java.lang.NullPointerException: 尝试在空对象引用上调用接口方法'java.util.List.iterator()'

Question

我目前正在开发一款从Parse服务器获取数据的Android应用程序。我想使用SearchView过滤我的RecyclerView。但是在搜索时，它没有显示任何内容。这一行代码*for (ParseObject parseObject : mRooms){ *给我报错。请帮助我编辑代码以解决此问题。

RoomCardRecyclerViewAdapter

private List<ParseObject> mRooms = new ArrayList<>();
private ArrayList<ParseObject> filterlist;
private ParseObject room;
private String mSection;
private Context context;

public RoomCardRecyclerViewAdapter(){
    super(DIFF_CALLBACK);
}
public static final DiffUtil.ItemCallback<ParseObject>  DIFF_CALLBACK = new 
DiffUtil.ItemCallback<ParseObject>() {
    @Override
    public boolean areItemsTheSame(@NonNull ParseObject oldItem, @NonNull ParseObject newItem) {
        return oldItem.getObjectId() == newItem.getObjectId();
    }

    @Override
    public boolean areContentsTheSame(@NonNull ParseObject oldItem, @NonNull ParseObject newItem) {
        return (oldItem.getUpdatedAt().equals(newItem.getUpdatedAt()) && 
oldItem.getCreatedAt().equals(newItem.getCreatedAt()));
    }
};


public RoomCardRecyclerViewAdapter(String section) {
    this();

    this.mSection = section;
}
public RoomCardRecyclerViewAdapter(Context context, List<ParseObject>arrayList) {
    this();

    this.context = context;
    mRooms = arrayList;
    filterlist = (ArrayList<ParseObject>) arrayList;
}


public class RoomViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener{
    protected ImageView mRoomImage;
    protected TextView mRoomPrice;
    protected TextView mInclusiveOrNot;
    protected TextView mPropertyType;
    protected TextView mNumOfBeds;
    protected TextView mNumOfBaths;
    protected TextView mRoomLocation;

    private Context context;

    public RoomViewHolder(Context context, View itemView) {
        super(itemView);
        mRoomImage = itemView.findViewById(R.id.room_image);
        mRoomPrice = itemView.findViewById(R.id.price_label);
        mInclusiveOrNot = itemView.findViewById(R.id.incl_excl_label);
        mPropertyType = itemView.findViewById(R.id.propertyType_label);
        mNumOfBeds = itemView.findViewById(R.id.num_beds_label);
        mNumOfBaths = itemView.findViewById(R.id.details_num_baths_label);
        mRoomLocation = itemView.findViewById(R.id.location_label);
        this.context = context;
        //set onclick listener
        itemView.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        Log.i("Click event: ", "My room has been clicked.");
        int pos = getAdapterPosition();
        Intent intent;
        ParseObject room = getCurrentList().get(pos);

        //create the ParseObject proxy
        ParseProxyObject roomProxy = new ParseProxyObject(room);
        Toast.makeText(context, room.getString("roomSuburb"), Toast.LENGTH_LONG).show();
        //fork to corresponding activity
        if(mSection != null) {
            Log.i("mSection text: ", "mSection text is: " + mSection);
            if (mSection.equals("My Rooms")) {
                //start my rooms detail activity
                Log.i("My room: ", "Room selected " + roomProxy.getObjectId());
                intent = new Intent(context, MyRoomDetailActivity.class);
                //add the room to the intent
                intent.putExtra("currentSelectedRoomObject", room);
                Log.i("Selected room", "Put Extra, " + room);
                intent.putExtra("roomObject", roomProxy);
                context.startActivity(intent);
            }
        }else {
            Log.i("My room:", "RoomDetailActivity loaded for MyRoomDetail Activity instead");
            intent = new Intent(context, RoomDetailActivity.class);
            //add the proxy to the intent
            intent.putExtra("roomObject", roomProxy);
            context.startActivity(intent);
        }

