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android - 用户登录时的空对象引用

问题描述

当用户注销然后尝试登录另一个帐户时出现空对象引用错误

我没有找到任何解决方案我仍在努力解决这个问题 2 天

消息活动.java

public class MessageActivity extends AppCompatActivity {

    CircleImageView profile_image;
    TextView username;

    FirebaseUser fuser;
    DatabaseReference reference;
    ValueEventListener seenListener;
    ImageButton btn_send;
    EditText text_send;
    String userid;
    MessageAdapter messageAdapter;
    List<Chat> mchat;

    RecyclerView recyclerView;

    Intent intent;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activiy_message);

        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setTitle("");
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //and this
                startActivity(new Intent(MessageActivity.this,MainActivity.class).setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP));
            }
        });

        recyclerView = findViewById(R.id.recycler_view);
        recyclerView.setHasFixedSize(true);
        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getApplicationContext());
        linearLayoutManager.setStackFromEnd(true);
        recyclerView.setLayoutManager(linearLayoutManager);

        profile_image = findViewById(R.id.profile_image);
        username = findViewById(R.id.username);
        btn_send = findViewById(R.id.btn_send);
        text_send = findViewById(R.id.text_send);

        intent = getIntent();
        userid = intent.getStringExtra("userid");
        fuser = FirebaseAuth.getInstance().getCurrentUser();

        btn_send.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String msg = text_send.getText().toString();
                if (!msg.equals("")) {

                    sendMessage(fuser.getUid(), userid, msg);
                } else {

                    Toast.makeText(MessageActivity.this, "You can't send an empty message",
                            Toast.LENGTH_SHORT).show();
                }

                text_send.setText("");
            }
        });

        fuser = FirebaseAuth.getInstance().getCurrentUser();
        reference = FirebaseDatabase.getInstance().getReference("Users").child(userid);

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                User user = dataSnapshot.getValue(User.class);
                username.setText(user.getUsername());
                if (user.getImageURL().equals("default")) {
                    profile_image.setImageResource(R.mipmap.ic_launcher);

                } else {

                    Glide.with(getApplicationContext()).load(user.getImageURL()).into(profile_image);
                }

                readMessages(fuser.getUid(), userid, user.getImageURL());
            }

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

            }
        });
    }
    private void seenMessage(final String userid){
        reference = FirebaseDatabase.getInstance().getReference("Chats");
        seenListener = reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                    Chat chat = snapshot.getValue(Chat.class);
                    if (chat.getReceiver().equals(fuser.getUid()) && chat.getSender().equals(userid)){
                        HashMap<String, Object> hashMap = new HashMap<>();
                        hashMap.put("isseen", true);
                        snapshot.getRef().updateChildren(hashMap);
                    }
                }
            }

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

            }
        });
        seenMessage(userid);
    }
    private void sendMessage(String sender, String receiver, String message) {

        DatabaseReference reference = FirebaseDatabase.getInstance().getReference();

        HashMap<String, Object> hashMap = new HashMap<>();
        hashMap.put("sender", sender);
        hashMap.put("receiver", receiver );
        hashMap.put("message", message);
        hashMap.put("isseen", false);

        reference.child("Chats").push().setValue(hashMap);
        // add user to chat
        final DatabaseReference chatRef = FirebaseDatabase.getInstance().getReference("Chatlist")
                .child(fuser.getUid())
                .child(userid);
        chatRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if(!dataSnapshot.exists()){
                    chatRef.child("id").setValue(userid);
                }

            }

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

            }
        });

    }

    private void readMessages(final String myid, final String userid, final String imageurl) {
        mchat = new ArrayList<>();

        reference = FirebaseDatabase.getInstance().getReference("Chats");
        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                mchat.clear();
                for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                    Chat chat = snapshot.getValue(Chat.class);

                    if (chat.getReceiver().equals(myid) && chat.getSender().equals(userid) ||
                            chat.getReceiver().equals(userid) && chat.getSender().equals(myid)) {

                        mchat.add(chat);
                    }

                    messageAdapter = new MessageAdapter(MessageActivity.this, mchat, imageurl);
                    recyclerView.setAdapter(messageAdapter);
                }
            }

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

            }
        });
    }

    private void status(String status){
        reference = FirebaseDatabase.getInstance().getReference("Users").child(fuser.getUid());

        HashMap<String,Object> hashMap = new HashMap<>();
        hashMap.put("status",status);
        reference.updateChildren(hashMap);

    }

    @Override
    protected void onResume() {
        super.onResume();
        status("online");
    }

    @Override
    protected void onPause() {

        if (seenListener != null && reference!=null) {
            reference.removeEventListener(seenListener);
        }
        super.onPause();
        status("offline");
    }

MainActivity.java

public class MainActivity extends AppCompatActivity {


    CircleImageView profile_image;
    TextView username;
    User user;
    FirebaseUser firebaseUser;
    DatabaseReference reference;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setTitle("");

        profile_image = findViewById(R.id.profile_image);
        username = findViewById(R.id.username);

        firebaseUser = FirebaseAuth.getInstance().getCurrentUser();
        reference = FirebaseDatabase.getInstance().getReference("Users").child(firebaseUser.getUid());

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                User user = dataSnapshot.getValue(User.class);
                username.setText("");
                username.setText(user.getUsername());
                if (user.getImageURL().equals("default")){
                    profile_image.setImageResource(R.mipmap.ic_launcher);
                } else {

                    //change this
                    Glide.with(getApplicationContext()).load(user.getImageURL()).into(profile_image);
                }
            }

