我的列表大小和索引大小相同，但我仍然得到 IndexOutOfBoundsException

Question

StackTrace 錯誤

2020-04-18 14:50:38.339 11953-12018/com.shivam.chatapp2 E/AndroidRuntime: FATAL EXCEPTION: Download_Thread
Process: com.shivam.chatapp2, PID: 11953
java.lang.IndexOutOfBoundsException: Index: 2, Size: 2
    at java.util.ArrayList.get(ArrayList.java:437)
    at com.shivam.chatapp2.Fragments.Chats.UserRef2(Chats.java:282)
    at com.shivam.chatapp2.Fragments.Chats.access$200(Chats.java:48)
    at com.shivam.chatapp2.Fragments.Chats$2$1.run(Chats.java:167)
    at java.lang.Thread.run(Thread.java:919)

我使用的標簽值

    2020-04-18 14:50:34.314 11953-12018/com.shivam.chatapp2 D/Tag2: Size for each  loop   1
2020-04-18 14:50:34.315 11953-12018/com.shivam.chatapp2 D/Tag2: Size for each  loop   2
2020-04-18 14:50:38.317 11953-12018/com.shivam.chatapp2 D/Tag2: size outside looper :   2
2020-04-18 14:50:38.319 11953-12018/com.shivam.chatapp2 D/Tag2: User List Size Two : 0
2020-04-18 14:50:38.320 11953-11953/com.shivam.chatapp2 D/Tag2: UserListSize inside Handler :  2

我正在嘗試從 Firebase 實時數據庫中獲取節點的大小，我正在后台線程上計算它的大小，並根據我從該節點獲得的大小，我正在將偵聽器添加到 FireBase 實時數據庫上的另一個節點並打印該值但它顯示當我調用方法 UserRef2() 時出現 IndexOutOfBoundsException。

我的代碼

    public class Chats extends Fragment {

    private RecyclerView recyclerView;
    private UserAdapter mUserAdapter;

    DatabaseReference reference;
    DatabaseReference rootRef;

    FloatingActionButton profile, credentials;

    private ChatsFragmentAdapter chatsFragmentAdapter;

    List<String> UserChatList = new ArrayList<>();
    List<Chatlist> usersList;
    private List<User> mUsers;

    String TAG = "Tag2";
    private static final String MESSAGE_KEY = "Message_Key";

    private Handler mHandler;

    int UserListSize;

    public Chats() {
        // 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_chats, container, false);

        mHandler = new Handler(getMainLooper()) {
            @Override
            public void handleMessage(@NonNull Message msg) {
                super.handleMessage(msg);


                UserListSize = msg.getData().getInt(MESSAGE_KEY);
                Log.d(TAG, "UserListSize inside Handler :  " + UserListSize);


            }
        };
        String fuser;
        fuser = FirebaseAuth.getInstance().getCurrentUser().getUid();

        rootRef = FirebaseDatabase.getInstance().getReference();
        DatabaseReference ChatlistRef = rootRef.child("Chatlist").child(fuser);

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


                Runnable runnable = new Runnable() {
                    @Override
                    public void run() {

                        for (DataSnapshot ds : dataSnapshot.getChildren()) {
                            String userIDS = ds.getKey();

                            UserChatList.add(userIDS);
                            Log.d(TAG, "Size for each  loop   " + UserChatList.size());
                        }
                        try {
                            Thread.sleep(2000);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        int size_from_looper = UserChatList.size();

                        Log.d(TAG, "size outside looper :   " + size_from_looper);


                        Message message = new Message();
                        Bundle bundle = new Bundle();
                        bundle.putInt(MESSAGE_KEY, size_from_looper);
                        message.setData(bundle);

                        mHandler.sendMessage(message);


                    }
                };

                Thread thread = new Thread(runnable);
                thread.setName("Download_Thread");
                thread.start();

                Log.d(TAG, "User List Size Two : " + UserListSize);


                if (UserListSize == 0) {
                    Toast.makeText(getContext(), "No Chats Available", Toast.LENGTH_SHORT).show();
                }

                if (UserListSize == 1) {

                    UserRef1();
                } else if (UserListSize == 2) {

                    UserRef1();
                    UserRef2();

                } else if (UserListSize == 3) {

                    UserRef1();
                    UserRef2();
                    UserRef3();
                }


            }

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

            }
        };
        ChatlistRef.addValueEventListener(mValueEventListener);

        return view;
    }

    private void UserRef1() {

        DatabaseReference UserRef1 = rootRef.child("Users").child(UserChatList.get(1));
        ValueEventListener eventListener1 = new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                User user = dataSnapshot.getValue(User.class);
                mUsers.add(user);
                mUserAdapter.notifyDataSetChanged();
                String name = dataSnapshot.child("First").getValue(String.class);
                Log.d(TAG, "Name called  " + name);

            }

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

            }
        };
        UserRef1.addValueEventListener(eventListener1);

    }

    private void UserRef2() {

        DatabaseReference UserRef2 = rootRef.child("Users").child(UserChatList.get(2));
        ValueEventListener eventListener2 = new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                User user = dataSnapshot.getValue(User.class);
                mUsers.add(user);
                mUserAdapter.notifyDataSetChanged();
                String name = dataSnapshot.child("First").getValue(String.class);
                Log.d(TAG, "Name called  " + name);

            }

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

            }
        };
        UserRef2.addValueEventListener(eventListener2);
    }

    private void UserRef3() {
        DatabaseReference UserRef3 = rootRef.child("Users").child(UserChatList.get(3));
        ValueEventListener eventListener3 = new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                User user = dataSnapshot.getValue(User.class);
                mUsers.add(user);
                mUserAdapter.notifyDataSetChanged();
                String name = dataSnapshot.child("First").getValue(String.class);
                Log.d(TAG, "Name called  " + name);

            }

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

            }
        };
        UserRef3.addValueEventListener(eventListener3);
    }
}

Answer 1

不可以。如果索引大小為 2，則必須使用小於或等於 1 (i <= 1) 的索引。

因為索引號從 0 開始，所以 size 從 1 開始。

Answer 2

當大小為 1 時，索引應該為 0，因為索引總是從 0 開始

因此，在您的代碼更改中：

UserChatList.get(1) to UserChatList.get(0)
UserChatList.get(2) to UserChatList.get(1)
UserChatList.get(3) to UserChatList.get(2)