    }
}

@Override
public RoomViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    //inflating the viewholder with the appropriate views
    View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.room_cardview, parent, 
false);

    return new RoomViewHolder(parent.getContext(), view);
}

@Override
public void onBindViewHolder(@NonNull RoomViewHolder holder, int position) {
    room = getItem(position);
    holder.mRoomLocation.setText(room.getString("roomSuburb"));
    holder.mRoomPrice.setText(Integer.toString(room.getInt("roomMonthlyRent")));
    holder.mInclusiveOrNot.setText(room.getString("roomRentInclusiveOfBills"));
    holder.mPropertyType.setText(room.getString("roomPropertyType"));
    holder.mNumOfBeds.setText(Integer.toString(room.getInt("roomBedrooms")));
    holder.mNumOfBaths.setText(Integer.toString(room.getInt("roomBathrooms")));

   

@Override
public Filter getFilter(){
  return new Filter() {
      @Override
      protected FilterResults performFiltering(CharSequence charSequence) {

String charString = charSequence.toString();
if (charString.isEmpty()){
room = (ParseObject) mRooms;

}else {
List<ParseObject> filteredList = new ArrayList<>();
for (ParseObject parseObject : mRooms){
    if (parseObject.getString("roomSuburb").toLowerCase().contains(charString.toLowerCase())){
        filteredList.add(parseObject);
    }
}
room = (ParseObject) filteredList;
}
FilterResults filterResults = new FilterResults();
filterResults.values = room;

return filterResults;

      }

      @Override
      protected void publishResults(CharSequence charSequence, FilterResults filterResults) {
          room = (ParseObject) filterResults.values;
          notifyDataSetChanged();
      }
  };

}

主活动

mHomeSectionsPagerAdapter = new 
HomeSectionsPagerAdapter(getSupportFragmentManager());
    roomCardRecyclerViewAdapter = new RoomCardRecyclerViewAdapter(this, 
mRooms);
    mRooms = new ArrayList<>();
    // Set up the ViewPager with the sections adapter.
    mViewPager = findViewById(R.id.container);
    mViewPager.setAdapter(mHomeSectionsPagerAdapter);

    
     
    

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    final ParseQuery<ParseUser> query = ParseUser.getQuery();

    
    //get the search view and set the searchable configuration
    SearchManager searchManager = (SearchManager) 
getSystemService(Context.SEARCH_SERVICE);
   
    MenuItem item = menu.findItem(R.id.search);
    SearchView searchView = (SearchView) MenuItemCompat.getActionView(item);

    //assumes the current activity is the searchable activity
    

searchView.setSearchableInfo(searchManager.getSearchableInfo
(getComponentName());

    searchView.setSubmitButtonEnabled(true);

    searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
        @Override
        public boolean onQueryTextSubmit(String query) {
            roomCardRecyclerViewAdapter.getFilter().filter(query);
            return false;
        }

        @Override
        public boolean onQueryTextChange(String newText) {
            roomCardRecyclerViewAdapter.getFilter().filter(newText);
            return false;
        }
    });

    return true;
}


@Override
public boolean onSearchRequested() {

    //pauseSomeStuff();

    //roomCardRecyclerViewAdapter.getFilter().filter(query);
    return super.onSearchRequested();
}

- raihan saifullah

---

你能分享一下你使用 RoomCardRecyclerViewAdapter 的代码吗？ - Tugay

我分享了文件。 - raihan saifullah

请尽量减少代码量。删除您认为不重要的部分。 - Tugay

在声明 List 后，您可以使用 Java 异常 - Try...Catch 块。 - Mahsin

Answer 1

我认为问题出在这里：

你没有分享 MainActivity 的前几行代码，其中你声明了变量。因此，我假设你有以下类似的代码：

List mRooms;

我假设在给这个mRooms赋值之前，你已经将其传递给了RoomCardRecyclerViewAdapter：

roomCardRecyclerViewAdapter = new RoomCardRecyclerViewAdapter(this, mRooms);
mRooms = new ArrayList<>();

您已经给 mRooms 赋值了，这就是为什么会出现 NPE 的原因。您可以像这样解决这个问题：

mRooms = new ArrayList<>();
roomCardRecyclerViewAdapter = new RoomCardRecyclerViewAdapter(this, mRooms);

- Tugay