数据库dao中

private Dbhelpher myHelper;private SQLiteDatabase db;//实现一个方法传递一个布局public Dao(Context context) {    myHelper = new Dbhelpher(context);}//添加历史记录public int   add(String json){    db = myHelper.getWritableDatabase();    db.execSQL("insert into jilu values(null,?)",new String[]{json});    //成功返回1    return 1;}//查询数据库public List<String> sel(){    List<String> list=new ArrayList<>();    db = myHelper.getWritableDatabase();    Cursor cursor = db.rawQuery("select * from jilu", null);    while (cursor.moveToNext()){        //String id = cursor.getString(cursor.getColumnIndex("id"));        String json = cursor.getString(cursor.getColumnIndex("json"));        list.add(json);    }    return list;}//清空历史记录public void del(){    db = myHelper.getWritableDatabase();    db.execSQL("delete from jilu");}