
            android:id="@+id/password_edit_text"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="请输入密码"
            android:inputType="textPassword"/>

    <Button
            android:id="@+id/login_button"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="登录"/>

</LinearLayout>

activity_cart.xml

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas