Develop Reference

Topic: querying in Firebase Android - data retrieval

This is my firebase database format:
-groups
  -groupName
    -groupId
      -key1:"value1"       -key2:"value2"
      -key3:"value3"
I need to access value3 of key3.
I have the groupId.
Below is the code I've come up with. But when I run this my screen keeps crashing.
private void method_does_user_belong_to_this_group() {
expected_key3 = editText_expected_key3.getText().toString();
reference.child("groups").orderByChild(groupId+"/"+"key3").addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot snapshot) {
if (snapshot.exists()) {
key3 = snapshot.getValue().toString();
if (key3.equals(expected_key3)) {
Toast.makeText(RoomJoiningPage.this, "Key3 found.", Toast.LENGTH_SHORT).show();
}
else {
Toast.makeText(RoomJoiningPage.this, "Key3 not found"+key3, Toast.LENGTH_SHORT).show();
}
}
else {
Toast.makeText(RoomJoiningPage.this, "Key3 not found"+key3, Toast.LENGTH_SHORT).show();;
}
}
#Override
public void onCancelled(#NonNull DatabaseError error) {
}
});
I've even tried this part too:
GroupDataClass groupDataClass = snapshot.getValue(GroupDataClass.class);
String key3 = groupDataClass.getKey();
by omitting this part from the above method:
key3 = snapshot.getValue().toString();
But even then my screen keeps crashing.
Any help is sincerely appreciated.
Edit-1
I've tried editing the code a bit so far. Now the crashing has stopped but the value I'm receiving is "null".
Log.d(LOG_TAG,"Debug Count - 12");
Log.d(LOG_TAG,"Entered else but didnt run the code below");
Toast.makeText(this, "Code entered.", Toast.LENGTH_SHORT).show();
reference_to_rooms_db = FirebaseDatabase.getInstance().getReference("rooms");
reference_to_rooms_db.orderByChild(string_current_users_number).addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot snapshot) {
Log.d(LOG_TAG,"went inside \"onDataChanged\"");
if (snapshot.exists()) {
Log.d(LOG_TAG,"went inside \"if\"");
Log.d(LOG_TAG,"snapshot existed");
code = snapshot.child("code").getValue(String.class);
Log.d(LOG_TAG,"code copied - "+code+".");
Toast.makeText(RoomJoiningPage.this, "added code:"+code, Toast.LENGTH_SHORT).show();
Log.d(LOG_TAG,"toasted within \"if\"");
}
else {
Log.d(LOG_TAG,"went inside \"else\"");
}
}
#Override
public void onCancelled(#NonNull DatabaseError error) {
Log.d(LOG_TAG,"went inside \"onCancelled\"");
}
});

