当我尝试从我的应用程序注销并关闭它并再次打开它时，我遇到了问题。该应用程序在那里崩溃了，我认为是因为MainActivity.class上的 onStart 无法正常工作，所以我花了很多时间来解决这个问题，但是我不能，我需要太多帮助。
这是错误
12-27 19:32:01.753 27788-27788/com.suryaaditya.dissent E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.suryaaditya.dissent, PID: 27788
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()' on a null object reference
at com.suryaaditya.dissent.FriendsFragment.onCreateView(FriendsFragment.java:60)
at android.support.v4.app.Fragment.performCreateView(Fragment.java:2346)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1428)
at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1759)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1827)
at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:797)
at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2596)
at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2383)
at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2338)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2245)
at android.support.v4.app.FragmentManagerImpl.dispatchStateChange(FragmentManager.java:3248)
at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:3200)
at android.support.v4.app.FragmentController.dispatchActivityCreated(FragmentController.java:195)
at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:597)
at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:177)
at com.suryaaditya.dissent.MainActivity.onStart(MainActivity.java:113)
at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1392)
at android.app.Activity.performStart(Activity.java:7157)
at android.app.ActivityThread.handleStartActivity(ActivityThread.java:2940)
at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:180)
at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:165)
at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:142)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1811)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6694)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
此MainActivity
public class MainActivity extends AppCompatActivity {
private Toolbar mToolbar;
private ViewPager myViewPager;
private TabLayout myTabLayout;
private TabsAccessorAdapter myTabsAccessorAdapter;
private FirebaseAuth mAuth;
private DatabaseReference RootRef;
private String currentUserID;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mAuth = FirebaseAuth.getInstance();
RootRef = FirebaseDatabase.getInstance().getReference();
mToolbar = (Toolbar) findViewById(R.id.main_page_toolbar);
setSupportActionBar(mToolbar);
getSupportActionBar().setTitle("Dissent");
BottomNavigationView navigationView= findViewById(R.id.bottom_navigation);
final ChatsFragment chatsFragment = new ChatsFragment();
final NotificationsFragment notificationsFragment = new NotificationsFragment();
final FriendsFragment friendsFragment = new FriendsFragment();
final GroupsFragment groupsFragment = new GroupsFragment();
navigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
int id = menuItem.getItemId();
if (id == R.id.navigation_friends){
setFragment(friendsFragment);
return true;
}
else if(id == R.id.navigation_group){
setFragment(groupsFragment);
return true;
}
else if (id == R.id.navigation_chats){
setFragment(chatsFragment);
return true;
}
else if(id == R.id.navigation_notif){
setFragment(notificationsFragment);
return true;
}
return false;
}
});
navigationView.setSelectedItemId(R.id.navigation_friends);
}
private void setFragment(Fragment fragment){
FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.frame, fragment);
fragmentTransaction.commit();
}
@Override
protected void onStart() {
super.onStart();
FirebaseUser currentUser = mAuth.getCurrentUser();
if (currentUser == null){
SendUserToLoginActivity();
}
else{
updateUserStatus("online");
VerifyUserExistence();
}
}
@Override
protected void onStop() {
super.onStop();
FirebaseUser currentUser = mAuth.getCurrentUser();
if (currentUser != null){
updateUserStatus("offline");
}
}
@Override
protected void onDestroy() {
super.onDestroy();
FirebaseUser currentUser = mAuth.getCurrentUser();
if (currentUser != null){
updateUserStatus("offline");
}
}
private void VerifyUserExistence() {
String currentUserID = mAuth.getCurrentUser().getUid();
RootRef.child("Users").child(currentUserID).addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
if ((dataSnapshot.child("name").exists())){
}
else {
SendUserToSettingsActivity();
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
private void SendUserToLoginActivity() {
Intent loginIntent = new Intent(MainActivity.this, LoginActivity.class);
loginIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
startActivity(loginIntent);
}
private void SendUserToFindFriendsActivity() {
Intent findFriendsIntent = new Intent(MainActivity.this , FindFriendsActivity.class);
startActivity(findFriendsIntent);
}
private void SendUserToSettingsActivity() {
Intent settingsIntent = new Intent(MainActivity.this , SettingsActivity.class);
startActivity(settingsIntent);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
super.onCreateOptionsMenu(menu);
getMenuInflater().inflate(R.menu.option_menu, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
super.onOptionsItemSelected(item);
if (item.getItemId() == R.id.main_logout_option){
updateUserStatus("offline");
mAuth.signOut();
SendUserToLoginActivity();
}
if (item.getItemId() == R.id.main_settings_option){
SendUserToSettingsActivity();
}
if (item.getItemId() == R.id.main_create_group_option){
RequestNewGroup();
}
if (item.getItemId() == R.id.main_find_friends_option){
