android studio SQLite获取数据

要从SQLite数据库中获取数据，需要执行以下步骤：

1. 创建SQLiteOpenHelper类，该类将帮助我们打开和创建数据库。

public class DatabaseHelper extends SQLiteOpenHelper {

    private static final String DATABASE_NAME = "mydb.db";
    private static final int DATABASE_VERSION = 1;

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

    @Override
    public void onCreate(SQLiteDatabase db) {
        // 创建表格
        db.execSQL("CREATE TABLE users (_id INTEGER PRIMARY KEY AUTOINCREMENT, name TEXT, age INTEGER)");
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // 更新表格
        db.execSQL("DROP TABLE IF EXISTS users");
        onCreate(db);
    }
}

2. 创建数据模型类，该类将表示表格中的一行。

public class User {
    private int id;
    private String name;
    private int age;

    public User(int id, String name, int age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }
}

3. 创建数据访问类，该类将提供从数据库中获取数据的方法。

public class UserDAO {
    private SQLiteDatabase db;

    public UserDAO(Context context) {
        db = new DatabaseHelper(context).getWritableDatabase();
    }

    public List<User> getUsers() {
        List<User> users = new ArrayList<>();
        Cursor cursor = db.rawQuery("SELECT * FROM users", null);
        if (cursor.moveToFirst()) {
            do {
                int id = cursor.getInt(cursor.getColumnIndex("_id"));
                String name = cursor.getString(cursor.getColumnIndex("name"));
                int age = cursor.getInt(cursor.getColumnIndex("age"));
                users.add(new User(id, name, age));
            } while (cursor.moveToNext());
        }
        cursor.close();
        return users;
    }
}

4. 在Activity中使用数据访问类获取数据。

public class MainActivity extends AppCompatActivity {
    private UserDAO userDAO;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        userDAO = new UserDAO(this);
        List<User> users = userDAO.getUsers();
        // 使用获取到的数据
    }
}

这样就可以从SQLite数据库中获取数据了。