Add extra data to recyclerView

I've just recently started to learn some coding, a little python and java.
I'm trying to make a whatsapp clone as a test, and I've hit a wall.
Basically I have a UID showing as the name of a group chat, and I want to swap it for a groupId I have saved in firebase at the same level as the Uid.
FirebaseDatabase.getInstance().getReference().child("chat").child(chatId).child("info").child("groupId)
I've been following simcoder youtube video and there's a lot of checks and balances happening between user and chat UIDs and I'm getting lost in the middle of all that.
I've tried adding groupId to the adapter and the chatObject, and I can see the data in debug, but I just can't get it to populate the recycler view. I've had a search of the forums here, but can't quite get it to work.
I'm hoping someone can help me out, many thanks
private RecyclerView mChatList;
private RecyclerView.Adapter mChatListAdapter;
private RecyclerView.LayoutManager mChatListLayoutManager;
ArrayList<ChatObject> chatList;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_group_chat);
OneSignal.startInit(this).init();
OneSignal.setSubscription(true);
OneSignal.idsAvailable(new OneSignal.IdsAvailableHandler() {
#Override
public void idsAvailable(String userId, String registrationId) {
FirebaseDatabase.getInstance().getReference().child("user").child(FirebaseAuth.getInstance().getUid()).child("notificationKey").setValue(userId);
}
});
OneSignal.setInFocusDisplaying(OneSignal.OSInFocusDisplayOption.Notification);
Button mLogout = findViewById(R.id.logout);
Button mFindUser = findViewById(R.id.finduser);
mFindUser.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
startActivity(new Intent(getApplicationContext(), FindUserActivity.class));
}
});
mLogout.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
OneSignal.setSubscription(false);
FirebaseAuth.getInstance().signOut();
Intent intent = new Intent(getApplicationContext(), LoginActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);
finish();
return;
}
});
getPermissions();
initializeRecyclerView();
getUserChatList();
}
private void getUserChatList(){
DatabaseReference mUserChatDB = FirebaseDatabase.getInstance().getReference().child("user").child(FirebaseAuth.getInstance().getUid()).child("chat");
mUserChatDB.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
if (dataSnapshot.exists()){
for (DataSnapshot childSnapshot : dataSnapshot.getChildren()){
ChatObject mChat = new ChatObject(childSnapshot.getKey());
boolean exists = false;
for (ChatObject mChatIterator : chatList){
if(mChatIterator.getChatId().equals(mChat.getChatId()))
exists = true;
}
if (exists)
continue;
chatList.add(mChat);
getChatData(mChat.getChatId());
}
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
}
private void getChatData(String chatId) {
DatabaseReference mChatDB = FirebaseDatabase.getInstance().getReference().child("chat").child(chatId).child("info");
mChatDB.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
if(dataSnapshot.exists()){
String chatId = "";
if(dataSnapshot.child("id").getValue() != null)
chatId = dataSnapshot.child("id").getValue().toString();
for(DataSnapshot userSnapshot : dataSnapshot.child("users").getChildren()){
for(ChatObject mChat : chatList)
if(mChat.getChatId().equals(chatId)){
UserObject mUser = new UserObject(userSnapshot.getKey());
mChat.addUserToArrayList(mUser);
getUserData(mUser);
}
}
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
}
private void getUserData(UserObject mUser) {
DatabaseReference mUserDb = FirebaseDatabase.getInstance().getReference().child("user").child(mUser.getUid());
mUserDb.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
UserObject mUser = new UserObject(dataSnapshot.getKey());
if(dataSnapshot.child("notificationKey").getValue() != null)
mUser.setNotificationKey(dataSnapshot.child("notificationKey").getValue().toString());
for(ChatObject mChat : chatList)
for(UserObject mUserIt : mChat.getUserObjectArrayList()){
if(mUserIt.getUid().equals(mUser.getUid())){
mUserIt.setNotificationKey(mUser.getNotificationKey());
}
}
mChatListAdapter.notifyDataSetChanged();
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
}
private void initializeRecyclerView() {
chatList = new ArrayList<>();
mChatList = findViewById(R.id.chatList);
mChatList.setNestedScrollingEnabled(false);
mChatList.setHasFixedSize(false);
mChatListLayoutManager = new LinearLayoutManager(getApplicationContext(), RecyclerView.VERTICAL,false);
mChatList.setLayoutManager(mChatListLayoutManager);
mChatListAdapter = new ChatListAdapter(chatList);
mChatList.setAdapter(mChatListAdapter);
}

From what I see you are passing to the adapter an empty list so that's why there's not data to be displayed.
And also, where you are using mChatListAdapter.notifyDataSetChanged();, you don't set before a populated list to the adapter.
I'm not sure which is the data you want to display but you have to collect in a list from the firebase all the information you want to display, for example, if UserObject is what you want, you have to make an ArrayList<UserObject>, to add there all the objects, pass it to the adapter and then call adapter.notifyDataSetChanged() .
You can make function in the adapter, like :
public void setData(ArrayList<YourObject> list){
yourAdapterList = list
this.notifyDataSetChanged()
}
And then call it from your activity/fragment like adapter.setData( /* your populated list */)

Unable to get the value of child in firebase other than null

Not sure why, but with the code I have, I cannot seem to get the value of 'isOnline':
dolRef = DatabaseReference dolRef = FirebaseDatabase.getInstance().getReference("DriversOnline");
dolRef = dolRef.child("iosDriver");
dolRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot ds : dataSnapshot.getChildren()) {
String driverid = ds.getKey();
// get value of 'isOnline'
dolRef = dolRef.child(driverid);
Log.e(TAG, "dolRef: " + dolRef);
dolRef.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot dss : dataSnapshot.getChildren()) {
String online = dss.child("isOnline").getValue(String.class);
Log.e(TAG, "Online: " + online);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
Below is a part of my firebase db:
"DriversOnline" : {
"iosDriver" : {
"BruEGfToc8axIWJk1o01fxcwd8I2" : { // driverId
"isOnline" : "true",
"latitude" : 45.276,
"longitude" : -66.064
}
}
}
Any idea why I can't get the value of 'isOnline' other than null ?

I think you're nesting your listeners one level deeper than needed.
DatabaseReference iosRef = FirebaseDatabase.getInstance().getReference("DriversOnline/iosDriver");
iosRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot driverSnapshot: dataSnapshot.getChildren()) {
String driverid = driverSnapshot.getKey();
DataSnapshot isOnlineSnapshot = driverSnapshot.child("isOnline");
System.out.println(isOnlineSnapshot.getValue(String.class));
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException();
}
});
Some things to note:
Reassigning one generically named dolRef variable 4 times in a block of code like this is a code-smell. It makes it much harder to follow what's going on, and to check if the variable points to the right location. Give each of them a name that clearly indicates what they point to, as I've done above.
There is no need to attach a second listener, as the value of isOnline is right in the driverSnapshot. You can just request the child snapshot with the right name, and then the value from that.
Don't ignore error, as that hides potential problems. At the very least throw them, as I've done above.

driverid = FirebaseDatabase.getInstance().getReference("driver");//If there is another driver table, the path must be two layers.
driverid.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Driver driver = dataSnapshot.getValue(Driver.class);//class model
dolRef = FirebaseDatabase.getInstance().getReference("DriversOnline/iosDriver").child(driver.getDriverID);
dolRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
DriversOnline driversOnline= dataSnapshot.getValue(DriversOnline .class);//class model
log.d("driverid","isOnline :"+driversOnline.getisOnline)
//display -> isOnline : true
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});

How do i get entire object in Firebase?

Im creating an app with Firebase database here is my data structure:
{
"teachers" : {
"USYSacnOjDR5EAPwljZMHtggN9I2" : {
"name" : {
"teacher_name" : "Alison"
},
"members": {
"sIrfMA3bm0R9uj55nDUFpFyabcL2" : true
}
}
},
"users" : {
"sIrfMA3bm0R9uj55nDUFpFyabcL2" : {
"position" : {
"bpos" : "0"
},
"name" : {
"email" : "m#m.com",
"name" : "John",
"surname" : "Clash"
}
}
So now in RecyclerView im trying to display all of the Alison teacher members, as you can see John Clash is set to true. Anyway when i try to retrieve user this way:
reference = FirebaseDatabase.getInstance().getReference("teachers").child(teacherkey).child("members");
reference.addChildEventListener(new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
User user = dataSnapshot.child("name").getValue(User.class);
result.add(user);
adapter.notifyDataSetChanged();
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
User user = dataSnapshot.child("name").getValue(User.class);
result.remove(user);
adapter.notifyDataSetChanged();
}
the app crash because of NPE, datasnapshot key is valid, it is John's account key but value of snapshot is true. How can i solve that and get from John's data name and surname?
#Edit:
Here is my retrieving code:
public void updateList() {
reference.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot ds : dataSnapshot.getChildren()) {
String userId = ds.getKey();
DatabaseReference userIdRef = FirebaseDatabase.getInstance().getReference().child("users").child(userId);
userIdRef.addChildEventListener(new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
User user = dataSnapshot.child("name").getValue(User.class);
result.add(user);
adapter.notifyDataSetChanged();
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}

To solve this, you need to query your database twice. Please see the code below:
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference membersRef = rootRef.child(teachers).child(teacherId).child("members");
ValueEventListener valueEventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot ds : dataSnapshot.getChildren()) {
String userId = ds.key();
DatabaseReference userIdRef = rootRef.child("users").child(userId);
ValueEventListener eventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
User user = dataSnapshot.getValue(User.class);
Log.d("TAG", user.getName());
}
#Override
public void onCancelled(DatabaseError databaseError) {}
};
userIdRef.addListenerForSingleValueEvent(eventListener);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {}
};
membersRef.addListenerForSingleValueEvent(valueEventListener);