SendUserToFindFriendsActivity();
}
return true;
}
private void RequestNewGroup() {
AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this, R.style.AlertDialog);
builder.setTitle("Enter Group Name : ");
final EditText groupNameField = new EditText(MainActivity.this);
groupNameField.setHint("e.g Dissent");
builder.setView(groupNameField);
builder.setPositiveButton("Create", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
String groupName = groupNameField.getText().toString();
if (TextUtils.isEmpty(groupName)){
Toast.makeText(MainActivity.this, "Please enter Group Name !" , Toast.LENGTH_SHORT).show();
}
else {
CreateNewGroup(groupName);
}
}
});
builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener()
{
@Override
public void onClick(DialogInterface dialog, int which) {
dialog.cancel();
}
});
builder.show();
}
private void CreateNewGroup(final String groupName) {
RootRef.child("Groups").child(groupName).setValue("")
.addOnCompleteListener(new OnCompleteListener<Void>() {
@Override
public void onComplete(@NonNull Task<Void> task) {
if (task.isSuccessful()){
Toast.makeText(MainActivity.this, groupName + " is Created Successfully !", Toast.LENGTH_SHORT).show();
}
}
});
}
private void updateUserStatus(String state){
String saveCurrentTime, saveCurrentDate;
Calendar calendar = Calendar.getInstance();
SimpleDateFormat currentTime = new SimpleDateFormat("hh:mm a");
saveCurrentTime = currentTime.format(calendar.getTime());
SimpleDateFormat currentDate = new SimpleDateFormat("MMM dd, yyyy");
saveCurrentDate = currentDate.format(calendar.getTime());
HashMap<String , Object> onlineStateMap = new HashMap<>();
onlineStateMap.put("time", saveCurrentTime);
onlineStateMap.put("date", saveCurrentDate);
onlineStateMap.put("state", state);
currentUserID = mAuth.getCurrentUser().getUid();
RootRef.child("Users").child(currentUserID).child("userState")
.updateChildren(onlineStateMap);
}
}
这是FriendsFragment
public class FriendsFragment extends Fragment {
private View FriendsView;
private RecyclerView friendsList ;
private DatabaseReference friendsRef, usersRef ;
private FirebaseAuth mAuth;
private String currentUserID;
public FriendsFragment() {
// Required empty public constructor
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
FriendsView = inflater.inflate(R.layout.fragment_friends, container, false);
friendsList = (RecyclerView) FriendsView.findViewById(R.id.friends_list);
friendsList.setLayoutManager(new LinearLayoutManager(getContext()));
mAuth = FirebaseAuth.getInstance();
currentUserID = mAuth.getCurrentUser().getUid();
friendsRef = FirebaseDatabase.getInstance().getReference().child("Contacts").child(currentUserID);
usersRef = FirebaseDatabase.getInstance().getReference().child("Users");
return FriendsView;
}
@Override
public void onStart() {
super.onStart();
FirebaseRecyclerOptions options =
new FirebaseRecyclerOptions.Builder<Contacts>()
.setQuery(friendsRef, Contacts.class)
.build();
FirebaseRecyclerAdapter<Contacts, FriendsViewHolder> adapter =
new FirebaseRecyclerAdapter<Contacts, FriendsViewHolder>(options)
{
@Override
protected void onBindViewHolder(@NonNull final FriendsViewHolder holder, int position, @NonNull Contacts model) {
String userID = getRef(position).getKey();
usersRef.child(userID).addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
if (dataSnapshot.exists()){
if (dataSnapshot.child("userState").hasChild("state")) {
String state = dataSnapshot.child("userState").child("state").getValue().toString();
String date = dataSnapshot.child("userState").child("date").getValue().toString();
String time = dataSnapshot.child("userState").child("time").getValue().toString();
if (state.equals("online")){
holder.onlineIcon.setVisibility(View.VISIBLE);
}
else if (state.equals("offline")){
holder.onlineIcon.setVisibility(View.INVISIBLE);
}
}
else {
holder.onlineIcon.setVisibility(View.INVISIBLE);
}
if (dataSnapshot.hasChild("image")){
String userImage = dataSnapshot.child("image").getValue().toString();
String profileName = dataSnapshot.child("name").getValue().toString();
String profileStatus = dataSnapshot.child("status").getValue().toString();
holder.userName.setText(profileName);
holder.userStatus.setText(profileStatus);
Picasso.get().load(userImage).placeholder(R.drawable.profile_image).into(holder.profileImage);
}
else {
String profileName = dataSnapshot.child("name").getValue().toString();
String profileStatus = dataSnapshot.child("status").getValue().toString();
holder.userName.setText(profileName);
holder.userStatus.setText(profileStatus);
}
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
@NonNull
@Override
public FriendsViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int viewType) {
View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.users_display_layout, viewGroup,false);
FriendsViewHolder viewHolder = new FriendsViewHolder(view);
return viewHolder;
}
};
friendsList.setAdapter(adapter);
adapter.startListening();
}
public static class FriendsViewHolder extends RecyclerView.ViewHolder{
TextView userName, userStatus;
CircleImageView profileImage;
ImageView onlineIcon;
public FriendsViewHolder(View itemView) {
super(itemView);
userName = itemView.findViewById(R.id.user_profile_name);
userStatus = itemView.findViewById(R.id.user_status);
profileImage = itemView.findViewById(R.id.user_profile_image);
onlineIcon = (ImageView) itemView.findViewById(R.id.user_online_status);
}
}
}
答案 0 :(得分：0)
问题出在FriendsFragment上。您正在尝试在空对象引用上获取Uid，这意味着在您与FriendsFragment进行交易时，该用户尚未登录。