空指針異常Android Java對已過濾項目

任何人都可以告訴我如何解決此問題嗎？我一直在試圖解決這個問題，但它一直提示我這個錯誤信息。空指針異常Android Java對已過濾項目

java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference at com.example.apple.qs.Activity.adapter.MessageListAdapter.getItemCount(MessageListAdapter.java:173)

，讓我錯誤的方法。

@Override 
    public int getItemCount() { 
     System.out.println(filtered_items); 
     if(filtered_items.size()==0 || filtered_items==null) { 
      return 1; 
     } 
     else return filtered_items.size(); 
    }

173線是

if(filtered_items.size()==0 || filtered_items==null) {

MessageListAdapter.java

import android.content.Context; 
import android.content.DialogInterface; 
import android.content.Intent; 
import android.graphics.Typeface; 
import android.support.design.widget.Snackbar; 
import android.support.v7.app.AlertDialog; 
import android.support.v7.widget.RecyclerView; 
import android.util.TypedValue; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.Filter; 
import android.widget.Filterable; 
import android.widget.ImageView; 
import android.widget.LinearLayout; 
import android.widget.TextView; 
import android.widget.Toast; 

import com.example.apple.qs.Activity.Activity.ActivityMain_txt; 
import com.example.apple.qs.Activity.Activity.ActivityMessageDetails_txt; 
import com.example.apple.qs.Activity.db.Constant; 
import com.example.apple.qs.Activity.db.model.Message; 
import com.example.apple.qs.Activity.db.Store.MessageStore; 
import com.example.apple.qs.Activity.widget.CircleTransform; 
import com.squareup.picasso.Picasso; 
import com.example.apple.qs.Activity.R; 
import java.util.ArrayList; 
import java.util.List; 

public class MessageListAdapter extends RecyclerView.Adapter<MessageListAdapter.ViewHolder> implements Filterable { 

    private final int mBackground; 

    private List<Message> original_items = new ArrayList<>(); 
    private List<Message> filtered_items = new ArrayList<>(); 
    private ItemFilter mFilter = new ItemFilter(); 

    private final TypedValue mTypedValue = new TypedValue(); 

    private Context ctx; 
    private ActivityMain_txt act; 

    public class ViewHolder extends RecyclerView.ViewHolder { 
     // each data item is just a string in this case 
     public TextView title; 
     public TextView content; 
     public TextView time; 
     public ImageView image; 
     public LinearLayout lyt_parent; 

     public ViewHolder(View v) { 
      super(v); 
      title = (TextView) v.findViewById(R.id.title); 
      content = (TextView) v.findViewById(R.id.content); 
      time = (TextView) v.findViewById(R.id.time); 
      image = (ImageView) v.findViewById(R.id.image); 
      lyt_parent = (LinearLayout) v.findViewById(R.id.lyt_parent); 
     } 

    } 

    public Filter getFilter() { 
     return mFilter; 
    } 

    // Provide a suitable constructor (depends on the kind of dataset) 
    public MessageListAdapter(ActivityMain_txt act, Context ctx, List<Message> items) { 
     this.ctx = ctx; 
     this.act = act; 
     original_items = items; 
     filtered_items = items; 
     ctx.getTheme().resolveAttribute(R.attr.selectableItemBackground, mTypedValue, true); 
     mBackground = mTypedValue.resourceId; 
    } 

    @Override 
    public MessageListAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { 
     // create a new view 
     View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.row_message_txt, parent, false); 
     v.setBackgroundResource(mBackground); 
     // set the view's size, margins, paddings and layout parameters 
     ViewHolder vh = new ViewHolder(v); 
     return vh; 
    } 

    // Replace the contents of a view (invoked by the layout manager) 
    @Override 
    public void onBindViewHolder(ViewHolder holder, int position) { 
     try { 
      final Message c = filtered_items.get(position); 
      holder.title.setText(c.contact.getNameOrNumber()); 

      holder.time.setText(Constant.formatTime(c.date)); 
      holder.content.setText(c.snippet); 
      if (c.contact.photoUri.equals("")) { 
       holder.image.setImageResource(R.drawable.unknown_avatar); 
      } else { 
       Picasso.with(ctx).load(c.contact.photoUri) 
         .resize(100, 100) 
         .transform(new CircleTransform()) 
         .into(holder.image); 
      } 
      if (!c.read) { 
       holder.title.setTypeface(null, Typeface.BOLD); 
       holder.time.setTypeface(null, Typeface.BOLD); 
       holder.content.setTypeface(null, Typeface.BOLD); 
      } else { 
       holder.title.setTypeface(null, Typeface.NORMAL); 
       holder.time.setTypeface(null, Typeface.NORMAL); 
       holder.content.setTypeface(null, Typeface.NORMAL); 
      } 

      // view detail message conversation 
      holder.lyt_parent.setOnClickListener(new View.OnClickListener() { 
       @Override 
       public void onClick(View view) { 
        act.messageStore.markAsRead(c.threadId); 
        try { 
         act.f_message.mAdapter.notifyDataSetChanged(); 
        }catch (Exception e){ 