You have 2 approaches to solve this issue:
a.) Inside onChildAdded you already have, if the value you got is true, do another search in database, but this time in "users" node. Something like
newReference = FirebaseDatabase.getInstance().getReference("users").child(dataSnapshot.getKey).child("name")
b.) In the "members" node, write "John Clash" instead of "true". This approach will work for now, but you will fall into the first approach if someday you need to retrieve email address as well

Why is "If Condition" inside onClick not working when using Firebase Database in Android Studio?

a2.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
//below
DatabaseReference ref = FirebaseDatabase.getInstance().getReference();
ref.addListenerForSingleValueEvent(new com.google.firebase.database.ValueEventListener() {
#Override
public void onDataChange(com.google.firebase.database.DataSnapshot dataSnapshot) {
ra2 = dataSnapshot.child("a2").getValue(String.class);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
//fetching face value
ref.addListenerForSingleValueEvent(new com.google.firebase.database.ValueEventListener() {
#Override
public void onDataChange(com.google.firebase.database.DataSnapshot dataSnapshot) {
rface = dataSnapshot.child("face").getValue(String.class);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
//fetching nothingdb which is equals to "zero" in Firebase by default.
ref.addListenerForSingleValueEvent(new com.google.firebase.database.ValueEventListener() {
#Override
public void onDataChange(com.google.firebase.database.DataSnapshot dataSnapshot) {
nothing = dataSnapshot.child("nothingdb").getValue(String.class);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
//fetching addeddb which is equals to "one" in Firebase by default.
ref.addListenerForSingleValueEvent(new com.google.firebase.database.ValueEventListener() {
#Override
public void onDataChange(com.google.firebase.database.DataSnapshot dataSnapshot) {
added = dataSnapshot.child("addeddb").getValue(String.class);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
//starting if condition
if (ra2==nothing){
if (rface==nothing){
a2.setBackgroundResource(R.drawable.rounded);{
Firebase refChild = ref2.child("a2");
refChild.setValue("rounda2");
refChild = ref2.child("face");
refChild.setValue("one");
}
else if (rface==added) {
a2.setBackgroundResource(R.drawable.crossed);
{
Firebase refChild = ref2.child("a2");
refChild.setValue("crossa2");
refChild = ref2.child("face");
refChild.setValue("zero");
}
}
}
});
}
});
I am trying to use the above written code so that once I click on the button the background image of the button is changed based on the data present in the Firebase but the If Condition is not working for reason.
It just ignores if (ra2==nothing){ and also the next if conditions.

The listener onDataChange() callbacks are asynchronous. ra2, nothing, added, and rface will not contain valid results when you compare them for equality because the onDataChange() methods will not yet have executed. This answer to a related question explains the execution order in more detail.
In addition, to compare Strings for equality you cannot use the == operator. You must use the equals() method or TextUtils.equals().

#qbix's answer is true, your if condition actually work but it does not receive the value of ra2, rface, nothing, and added yet.
Also, why should you create 4 different ValueEventListener while you listening to the same reference? You can use just 1 listener then put your if condition inside onDataChange, like this:
a2.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
DatabaseReference ref = FirebaseDatabase.getInstance().getReference();
ref.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
ra2 = dataSnapshot.child("a2").getValue(String.class);
rface = dataSnapshot.child("face").getValue(String.class);
nothing = dataSnapshot.child("nothingdb").getValue(String.class);
added = dataSnapshot.child("addeddb").getValue(String.class);
//starting if condition
if (ra2.equals(nothing)) {
if (rface.equals(nothing)) {
a2.setBackgroundResource(R.drawable.rounded);
Firebase refChild = ref2.child("a2");
refChild.setValue("rounda2");
refChild = ref2.child("face");
refChild.setValue("one");
} else if (rface.equals(added)) {
a2.setBackgroundResource(R.drawable.crossed);
Firebase refChild = ref2.child("a2");
refChild.setValue("crossa2");
refChild = ref2.child("face");
refChild.setValue("zero");
}
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
Log.e(TAG, "onCancelled", databaseError.toException());
}
});
}
});