给出AppCompatActivity的代码

下面是一个简单的AppCompatActivity的代码示例：

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

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

该代码创建了一个名为MainActivity的类，它继承了AppCompatActivity类，并覆盖了onCreate()方法，该方法是Activity的生命周期方法之一。在onCreate()方法中，使用setContentView()方法指定了一个名为activity_main的布局文件，它定义了界面的外观和布局。注意，要使用AppCompatActivity类，需要在build.gradle文件中添加如下依赖项：

implementation 'androidx.appcompat:appcompat:1.2.0'

相关问题

给出Android 的 jni 示例代码

以下是一个简单的 Android JNI 示例代码:

Java 代码：

public class MainActivity extends AppCompatActivity {

// 载入名为 "native-lib" 的本地库
static {
System.loadLibrary("native-lib");
}

// 被本地方法调用的示例方法
public native String stringFromJNI();

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

// 调用本地方法并将返回结果显示在 TextView 上
TextView tv = findViewById(R.id.sample_text);
tv.setText(stringFromJNI());
}
}

C++ 代码：

#include <jni.h>
#include <string>

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_android_testapplication_MainActivity_stringFromJNI(JNIEnv *env, jobject /* this */) {
std::string hello = "Hello from C++";
return env->NewStringUTF(hello.c_str());
}

以上代码简单地在 Android Activity 上连接了一个名为 "native-lib" 的本地库，并在 Activity 的 onCreate 方法中调用本地方法 stringFromJNI，该方法返回一个字符串，并将结果显示在 TextView 上。具体的本地方法实现可以在 C++ 文件中进行。

根据上述电商app给出完整代码

很抱歉，由于篇幅限制，我无法在这里给出完整的代码。但是，我可以提供一些关键的代码片段，供您参考：

MainActivity.java:

public class MainActivity extends AppCompatActivity {
    private RecyclerView mRecyclerView;
    private ProductAdapter mAdapter;
    private List<Product> mProductList;
    private String mLoggedInUser;

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

        mRecyclerView = findViewById(R.id.recycler_view);
        mProductList = getProductList();
        mAdapter = new ProductAdapter(this, mProductList, new OnBuyClickListener() {
            @Override
            public void onBuyClick(Product product) {
                if (mLoggedInUser == null) {
                    Intent loginIntent = new Intent(MainActivity.this, LoginActivity.class);
                    startActivityForResult(loginIntent, REQUEST_LOGIN);
                } else {
                    addToCart(product);
                }
            }
        });
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));
        mRecyclerView.setAdapter(mAdapter);
    }

    private List<Product> getProductList() {
        // TODO: fetch product list from server
    }

    private void addToCart(Product product) {
        // TODO: add product to cart and show cart activity
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (requestCode == REQUEST_LOGIN && resultCode == RESULT_OK) {
            mLoggedInUser = data.getStringExtra(LoginActivity.EXTRA_USERNAME);
            Toast.makeText(this, "Logged in as " + mLoggedInUser, Toast.LENGTH_SHORT).show();
        }
    }
}

LoginActivity.java:

public class LoginActivity extends AppCompatActivity {
    public static final String EXTRA_USERNAME = "username";

    private static LoginActivity sInstance;

    private EditText mUsernameEditText;
    private EditText mPasswordEditText;
    private Button mLoginButton;

    public static LoginActivity getInstance() {
        if (sInstance == null) {
            sInstance = new LoginActivity();
        }
        return sInstance;
    }

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

        mUsernameEditText = findViewById(R.id.username_edit_text);
        mPasswordEditText = findViewById(R.id.password_edit_text);
        mLoginButton = findViewById(R.id.login_button);

        mLoginButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String username = mUsernameEditText.getText().toString();
                String password = mPasswordEditText.getText().toString();

                // TODO: validate username and password with server

                Intent resultIntent = new Intent();
                resultIntent.putExtra(EXTRA_USERNAME, username);
                setResult(RESULT_OK, resultIntent);
                finish();
            }
        });
    }
}

CartActivity.java:

public class CartActivity extends AppCompatActivity {
    public static final String EXTRA_USERNAME = "username";
    public static final String EXTRA_PRODUCT_NAME = "product_name";
    public static final String EXTRA_PRODUCT_PRICE = "product_price";

    private TextView mCartTextView;

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

        mCartTextView = findViewById(R.id.cart_text_view);

        Intent intent = getIntent();
        String username = intent.getStringExtra(EXTRA_USERNAME);
        String productName = intent.getStringExtra(EXTRA_PRODUCT_NAME);
        String productPrice = intent.getStringExtra(EXTRA_PRODUCT_PRICE);

        mCartTextView.setText("User: " + username + "\nProduct: " + productName + "\nPrice: " + productPrice);
    }
}

这些代码片段只是提供了电商App的基本框架，具体实现还需要根据您的需求进行设计和开发。希望能对您有所帮助。