嘗試刪除sqlite數據庫中的項目時出現空指針異常

Question

確實有人知道如何解決此問題嗎？嗯，我想要的是在我的SQLite數據庫和我在recyclerview項目，但我相處刪除行是此錯誤

java.lang.NullPointerException: Attempt to invoke virtual method 'int petguide.com.petguide101.database_item.reg_Adapter.getItemId()' on a null object reference 
       at petguide.com.petguide101.database_item.reg_ViewHolder$1.onClick(reg_ViewHolder.java:35) 
       at android.view.View.performClick(View.java:5198) 
       at android.view.View$PerformClick.run(View.java:21147) 
       at android.os.Handler.handleCallback(Handler.java:739) 
       at android.os.Handler.dispatchMessage(Handler.java:95) 
       at android.os.Looper.loop(Looper.java:148) 
       at android.app.ActivityThread.main(ActivityThread.java:5417) 
       at java.lang.reflect.Method.invoke(Native Method) 
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

這裏是我的代碼爲我的DB

public class Register_pet_database extends SQLiteOpenHelper { 
    public static final String DATABASE_NAME="Registered_pet.db"; 
    public static final String TABLE_NAME="Pet_Registered"; 
    public static final String ROWID="ID"; 
    public static final String COl_1="Pet_name"; 
    public static final String COl_2="Pet_type"; 
    public static final String COl_3="Pet_breed"; 
    public SQLiteDatabase myDB=getReadableDatabase(); 
    public Register_pet_database(Context context) { 
     super(context, DATABASE_NAME, null, 1); 

    } 

    @Override 
    public void onCreate(SQLiteDatabase db) { 
    db.execSQL("create table "+TABLE_NAME+"("+ROWID+"INTEGER PRIMARY KEY AUTOINCREMENT, Pet_name TEXT, Pet_type TEXT, Pet_breed TEXT)"); 
    } 

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

    public boolean insertDataToPetRegister(String pet_name, String pet_type, String pet_breed){ 
     SQLiteDatabase db=getWritableDatabase(); 
     ContentValues contentValues= new ContentValues(); 
     contentValues.put(COl_1,pet_name); 
     contentValues.put(COl_2,pet_type); 
     contentValues.put(COl_3,pet_breed); 

     long result= db.insert(TABLE_NAME,null,contentValues); 
     if(result == -1) 
      return false; 
      else 
      return true; 
    } 
    public Integer deleteData(String id){ 
     SQLiteDatabase db=this.getWritableDatabase(); 
     return db.delete(TABLE_NAME,"ID=?", new String[]{id}); 
     } 

    public Cursor quearyData(String query){ 
     return myDB.rawQuery(query,null); 
    } 
    public int getId(){ 
     String query = "SELECT id FROM " + TABLE_NAME+ " WHERE ID=?" + ROWID + "'"; 
     SQLiteDatabase db = this.getReadableDatabase(); 
     Cursor res = db.rawQuery(query, null); 
     int id=-1; 
     if(res!=null&&res.moveToFirst()) 
     id=res.getInt(res.getColumnIndex("ID")); 
     return id; 
    } 
} 

我爲我的適配器

public class reg_Adapter extends RecyclerView.Adapter<reg_ViewHolder>{ 
    Activity activity; 
    List<db_getItem> items; 
    ArrayList<db_getItem> arrayList=new ArrayList<>(); 
    db_getItem dbitem; 

    public reg_Adapter(Activity activity, List<db_getItem> items){ 
     this.activity=activity; 
     this.items=items; 
    } 
    public reg_ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { 
     View view= LayoutInflater.from(parent.getContext()).inflate(R.layout.reg_pet_recycler,parent,false); 
     return new reg_ViewHolder(view); 
    } 
    public void delete(int position) { //removes the row 
     arrayList.remove(position); 
     notifyItemRemoved(position); 
    } 
    @Override 
    public void onBindViewHolder(reg_ViewHolder holder, int position) { 
     holder.pet_name.setText(items.get(position).getPet_name()); 
     holder.pet_type.setText(items.get(position).getPet_type()); 
     holder.pet_breed.setText(items.get(position).getPet_breed()); 

    } 

    @Override 
    public int getItemCount() { 
     return items.size(); 
    } 
    public int getItemId() { 
     return dbitem.getPetID(); 
    } 
} 

我的代碼代碼爲我viewHolder

public class reg_ViewHolder extends RecyclerView.ViewHolder { 
    Register_pet_database db; 
    TextView pet_name; 
    TextView pet_type; 
    TextView pet_breed; 
    Button btndel; 
    Register_pet_database register_pet_database; 
    reg_Adapter reg_adapter; 
    DBAdapter dbAdapter; 

    public reg_ViewHolder(final View itemView) { 
     super(itemView); 
     pet_name = (TextView) itemView.findViewById(R.id.pet_reg_name); 
     pet_type = (TextView) itemView.findViewById(R.id.pet_reg_type); 
     pet_breed = (TextView) itemView.findViewById(R.id.pet_reg_breed); 
     btndel =(Button) itemView.findViewById(R.id.btndel); 
     btndel.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       int id=reg_adapter.getItemId(); 
       Integer delete=register_pet_database.deleteData(String.valueOf(id)); 
       if(delete>0) 
        Toast.makeText(itemView.getContext(),"Deleted", Toast.LENGTH_LONG).show(); 
       else 
        Toast.makeText(itemView.getContext(),"Not Deleted", Toast.LENGTH_LONG).show(); 

      } 
     }); 
    } 
} 

能告訴我如何解決這個問題嗎？

Answer 1

你需要解決這個問題。

public void onClick(View v) { 
      int id=reg_adapter.getItemId(); // your adapter is null 

一個可能的選項，通過ID在該ViewHolder

public class reg_ViewHolder extends RecyclerView.ViewHolder { 
    int id; // See here 
    TextView pet_name; 
    TextView pet_type; 
    TextView pet_breed; 

的構造函數，並把它

@Override 
public void onBindViewHolder(reg_ViewHolder holder, int position) { 
    holder.id = getItemId(); 
    holder.pet_name.setText(items.get(position).getPet_name()); 
    holder.pet_type.setText(items.get(position).getPet_type()); 
    holder.pet_breed.setText(items.get(position).getPet_breed()); 

} 

注意：您有很多交叉類和變量在這裏的。 .. ViewHolder應該只能容納視圖，不是數據庫，不是適配器（特別是不是兩個他們

）也db_Item爲空太...

而且你也不需要兩個列表...

List<db_getItem> items; 
ArrayList<db_getItem> arrayList=new ArrayList<>(); 

而Java不姓用下劃線類或變量，以及所有類應該是CapitalizedNames ...也許在深入Android之前學習更多的Java？