Android app 连接数据库的代码

连接数据库的代码需要在 Android 应用程序的后端服务器上运行。常用的数据库包括 MySQL, SQLite, PostgreSQL 等。以下是一个使用 SQLite 数据库的示例代码，供参考：

1. 添加依赖库

在 app 的 build.gradle 文件中添加以下依赖库：

dependencies {
    implementation 'com.android.support:appcompat-v7:26.1.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    implementation 'com.squareup.retrofit2:retrofit:2.3.0'
    implementation 'com.squareup.retrofit2:converter-gson:2.3.0'
    implementation 'com.squareup.okhttp3:logging-interceptor:3.8.0'
    implementation 'com.squareup.okhttp3:okhttp:3.8.0'
}

2. 创建数据库和表

public class DBHelper extends SQLiteOpenHelper {
    private static final String DATABASE_NAME = "mydatabase.db";
    private static final int DATABASE_VERSION = 1;
    private static final String TABLE_NAME = "mytable";
    private static final String COLUMN_ID = "id";
    private static final String COLUMN_NAME = "name";
    private static final String COLUMN_AGE = "age";

    public DBHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String query = "CREATE TABLE " + TABLE_NAME + " (" +
                COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                COLUMN_NAME + " TEXT, " +
                COLUMN_AGE + " INTEGER)";
        db.execSQL(query);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
        onCreate(db);
    }
}

3. 连接数据库

public class DBManager {
    private DBHelper dbHelper;
    private Context context;
    private SQLiteDatabase database;

    public DBManager(Context c) {
        context = c;
    }

    public DBManager open() throws SQLException {
        dbHelper = new DBHelper(context);
        database = dbHelper.getWritableDatabase();
        return this;
    }

    public void close() {
        dbHelper.close();
    }

    public void insert(String name, int age) {
        ContentValues contentValue = new ContentValues();
        contentValue.put(DBHelper.COLUMN_NAME, name);
        contentValue.put(DBHelper.COLUMN_AGE, age);
        database.insert(DBHelper.TABLE_NAME, null, contentValue);
    }

    public Cursor fetch() {
        String[] columns = new String[] { DBHelper.COLUMN_ID, DBHelper.COLUMN_NAME, DBHelper.COLUMN_AGE };
        Cursor cursor = database.query(DBHelper.TABLE_NAME, columns, null, null, null, null, null);
        if (cursor != null) {
            cursor.moveToFirst();
        }
        return cursor;
    }
}

4. 在 Activity 中使用

public class MainActivity extends AppCompatActivity {
    private DBManager dbManager;

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

        dbManager = new DBManager(this);
        dbManager.open();
        dbManager.insert("John", 30);

        Cursor cursor = dbManager.fetch();
        if (cursor.moveToFirst()) {
            do {
                String name = cursor.getString(cursor.getColumnIndex(DBHelper.COLUMN_NAME));
                int age = cursor.getInt(cursor.getColumnIndex(DBHelper.COLUMN_AGE));
                Log.d("TAG", "Name: " + name + ", Age: " + age);
            } while (cursor.moveToNext());
        }

        dbManager.close();
    }
}

注意，以上代码仅供参考，需要根据自己的实际需求进行修改和适配。同时，在实际开发中，为了保证数据安全和隐私，需要对数据库连接进行加密和权限控制。