java - 无法在 Android 中显示来自 Firebase 实时数据库的随机数据

Question

我正在尝试将 Firebase 中的用户随机分配到我的 RecyclerView。我尝试在注册时为用户创建一个随机 ID，并添加相关查询，但会使用大量内存来下载所有可用数据从 Firebase 中，我尝试的第二种方法在以下行中显示错误

 DatabaseReference productIdRef = rootRef.child("Users").child(UserIdsList.get(new Random().nextInt(UserListSize)));//Line149

堆栈跟踪

2020-04-06 23:39:33.277 29631-29631/com.shivam.chatapp2 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.shivam.chatapp2, PID: 29631
java.lang.IllegalArgumentException: bound must be positive
    at java.util.Random.nextInt(Random.java:388)
    at com.shivam.chatapp2.Fragments.UsersFragment$1.onDataChange(UsersFragment.java:90)
    at com.google.firebase.database.Query$1.onDataChange(com.google.firebase:firebase-database@@19.2.1:179)
    at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@19.2.1:75)
    at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@19.2.1:63)
    at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@19.2.1:55)
    at android.os.Handler.handleCallback(Handler.java:883)
    at android.os.Handler.dispatchMessage(Handler.java:100)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7356)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

错误是

代码

UserFragment.java

public class UsersFragment extends Fragment {
    private RecyclerView recyclerView;
    private UserAdapter mUserAdapter;
    private List<User> mUsers;
    String TAG = "MyTag";
    ValueEventListener mValueEventListener;

    public UsersFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment

        View view = inflater.inflate(R.layout.fragment_users, container, false);
        recyclerView = view.findViewById(R.id.recycler_view);
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
        mUsers = new ArrayList<>();


        //readUser();
        RandomUsers();


        return view;
    }


    private void RandomUsers() {

        mUserAdapter = new UserAdapter(getContext(), mUsers, false);
        recyclerView.setAdapter(mUserAdapter);
        mUserAdapter.notifyDataSetChanged();

        DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
        DatabaseReference productIdsRef = rootRef.child("UserIds");

        ValueEventListener valueEventListener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                List<String> UserList = new ArrayList<>();

                List<String> UserIdsList = new ArrayList<>();
                for (DataSnapshot ds : dataSnapshot.getChildren()) {
                    String productId = ds.getKey();
                    UserIdsList.add(productId);
                }

                int UserListSize = UserList.size();
                List<String> randomProductList = new ArrayList<>();

                DatabaseReference productIdRef = rootRef.child("Users").child(UserIdsList.get(new Random().nextInt(UserListSize)));
                ValueEventListener eventListener = new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        String name = dataSnapshot.child("Firs").getValue(String.class);
                        Log.d("TAG", name);
                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {
                        Log.d(TAG, "Error: " + databaseError.toException()); //Don't ignore errors!
                    }
                };
                productIdRef.addListenerForSingleValueEvent(eventListener);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Log.d(TAG, "Error: " + databaseError.getMessage()); //Don't ignore errors!
            }
        };
        productIdsRef.addListenerForSingleValueEvent(valueEventListener);


    }

}

UserAdapter

    public class UserAdapter extends RecyclerView.Adapter<UserAdapter.ViewHolder> {

    private Context mContext;
    private List<User> mUsers;
    private boolean ischat;

    String theLastMessage;

    public UserAdapter(Context mContext, List<User> mUsers,boolean ischat) {
        this.mContext = mContext;
        this.mUsers = mUsers;
        this.ischat=ischat;
    }

    @NonNull
    @Override
    public UserAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view= LayoutInflater.from(mContext).inflate(R.layout.user_item,parent,false);

        return new UserAdapter.ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull UserAdapter.ViewHolder holder, int position) {

        final User user=mUsers.get(position);
        holder.username.setText(user.getFirst());

        if (user.getImageURL().equals("default")){
            holder.profile_image.setImageResource(R.mipmap.ic_launcher);
        } else {
            Glide.with(mContext).load(user.getImageURL()).into(holder.profile_image);
        }

        if (ischat){
            lastMessage(user.getId(), holder.last_msg);
        } else {
            holder.last_msg.setVisibility(View.GONE);
        }

        if (ischat){
            if (user.getStatus().equals("online")){
                holder.img_on.setVisibility(View.VISIBLE);
                holder.img_off.setVisibility(View.GONE);
            } else {
                holder.img_on.setVisibility(View.GONE);
                holder.img_off.setVisibility(View.VISIBLE);
            }
        } else {
            holder.img_on.setVisibility(View.GONE);
            holder.img_off.setVisibility(View.GONE);
        }


        holder.itemView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent=new Intent(mContext, MessageActivity.class);
                intent.putExtra("UserName",user.getFirst());
                intent.putExtra("userid", user.getId());
                intent.putExtra("ImageURL",user.getImageURL());
                mContext.startActivity(intent);
            }
        });

    }

    @Override
    public int getItemCount() {
        return mUsers.size();
    }
    public class ViewHolder extends RecyclerView.ViewHolder{

        public TextView username;
        public ImageView profile_image;
        private ImageView img_on;
        private ImageView img_off;
        private  TextView last_msg;

        public ViewHolder(@NonNull View itemView) {
            super(itemView);
            username=itemView.findViewById(R.id.username);
            profile_image=itemView.findViewById(R.id.profile_image);
            img_on = itemView.findViewById(R.id.img_on);
            img_off = itemView.findViewById(R.id.img_off);
            last_msg=itemView.findViewById(R.id.last_msg);
        }
    }

    private void lastMessage(final String userid, final TextView last_msg){
        theLastMessage = "default";
        final FirebaseUser firebaseUser = FirebaseAuth.getInstance().getCurrentUser();
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Chats");

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                    Chat chat = snapshot.getValue(Chat.class);
                    if (firebaseUser != null && chat != null) {
                        if (chat.getReceiver().equals(firebaseUser.getUid()) && chat.getSender().equals(userid) ||
                                chat.getReceiver().equals(userid) && chat.getSender().equals(firebaseUser.getUid())) {
                            theLastMessage = chat.getMessage();
                        }
                    }
                }

                switch (theLastMessage){
                    case  "default":
                        last_msg.setText("No Message");
                        break;

                    default:
                        last_msg.setText(theLastMessage);
                        break;
                }

                theLastMessage = "default";
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }

}

Answer 1

因为您已经创建了 UserList 的新实例

List<String> UserList = new ArrayList<>();

但是你根本没有使用并得到 UserList.size()总是为零，它导致 new Random().nextInt(UserListSize))收到无效消息

java.lang.IllegalArgumentException: bound must be positive at java.util.Random.nextInt(Random.java:388)

因此，通过获取 UserIdsList 的大小即可简单修复这可以在列表中获得随机用户。删除List<String> UserList = new ArrayList<>();的使用

List<String> UserIdsList = new ArrayList<>();
for (DataSnapshot ds : dataSnapshot.getChildren()) {
    String productId = ds.getKey();
    UserIdsList.add(productId);
}

int UserListSize = UserIdsList.size();