We Keep Coding

Data retrieving error while i want to get all child from a certain Key

I want to read some specific child from the parent-child, by getchild() function but this will not work properly.
FirebaseUser FUser = mAuth.getCurrentUser();
String userid = FUser.getUid();
DatabaseReference DR;
DR = FirebaseDatabase.getInstance().getReference().child("HistoryTable").child(userid);
DR.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
// Iterable<DataSnapshot> root = dataSnapshot.getChildren();
// Toast.makeText(getApplicationContext(), "ds "+dataSnapshot.getChildren(),Toast.LENGTH_LONG).show();
for (DataSnapshot ds: dataSnapshot.getChildren()) {
// Toast.makeText(getApplicationContext(), "ds "+ds,Toast.LENGTH_LONG).show();
for (DataSnapshot d: ds.getChildren()) {
String Height = d.getKey() + d.getValue() + "\n".toString();
String ch = d.child("1Height:").getValue(String.class);
// tv.append(Height);
tv.append(ch);
}
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
I want to get this four child from every key.

you have to add your push id in reference :-
DR = FirebaseDatabase.getInstance().getReference().child("HistoryTable").child(userid).child("push id");
DR.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
// Iterable<DataSnapshot> root = dataSnapshot.getChildren();
// Toast.makeText(getApplicationContext(), "ds "+dataSnapshot.getChildren(),Toast.LENGTH_LONG).show();
for (DataSnapshot ds: dataSnapshot.getChildren()) {
// Toast.makeText(getApplicationContext(), "ds "+ds,Toast.LENGTH_LONG).show();
for (DataSnapshot d: ds.getChildren()) {
String Height = d.getKey() + d.getValue() + "\n".toString();
String ch = d.child("1Height:").getValue(String.class);
// tv.append(Height);
tv.append(ch);
}
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});

Try like this if you just want to read the value. if you want to read data only once use addListenerForSingleValueEvent()
*Try to read values by object stucture https://firebase.google.com/docs/database/android/read-and-write#basic_write
DR.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
for (DataSnapshot d: ds.getChildren()) {
//below line may cause null pointer Exception
String Height = d.getKey() + d.getValue() + "\n".toString();
if(d.child("1Height:").getValue()!=null){
String ch = d.child("1Height:").getValue(String.class);
//tv.append(Height);
tv.append(ch);}
}
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});

To get the value of your 4UserId property, simply use the following lines of code:
String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference uidRef = rootRef.child("HistoryTable").child(uid);
ValueEventListener valueEventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot ds : dataSnapshot.getChildren()) {
String userId = ds.child("4UserId").getValue(String.class);
Log.d(TAG, userId);
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
Log.d(TAG, databaseError.getMessage()); //Don't ignore errors!
}
};
uidRef.addListenerForSingleValueEvent(valueEventListener);
The result in your logcat will be:
2Wwrjx...P2obFO83

Why am I receiving empty strings after receiving valid data from Firebase?