        } 
        Intent intent = new Intent(ctx, ActivityMessageDetails_txt.class); 
        intent.putExtra("obj_conv", c); 
        ctx.startActivity(intent); 
       } 
      }); 

      holder.lyt_parent.setOnLongClickListener(new View.OnLongClickListener() { 
       @Override 
       public boolean onLongClick(View view) { 
        dialogDeleteMessageConfirm(view, c); 
        return false; 
       } 
      }); 
     }catch (Exception e){ 
      Toast.makeText(ctx, "Ups.., Something error", Toast.LENGTH_SHORT).show(); 
     } 
    } 


    private void dialogDeleteMessageConfirm(final View view, final Message c) { 
     AlertDialog.Builder builder = new AlertDialog.Builder(ctx); 
     builder.setTitle("Delete Confirmation"); 
     builder.setMessage("All message from : " + c.contact.getNameOrNumber() + " will be deleted?"); 
     builder.setPositiveButton("Delete", new DialogInterface.OnClickListener() { 
      @Override 
      public void onClick(DialogInterface dialogInterface, int i) { 
       boolean result = MessageStore.deleteMessage(ctx, c.threadId + ""); 
       if (result) { 
        try { 
         Snackbar.make(view, "Delete success", Snackbar.LENGTH_SHORT).show(); 
         ActivityMain_txt.messageList.remove(c.threadId); 
         ActivityMain_txt.f_message.mAdapter.notifyDataSetChanged(); 
        } catch (Exception e) { 
        } 
       } 
      } 
     }); 
     builder.setNegativeButton("Cancel", null); 
     builder.show(); 
    } 

    // Return the size of your dataset (invoked by the layout manager) 
    @Override 
    public int getItemCount() { 
     System.out.println(filtered_items); 
     if(filtered_items.size()==0 || filtered_items==null) { 
      return 1; 
     } 
     else return filtered_items.size(); 
    } 

    @Override 
    public long getItemId(int position) { 
     return filtered_items.get(position).threadId; 
    } 

    public int getItemPosition(long threadId){ 
     int pos = -1; 
     for (int i = 0; i < filtered_items.size(); i++) { 
      if(filtered_items.get(i).threadId == threadId){ 
       return i; 
      } 
     } 
     return pos; 
    } 

    private class ItemFilter extends Filter { 
     @Override 
     protected FilterResults performFiltering(CharSequence constraint) { 
      String query = constraint.toString().toLowerCase(); 

      FilterResults results = new FilterResults(); 
      final List<Message> list = original_items; 
      final List<Message> result_list = new ArrayList<>(list.size()); 

      for (int i = 0; i < list.size(); i++) { 
       String str_title = list.get(i).contact.getNameOrNumber(); 
       if (str_title.toLowerCase().contains(query)) { 
        result_list.add(list.get(i)); 
       } 
      } 

      results.values = result_list; 
      results.count = result_list.size(); 

      return results; 
     } 

     @SuppressWarnings("unchecked") 
     @Override 
     protected void publishResults(CharSequence constraint, FilterResults results) { 
      filtered_items = (List<Message>) results.values; 
      notifyDataSetChanged(); 
     } 

    } 
}

來源

2016-07-26 Apple

問題本身就是在這一行

if(filtered_items.size()==0 || filtered_items==null) {

你必須改變表達式的順序：

if(filtered_items==null || filtered_items.size()==0) {

而且讀JLS 15.24:

有條件-或操作員||運營商...評估其右手操作數只有在其左側操作數的值是假

來源

2016-07-26 07:14:45 Andremoniy

從改變你的聲明

if(filtered_items.size()==0 || filtered_items==null)

到：

if(filtered_items==null || filtered_items.size()==0)

爲有順序檢查條件。如果第一個是真的，那麼第二個將不會被檢查。

來源

2016-07-26 07:15:22

與

public int getItemCount() { 
    System.out.println(filtered_items); 
    if(filtered_items!=null || filtered_items.size()>0){ 
     return filtered_items.size(); 
    }else{ 
     return 1; 
    } 
}

來源

2016-07-26 07:20:22

更改您的語句來替換getItemCount()從

if(filtered_items.size()==0 || filtered_items==null)

到：

if(filtered_items !=null && filtered_items.size()==0)

，因爲這將首先是否在filtered_items值，如果不爲null檢查。如果值不爲空，那麼只有它會調用size（）方法。

或

你可以修改代碼爲：

if(filtered_items != null) { 
    switch(filtered_items.size()) { 
     case 0: 
      return 1; 
      break; 

     default: 
      return filtered_items.size(); 
      break; 
    } 
}

來源

2016-07-26 07:30:30

在這兩種情況下'和0''default'是'迴歸'聲明，所以我認爲你也不需要'break'（它永遠不會到達'break'）。 –

@Override 
public int getItemCount() { 
    int size = 1; 
    System.out.println(filtered_items); 
    if (filtered_items != null) { 
     if (filtered_items.size() > 0) { 
      size = filtered_items.size(); 
     } 
    } 
    return size; 
}

試試這個

來源

2016-07-26 08:45:48

空指針異常Android Java對已過濾項目

回答

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