dbhelper类增删改查

DBHelper类是一个用于操作数据库的工具类，通常包括增删改查（CRUD）等数据库操作方法。下面是一个DBHelper类的增删改查示例：

public class DBHelper extends SQLiteOpenHelper {

    // 数据库名称
    private static final String DB_NAME = "mydb.db";
    // 数据库版本号
    private static final int DB_VERSION = 1;
    // 表名
    private static final String TABLE_NAME = "mytable";
    // 列名
    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, DB_NAME, null, DB_VERSION);
    }

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

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

    // 添加数据
    public void insert(String name, int age) {
        SQLiteDatabase db = getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(COLUMN_NAME, name);
        values.put(COLUMN_AGE, age);
        db.insert(TABLE_NAME, null, values);
        db.close();
    }

    // 删除数据
    public void delete(int id) {
        SQLiteDatabase db = getWritableDatabase();
        String whereClause = COLUMN_ID + "=?";
        String[] whereArgs = new String[]{String.valueOf(id)};
        db.delete(TABLE_NAME, whereClause, whereArgs);
        db.close();
    }

    // 修改数据
    public void update(int id, String name, int age) {
        SQLiteDatabase db = getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(COLUMN_NAME, name);
        values.put(COLUMN_AGE, age);
        String whereClause = COLUMN_ID + "=?";
        String[] whereArgs = new String[]{String.valueOf(id)};
        db.update(TABLE_NAME, values, whereClause, whereArgs);
        db.close();
    }

    // 查询数据
    public List<Person> query() {
        SQLiteDatabase db = getReadableDatabase();
        List<Person> persons = new ArrayList<>();
        Cursor cursor = db.query(TABLE_NAME, null, null, null, null, null, null);
        while (cursor.moveToNext()) {
            int id = cursor.getInt(cursor.getColumnIndex(COLUMN_ID));
            String name = cursor.getString(cursor.getColumnIndex(COLUMN_NAME));
            int age = cursor.getInt(cursor.getColumnIndex(COLUMN_AGE));
            Person person = new Person(id, name, age);
            persons.add(person);
        }
        cursor.close();
        db.close();
        return persons;
    }
}

上述代码实现了一个名为`mytable`的表的增删改查操作，其中`Person`是一个包含`id`、`name`和`age`属性的JavaBean类。你可以根据需要修改表名、列名和JavaBean类的属性。