Using Firebase, I'm trying to display to the user people they have matched with. I already have valid data for testing this and I have already run tests with sample data to see if everything else works.
Now, when I use real data from Firebase, problems occur.
This is what I have for code:
public String username = "";
public String profileImage = "";
public String discussion = "";
private void FetchMatchInformation(final String key, final String choice) {
DatabaseReference matchDb = FirebaseDatabase.getInstance().getReference().child("answers").child(key);
matchDb.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
String opposite = postSnapshot.getValue(String.class);
if(opposite.equals("agree") && choice.equals("disagree")) {
DatabaseReference found = FirebaseDatabase.getInstance().getReference().child("users").child(postSnapshot.getKey());
found.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
if(postSnapshot.getKey().equals("username")) {
username = postSnapshot.getValue(String.class);
}
if(postSnapshot.getKey().equals("providerId")) {
String provider = postSnapshot.getValue(String.class);
if(provider.equals("google.com")) {
Uri photoUrl = FirebaseAuth.getInstance().getCurrentUser().getPhotoUrl();
String originalPieceOfUrl = "s96-c/photo.jpg";
String newPieceOfUrlToAdd = "s400-c/photo.jpg";
String photoPath = photoUrl.toString();
String newString = photoPath.replace(originalPieceOfUrl, newPieceOfUrlToAdd);
profileImage = newString;
} else if(provider.equals("facebook.com")) {
profileImage = FirebaseAuth.getInstance().getCurrentUser().getPhotoUrl().toString() + "?type=large";
}
}
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
FirebaseDatabase.getInstance().getReference().child("debates").addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
if(postSnapshot.getKey().equals(key)) {
discussion = postSnapshot.getValue(String.class);
break;
}
break;
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
MessagesObject objDisc = new MessagesObject(username, discussion, profileImage);
resultsMessages.add(objDisc);
mMessagesAdapter.notifyDataSetChanged();
} else if(opposite.equals("disagree") && choice.equals("agree")) {
DatabaseReference found = FirebaseDatabase.getInstance().getReference().child("users").child(postSnapshot.getKey());
found.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
if(postSnapshot.getKey().equals("username")) {
username = postSnapshot.getValue(String.class);
}
if(postSnapshot.getKey().equals("providerId")) {
String provider = postSnapshot.getValue(String.class);
if(provider.equals("google.com")) {
Uri photoUrl = FirebaseAuth.getInstance().getCurrentUser().getPhotoUrl();
String originalPieceOfUrl = "s96-c/photo.jpg";
String newPieceOfUrlToAdd = "s400-c/photo.jpg";
String photoPath = photoUrl.toString();
String newString = photoPath.replace(originalPieceOfUrl, newPieceOfUrlToAdd);
profileImage = newString;
} else if(provider.equals("facebook.com")) {
profileImage = FirebaseAuth.getInstance().getCurrentUser().getPhotoUrl().toString() + "?type=large";
}
}
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
FirebaseDatabase.getInstance().getReference().child("debates").addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
if(postSnapshot.getKey().equals(key)) {
discussion = postSnapshot.getValue(String.class);
break;
}
break;
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
MessagesObject objDisc = new MessagesObject(username, discussion, profileImage);
resultsMessages.add(objDisc);
mMessagesAdapter.notifyDataSetChanged();
}
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
}
So what's happening is I'm getting the user's username, their profile image, and figuring out the value, which I called discussion after determining the valid key. These values are all being accessed from different tree structures from the same real-time database.
Now, at the end of the if or else-if statement, I create and instantiate the MessagesObject by passing in the username, profile image, and discussion variables. I then add this object to the List<MessagesObject> called resultMessages. I then notify my custom adapter mMessagesAdapter that the data has changed.
Like I said, all the other pieces of code work perfectly fine. It's just when I pass username, discussion, and profileImage it always passes an empty string. I know this from using the debugger.
Why is that? It should not be doing that.

Firebase RealTime Database Delete First 20 item

DatabaseReference newReferance = database.getReference().child("Users");
Query query = newReferance.orderByChild("timestamp");
query.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
chatMessages.clear();
for (DataSnapshot dataSnapshot1 : dataSnapshot.getChildren()) {
HashMap<String, String> hashMap = (HashMap<String, String>) dataSnapshot1.getValue();
//String userMail = hashMap.get("useremail");
String userMessage = hashMap.get("usermessage");
String userCt = hashMap.get("usershow");
chatMessages.add(userCt + ": " + userMessage);
if (chatMessages.size() >= 10){
for (int i= 0; i < 5; i++){
//HOW ????
}
}
recyclerAdapter.notifyDataSetChanged();
}
}
Hi Dear guys. How do I select and delete the first 5 data in a real-time database? When the ArrayList reaches a certain limit, I want to delete the first 5 data from the database. I'd appreciate it if you could help with the problem.

How do I select and delete the first 5 data in a real-time database?
You can achieve this using Firebase Query's limitToFirst(int limit):
Create a query with limit and anchor it to the start of the window
And in code should look like this:
Query query = newReferance.orderByChild("timestamp").limitToFirst(5);
See I have used this limitToFirst() method that can help you find the first 5 items. To delete them, just attach a listener and inside the onDataChange() method use the following line of code:
dataSnapshot.getRef().removeValue();

final DatabaseReference newReferance = database.getReference().child("Users");
final Query query = newReferance.orderByChild("timestamp");
query.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
chatMessages.clear();
for (DataSnapshot dataSnapshot1 : dataSnapshot.getChildren()) {
HashMap<String, String> hashMap = (HashMap<String, String>) dataSnapshot1.getValue();
//String userMail = hashMap.get("useremail");
String userMessage = hashMap.get("usermessage");
String userCt = hashMap.get("usershow");
chatMessages.add(userCt + ": " + userMessage);
if (chatMessages.size() >= 10){
Query query1 = newReferance.limitToFirst(5);
query.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
database.getReference().child("Users").setValue(null);
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
}
recyclerAdapter.notifyDataSetChanged();
}
}
Although I'm set, it's all deleted. I can't erase the items I want. All cleared.

