HashMap无法强制转换为ArrayList

Question

I upload an ArrayList of objects on firebase database, and when I download that ArrayList and put in ArrayList value, I get message "HashMap cannot be cast to java.util.ArrayList". But I upload ArrayList , download ArrayList , put in ArrayList value, so why does the error say HashMap?

public class TileContentFragment extends Fragment {

    ArrayList<Item> mItems=new ArrayList<>();

    private FirebaseAuth mFirebaseAuth;
    private FirebaseUser mFirebaseUser;
    private DatabaseReference mDatabase;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {

        // Initialize Firebase Auth and Database Reference
        mFirebaseAuth = FirebaseAuth.getInstance();
        mFirebaseUser = mFirebaseAuth.getCurrentUser();
        mDatabase = FirebaseDatabase.getInstance().getReference();

        RecyclerView recyclerView = (RecyclerView) inflater.inflate(R.layout.recycler_view, container, false);
        ContentAdapter adapter = new ContentAdapter(recyclerView.getContext(),mItems);
        recyclerView.setAdapter(adapter);
        recyclerView.setHasFixedSize(true);
        // Set padding for Tiles
        int tilePadding = getResources().getDimensionPixelSize(R.dimen.tile_padding);
        recyclerView.setPadding(tilePadding, tilePadding, tilePadding, tilePadding);
        recyclerView.setLayoutManager(new GridLayoutManager(getActivity(), 2));
        return recyclerView;
    }
    @Override
    public void onStart() {
        super.onStart();

        mDatabase.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot snapshot) {
                if (snapshot.hasChild("ItemsArray")) {

                    final ValueEventListener userListener = new ValueEventListener() {
                        @Override
                        public void onDataChange(DataSnapshot dataSnapshot) {
                            mItems = new ArrayList<>((ArrayList) dataSnapshot.getValue());

                        }

                        @Override
                        public void onCancelled(DatabaseError databaseError) {
                            Log.w("AppInfo", "onCancelled: ",databaseError.toException());

                        }
                    };
                    mDatabase.child("ItemsArray").addValueEventListener(userListener);
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
    }

    public static class ViewHolder extends RecyclerView.ViewHolder {

        public ImageView picture;
        public TextView name;

        public ViewHolder(LayoutInflater inflater, ViewGroup parent) {
            super(inflater.inflate(R.layout.item_tile, parent, false));
            picture = (ImageView) itemView.findViewById(R.id.tile_picture);
            name = (TextView) itemView.findViewById(R.id.tile_title);
            itemView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    Context context = v.getContext();
                    Intent intent = new Intent(context, DetailActivity.class);
                    intent.putExtra(DetailActivity.EXTRA_POSITION, getAdapterPosition());
                    context.startActivity(intent);
                }
            });
        }
    }

    /**
     * Adapter to display recycler view.
     */
    public static class ContentAdapter extends RecyclerView.Adapter<ViewHolder> {
        // Set numbers of Tiles in RecyclerView.
        private static final int LENGTH = 18;

        private final String[] mPlaces;
        private ArrayList<Item> mPlacePictures;

        public ContentAdapter(Context context,ArrayList<Item> mPlacePictures) {
            Resources resources = context.getResources();
            mPlaces = resources.getStringArray(R.array.places);
            this.mPlacePictures=mPlacePictures;
        }

        @Override
        public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            return new ViewHolder(LayoutInflater.from(parent.getContext()), parent);
        }

        @Override
        public void onBindViewHolder(ViewHolder holder, int position) {
            Picasso.with(holder.picture.getContext()).load(mPlacePictures.get(position).getProfilePic()).into(holder.picture);
           // holder.picture.setImageDrawable(mPlacePictures[position % mPlacePictures.length]);
            holder.name.setText(mPlaces[position % mPlaces.length]);
        }

        @Override
        public int getItemCount() {
            return mPlacePictures.size();
        }
    }
}

public class Item {

    private String user;
    private String name;
    private String desription;
    private int price;
    private ArrayList<String> url;
    private String profilePic;

    public Item(){}
    public Item(String user,String name, String desription, int price, ArrayList<String> url){
        this.user=user;
        this.name=name;
        this.desription=desription;
        this.price=price;
        this.url=url;
    }


    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDesription() {
        return desription;
    }

    public void setDesription(String desription) {
        this.desription = desription;
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        this.price = price;
    }

    public ArrayList<String> getUrl() {

        return url;
    }

    public void setUrl(ArrayList<String> url) {

        this.url = url;
    }

    public String getUser() {
        return user;
    }

    public void setUser(String user) {
        this.user = user;
    }

    public String getProfilePic() {
        return profilePic;
    }

    public void setProfilePic(String profilePic) {
        this.profilePic = profilePic;
    }
}

    E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.packtpub.materialdesign4, PID: 713
              java.lang.ClassCastException: java.util.ArrayList cannot be cast to java.util.Map
                  at com.packtpub.materialdesign4.TileContentFragment$1$1.onDataChange(TileContentFragment.java:82)
                  at com.google.android.gms.internal.zzakg.zza(Unknown Source)
                  at com.google.android.gms.internal.zzalg.zzcxk(Unknown Source)
                  at com.google.android.gms.internal.zzalj$1.run(Unknown Source)
                  at android.os.Handler.handleCallback(Handler.java:739)
                  at android.os.Handler.dispatchMessage(Handler.java:95)
                  at android.os.Looper.loop(Looper.java:148)
                  at android.app.ActivityThread.main(ActivityThread.java:5527)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620)
DynamiteModule: Local module descriptor class for com.google.firebase.auth not found.

Answer 1

Have a look at the documentation for DataSnapshot here : https://www.firebase.com/docs/java-api/javadoc/com/firebase/client/DataSnapshot.html#getValue--

getValue() returns the data contained in this snapshot as native types. The possible types returned are:

• Boolean
• Long
• Double
• Map <String, Object>
• List<Object>

The method dataSnapshot.getValue() is returning a Map<String, Object> instead of the List<Object> you were expecting.

Casting to a Map first, then creating an ArrayList from the result should fix your problem

mItems = new ArrayList(((Map<String, Item>) dataSnapshot.getValue()).values());