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

            }
        });

        final TabLayout tabLayout = findViewById(R.id.tab_layout);
        final ViewPager viewPager = findViewById(R.id.view_pager);


        reference = FirebaseDatabase.getInstance().getReference("Chats");
        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                ViewPagerAdapter viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());
                int unread = 0;
                for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                    Chat chat = snapshot.getValue(Chat.class);
                    if (chat.getReceiver().equals(firebaseUser.getUid()) && !chat.isIsseen()){
                        unread++;
                    }
                }

                if (unread == 0){
                    viewPagerAdapter.addFragment(new ChatsFragment(), "Chats");
                } else {
                    viewPagerAdapter.addFragment(new ChatsFragment(), "("+unread+") Chats");
                }

                viewPagerAdapter.addFragment(new UsersFragment(), "Users");
                viewPagerAdapter.addFragment(new ProfileFragment(), "Profile");

                viewPager.setAdapter(viewPagerAdapter);

                tabLayout.setupWithViewPager(viewPager);

            }

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

            }
        });


    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()){

            case  R.id.logout:
                FirebaseAuth.getInstance().signOut();
                // change this code beacuse your app will crash
                startActivity(new Intent(MainActivity.this, StartActivity.class).setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP));
                return true;
        }

        return false;
    }

    class ViewPagerAdapter extends FragmentPagerAdapter {

        private ArrayList<Fragment> fragments;
        private ArrayList<String> titles;

        ViewPagerAdapter(FragmentManager fm){
            super(fm);
            this.fragments = new ArrayList<>();
            this.titles = new ArrayList<>();
        }

        @Override
        public Fragment getItem(int position) {
            return fragments.get(position);
        }

        @Override
        public int getCount() {
            return fragments.size();
        }

        public void addFragment(Fragment fragment, String title){
            fragments.add(fragment);
            titles.add(title);
        }

        // Ctrl + O

        @Nullable
        @Override
        public CharSequence getPageTitle(int position) {
            return titles.get(position);
        }
    }

    private void status(String status){
        reference = FirebaseDatabase.getInstance().getReference("Users").child(firebaseUser.getUid());

        HashMap<String, Object> hashMap = new HashMap<>();
        hashMap.put("status", status);

        reference.updateChildren(hashMap);
    }

    @Override
    protected void onResume() {
        super.onResume();
        status("online");
    }

    @Override
    protected void onPause() {
        super.onPause();
        status("offline");
    }

用户.java

public class User {


    private String id;
    private  String username;
    private  String imageURL;
    private String status;
    private String search;

    public User(String id, String username, String imageURL,String status,String search) {
        this.id = id;
        this.status=status;
        this.search=search;
        this.username = username;
        this.imageURL = imageURL;
    }
    public User(){

    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getImageURL() {
        return imageURL;
    }

    public void setImageURL(String imageURL) {
        this.imageURL = imageURL;
    }

    public String getStatus() {
        return status;
    }

    public void setStatus(String status) {
        this.status = status;
    }

    public String getSearch() {
        return search;
    }

    public void setSearch(String search) {
        this.search = search;
    }

错误

 java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
        at com.example.anonymousnow.Fragments.UsersFragment$3.onDataChange(UsersFragment.java:123)

在空对象引用 UserFragments.java上发生了同样的错误

public class UsersFragment extends Fragment {
    private RecyclerView recyclerView;
    private UserAdapter userAdapter;
    private List<User> mUsers;
    EditText search_users;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_users,container,false);
        recyclerView = view.findViewById(R.id.recycle_view);

        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
        mUsers = new ArrayList<>();

        readUsers();

        search_users = view.findViewById(R.id.search_users);

        search_users.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence charSequence, int start, int before, int count) {
                searchUsers(charSequence.toString().toLowerCase());
            }


            @Override
            public void afterTextChanged(Editable s) {

            }
        });
        return view;
    }

    private void searchUsers(String s) {
        final FirebaseUser fuser = FirebaseAuth.getInstance().getCurrentUser();
        Query query = FirebaseDatabase.getInstance().getReference("Users").orderByChild("search")
                .startAt(s)
                .endAt(s+"\uf8ff");

        query.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                mUsers.clear();


                for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                    User user = snapshot.getValue(User.class);

                    assert user!=null;
                    assert fuser!=null;
                    if (!user.getId().equals(fuser.getUid())){
                        mUsers.add(user);
                    }
                }
                userAdapter = new UserAdapter(getContext(),mUsers,false);
                recyclerView.setAdapter(userAdapter);
            }


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

            }
        });

    }


    private void readUsers(){
        final FirebaseUser firebaseUser = FirebaseAuth.getInstance().getCurrentUser();
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Users");

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                    mUsers.clear();
                    User user = snapshot.getValue(User.class);
                    assert user != null;
                    assert firebaseUser != null;
                    if (!user.getId().equals(firebaseUser.getUid())){

                        mUsers.add(user);
                    }
                }
                userAdapter = new UserAdapter(getContext(),mUsers,true);
                recyclerView.setAdapter(userAdapter);

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

            }
        });
    }

}

数据库屏幕截图

标签： androidfirebasefirebase-authentication

解决方案

您尝试检索的用户不包含在您的 User 类中指定的所有键，因此键 imageURL 为空，这会在此处产生 NullPointerException -

if (user.getImageURL().equals("default")){

因此，请确保您的 db 节点包含所有键或使用条件来检查 imageURL 是否不为空 -

User user = dataSnapshot.getValue(User.class);
if(user != null){
if (user.getImageURL() != null && user.getImageURL().equals("default")){
    // use getImageURL here
}
}

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