rootRef is the firebase database reference. whose instance we have to gained before trying to delete any value. you can see the code given below.
count = 0;
rootRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot snapshot) {
for (DataSnapshot postSnapshot : snapshot.getChildren()) {
CommonLocationClass user = postSnapshot.getValue(CommonLocationClass.class);
if (count < 5) {
rootRef.child(user.getPhNo).removeValue();
count++;
}
}
}
}
#Override
public void onCancelled (DatabaseError firebaseError){
}
});
the CommonLocationClass is a getter setter model class i have made to make the fire-base implementation easy
you can try this it works for me

public void getDataFirebase() {
/*if (chatMessages.size() == MAX_MSJ_LIMIT){
database.getReference().child("Users").removeValue();
}*/
final DatabaseReference newRefere = database.getReference().child("Users");
Query query = newRefere.orderByChild("timestamp");
query.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
chatMessages.clear();
for (DataSnapshot dataSnapshot1 : dataSnapshot.getChildren()) {
HashMap<String, String> hashMap = (HashMap<String, String>) dataSnapshot1.getValue();
//String userMail = hashMap.get("useremail");
String userMessage = hashMap.get("usermessage");
String userCt = hashMap.get("usershow");
chatMessages.add(userCt + ": " + userMessage);
recyclerAdapter.notifyDataSetChanged();
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
Toast.makeText(getApplicationContext(), databaseError.getMessage(), Toast.LENGTH_LONG).show();
}
});
if (chatMessages.size() >= 10) {
Query query1 = newRefere.orderByChild("timestamp").limitToFirst(5);
query1.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
dataSnapshot.getRef().removeValue();
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
}
}
It didn't just erase the first 5 data.

if (chatMessages.size() >= 10) {
Query query1 = newRefere.orderByChild("timestamp").limitToFirst(5);
query1.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
for (DataSnapshot ds : dataSnapshot.getChildren()){
ds.getRef().removeValue();
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
I solved the problem. With this code

how i can get my info through the random key by push

I have JsonString by receive DataSnapShot:
myRef.child("user").orderByChild("u_email").equalTo(name).addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
String c = dataSnapshot.getValue().toString();
Log.w(Login.TAG, "Data: "+ c);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
in normal if i know the key (-L3Mh7kKl04GSrndbrVD) ill code:
JSONObject a = new JSONObject(c);
JSONObject jsonUser = a.getJSONObject("-L3Mh7kKl04GSrndbrVD");
But i dont have that key, How i can get that value (name, pass, img, ...) with code, im newbie. Thanks all alot

try this:
DatabaseReference references=FirebaseDatabase.getInstance().getReference().child("user");
references.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot data: dataSnapshot.getChildren()){
String name=data.child("name").getValue().toString();
String password=data.child("pass").getValue().toString();
String img=data.child("img").getValue().toString();
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
Here the reference is the user node, then using the for loop it will iterate inside every push id to give you the values (name,img,pass,...).

Try this
myRef.child("user").orderByChild("u_email").equalTo(name).addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
String c = dataSnapshot.getValue().toString();
Log.w(Login.TAG, "Data: "+ c);
for(DataSnapshot ds : dataSnapshot.getChildren()) {
String u_img = ds.child("u_img").getValue(String.class);
String u_pass = ds.child("u_pass").getValue(String.class);
String u_address = ds.child("u_address").getValue(String.class);
Log.d("TAG",u_img + " / " + u_pass + " / " + u_address);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});

Firebase Database reference to event listener

Hi in my Firebase database I am trying to access one child "Chatmessages", gathering all its keys and using them to locate their corresponding keys in the child "Users" and gathering its information. I am doing this this through querying twice using eventlisteners. Although how would I apply this to chatIDref in my loadUserlist() method.
Structure of database:
rootRef = FirebaseDatabase.getInstance().getReference();
chatIdRef = rootRef.child("Chatmessages").child(last);
eventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot ds : dataSnapshot.getChildren()) {
String uid = ds.getKey();
usersRef = rootRef.child("Users").child(uid);
ValueEventListener eventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
String image = dataSnapshot.child("image").getValue(String.class);
String name = dataSnapshot.child("name").getValue(String.class);
Boolean onlineStatus = dataSnapshot.child("onlineStatus").getValue(Boolean.class);
String status = dataSnapshot.child("username").getValue(String.class);
Log.d("TAG", image + " / " + name + " / " + onlineStatus + " / " + status);
}
#Override
public void onCancelled(DatabaseError databaseError) {}
};
usersRef.addListenerForSingleValueEvent(eventListener);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {}
};
chatIdRef.addListenerForSingleValueEvent(eventListener);
chatIdRef.keepSynced(true);
userlist.setHasFixedSize(true);
userlist.setLayoutManager(new LinearLayoutManager(getContext()));
return userView;
}
public void loadUserList() {
FirebaseRecyclerAdapter<User, UserViewHolder> firebaseRecyclerAdapter = new FirebaseRecyclerAdapter<User, UserViewHolder>(
User.class,
R.layout.userlist_layout,
UserViewHolder.class,
chatIdRef
)