如何在 Android Studio 中同时从 recyclerview 和数据库中删除数据？ [关闭]

Posted 2023-03-22

【中文标题】如何在 Android Studio 中同时从 recyclerview 和数据库中删除数据？ [关闭]

【英文标题】：How can I delete data from recyclerview and database at the same time in Android Studio? [closed]

【发布时间】：2022-01-17 01:57:03

【问题描述】：

我有一个 recyclerview，我在 recyclerview 中显示城市名称，当我在 recyclerview 中长按城市名称时，我想删除我点击的城市。

除第一个和最后一个城市外，删除成功。当我想从我的 recyclerview 中删除最后一个城市和第一个城市时，它正在删除，但它没有从我的 sql 数据库中删除，它显示我的 toast 消息“出了点问题”。

如何从数据库中删除我想删除的城市？

“空对象引用上的Adapter.notifyDataSetChanged()'”

我的适配器类

public class Adapter extends RecyclerView.Adapter<Adapter.MyViewHolder>  

ArrayList<City> arrayList;
Context context;
SQLiteDatabase db ;
Cursor cursor;
int id;


public Adapter(ArrayList<City> arrayList ,Context context )
    this.arrayList = arrayList;
    this.context = context;

    db = context.openOrCreateDatabase("City",MODE_PRIVATE,null);

    Intent intent = ((Activity) context).getIntent();
    id = intent.getIntExtra("citId", 0);


@NonNull
@Override
public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) 

    RecyclerviewRowBinding recyclerviewRowBinding = 
    RecyclerviewRowBinding.inflate(
    LayoutInflater.from(parent.getContext()),
    parent,
    false);

    return new MyViewHolder(recyclerviewRowBinding);



@Override
public void onBindViewHolder(@NonNull Adapter.MyViewHolder holder, int position) 

    holder.binding.MytxtCities.setText(arrayList.get(position).cityName);

       /*when i long click it does the deletion here*/

    holder.itemView.setOnLongClickListener(new View.OnLongClickListener() 
        @Override
        public boolean onLongClick(View v) 
            AlertDialog.Builder builder = new AlertDialog.Builder(context);
            builder.setIcon(R.drawable.warningicon);
            builder.setMessage("Are you sure that you want to delete "+arrayList.get(position).cityName);
            builder.setPositiveButton("Ok", new DialogInterface.OnClickListener() 
                @Override
                public void onClick(DialogInterface dialog, int which) 
                    arrayList.remove(position);
                    notifyItemRemoved(position);
                    notifyItemRangeChanged(position,arrayList.size());


                    cursor = db.rawQuery("select * from city where id=?",new String[]String.valueOf(id));
                    Result(cursor);



                
            ).setNegativeButton("Cancel", new DialogInterface.OnClickListener() 
                @Override
                public void onClick(DialogInterface dialog, int which) 

                
            ).show();


            return true;
        


    );


    holder.itemView.setOnClickListener(v -> 
        Intent intent = new Intent(holder.itemView.getContext(),MainActivity.class);
        intent.putExtra("citId",arrayList.get(position).id);
        holder.itemView.getContext().startActivity(intent);
    );



private void Result(Cursor cursor)

    if(cursor.getCount() > 0)

        db.delete("City","id=?",new String[]String.valueOf(id));
        notifyDataSetChanged();
    
    else
        Toast.makeText(context,"something went wrong !",Toast.LENGTH_LONG).show();
    





@Override
public int getItemCount() 
    return arrayList.size();




public class MyViewHolder extends RecyclerView.ViewHolder 
    TextView Mytxt_cities;
    private RecyclerviewRowBinding binding;

    public MyViewHolder(@NonNull RecyclerviewRowBinding binding) 
        super(binding.getRoot());
        this.binding = binding;
        Mytxt_cities = itemView.findViewById(R.id.Mytxt_cities);



    


 

我的 Recyclerview 类是城市类

public class cities extends AppCompatActivity 

RecyclerView recyclerView ;
ArrayList<City> cityArrayList;
Adapter cityadapter;
ImageView cities_back_icon;



public void init()

    cities_back_icon = findViewById(R.id.Id_cities_back_icon);
    cities_back_icon_click_register();

    cityArrayList = new ArrayList<>();
    recyclerView = findViewById(R.id.recyclerview_id);



    SQLGet_Data();




@Override
protected void onCreate(Bundle savedInstanceState) 
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_cities);
    init();




private void cities_back_icon_click_register()

    cities_back_icon.setOnClickListener(new View.OnClickListener() 
        @Override
        public void onClick(View v) 
            Intent intent = new Intent(cities.this, MainActivity.class);
            startActivity(intent);
        
    );


private void SQLGet_Data()
    try 

        SQLiteDatabase sqLiteDatabase = this.openOrCreateDatabase("City",MODE_PRIVATE,null);
        Cursor cursor = sqLiteDatabase.rawQuery("SELECT*FROM city",null);
        int idIx = cursor.getColumnIndex("id");
        int nameIx = cursor.getColumnIndex("cityname");

        while(cursor.moveToNext())
            String cityname = cursor.getString(nameIx);
            int id = cursor.getInt(idIx);
            City city = new City(cityname,id);
            cityArrayList.add(city);
        
        cityadapter.notifyDataSetChanged();//  <--- There is an error in this code
        cursor.close();
    

    catch (Exception e )
       e.printStackTrace();
    

    /*---------------------- set recyclerview-----------------------------*/

    cityadapter = new Adapter(cityArrayList,this);
    recyclerView.setAdapter(cityadapter);
    recyclerView.setLayoutManager(new LinearLayoutManager(cities.this));



/*------- We drew a line between the data in the recyclerview ------*/

    DividerItemDecoration dividerItemDecoration = new 
    DividerItemDecoration(recyclerView.getContext(),
    DividerItemDecoration.VERTICAL);
    Drawable drawable = ContextCompat.getDrawable(getApplicationContext(), 
    R.drawable.custom_divider);
    dividerItemDecoration.setDrawable(drawable);
    recyclerView.addItemDecoration(dividerItemDecoration);

 


 

【参考方案1】：

您的问题是错误点的 cityAdapter 尚未实例化，因此为空。你稍后实例化它：-

cityadapter = new Adapter(cityArrayList,this);

您可以在例如之前移动实例化

private void SQLGet_Data()
    cityadapter = new Adapter(cityArrayList,this);
    try  
        ....