dbhelper类增删改查

DBHelper类通常用于数据库的操作，包括增删改查等。以下是一个简单的DBHelper类的示例代码，可以实现基本的增删改查操作。

public class DBHelper extends SQLiteOpenHelper {
    private static final String DATABASE_NAME = "mydb.db";
    private static final int DATABASE_VERSION = 1;

    public static final String TABLE_NAME = "mytable";
    public static final String COL_ID = "id";
    public static final String COL_NAME = "name";
    public static final String COL_AGE = "age";

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

    @Override
    public void onCreate(SQLiteDatabase db) {
        String sql = "CREATE TABLE " + TABLE_NAME +
                "(" + COL_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                COL_NAME + " TEXT, " +
                COL_AGE + " INTEGER)";
        db.execSQL(sql);
    }

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

    public void insert(String name, int age) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(COL_NAME, name);
        values.put(COL_AGE, age);
        db.insert(TABLE_NAME, null, values);
        db.close();
    }

    public void delete(int id) {
        SQLiteDatabase db = this.getWritableDatabase();
        String whereClause = COL_ID + " = ?";
        String[] whereArgs = {String.valueOf(id)};
        db.delete(TABLE_NAME, whereClause, whereArgs);
        db.close();
    }

    public void update(int id, String name, int age) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(COL_NAME, name);
        values.put(COL_AGE, age);
        String whereClause = COL_ID + " = ?";
        String[] whereArgs = {String.valueOf(id)};
        db.update(TABLE_NAME, values, whereClause, whereArgs);
        db.close();
    }

    public List<Person> query() {
        List<Person> list = new ArrayList<>();
        SQLiteDatabase db = this.getReadableDatabase();
        String[] columns = {COL_ID, COL_NAME, COL_AGE};
        Cursor cursor = db.query(TABLE_NAME, columns, null, null, null, null, null);
        while (cursor.moveToNext()) {
            int id = cursor.getInt(cursor.getColumnIndex(COL_ID));
            String name = cursor.getString(cursor.getColumnIndex(COL_NAME));
            int age = cursor.getInt(cursor.getColumnIndex(COL_AGE));
            Person person = new Person(id, name, age);
            list.add(person);
        }
        cursor.close();
        db.close();
        return list;
    }
}

其中，Person是一个简单的Java类，用于保存查询结果。

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

    public Person(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;
    }
}

使用DBHelper类进行增删改查操作的示例代码：

DBHelper dbHelper = new DBHelper(context);

// 插入数据
dbHelper.insert("张三", 18);

// 删除数据
dbHelper.delete(1);

// 更新数据
dbHelper.update(2, "李四", 20);

// 查询数据
List<Person> list = dbHelper.query();
for (Person person : list) {
    Log.d("DBHelper", person.getId() + " " + person.getName() + " " + person.getAge());
}