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"println needs a message" error in android

I want to read Music_ID of the group with Playlist_ID of 2 in firebase.
The following error occurs.
java.lang.NullPointerException: println needs a message
This is my firebase realtime database.
And this is my code.
database = FirebaseDatabase.getInstance();
storage = FirebaseStorage.getInstance();
dref = FirebaseDatabase.getInstance().getReference();
private void Startplaylist(String mood) {
DatabaseReference plist = dref.child("Playlist");
plist.orderByChild("Playlist_ID").equalTo(2).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
// Log.i("Value", dataSnapshot.getValue().toString());
String music_id = dataSnapshot.child("Music_ID").getValue(String.class);
Log.i("Value_id", music_id);
str_musictitle.setText(music_id);
}
#Override
public void onCancelled(#NonNull DatabaseError error) {
}
});
}
An alarm pops up that an error occurs in this part.
Log.i("Value_id", music_id);
I think "music_id" is not being read.
I tried to change part
String music_id = dataSnapshot.child("Music_ID").getValue(String.class);
to String music_ids = dataSnapshot.child("Music_ID").getValue().toString(); and run it, but I couldn't get the desired result.

When you execute a query against the Firebase Database, there will potentially be multiple results. So the snapshot contains a list of those results. Even if there is only a single result, the snapshot will contain a list of one result.
The code in your onDataChange will need to handle this list by looping over dataSnapshot.getChildren(). Something like this:
DatabaseReference plist = dref.child("Playlist");
plist.orderByChild("Playlist_ID").equalTo(2).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot: dataSnapshot.getChildren()) { // 👈 Loop over results
String music_id = snapshot.child("Music_ID").getValue(String.class); // 👈 Get value for this result
Log.i("Value_id", music_id);
str_musictitle.setText(music_id);
}
}
#Override
public void onCancelled(#NonNull DatabaseError error) {
throw error.toException(); // 👈 Never ignore possible errors
}
});

Detect value change with firebase ValueEventListener

I am trying to detect value change from my Firebase Database. Here is my code for initializing the ValueEventListener:
valueEventListener = new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
try {
String customerLocation = String.valueOf(dataSnapshot.getValue());
Point customerPoint = locationFounder.getPoint(customerLocation);
if (customerPoint != null) {
databaseReference.child("isActive").addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
boolean isActive = Boolean.valueOf(String.valueOf(dataSnapshot.getValue()));
displayPopUp(isActive, customerPoint, customerLocation);
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
}
} catch (Exception e) {
Log.d("Listener",e.toString());
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
};
destinationReference.addValueEventListener(valueEventListener);
Problem occurs when I want to call this listener in my activity. I've been trying with this:
destinationListener.getValueEventListener().onDataChange(REQUIRED_SNAPSHOT);
I do not know how can I get datasnapshot which is required for onDataChange. I would like to get this work with ValueEventListener, not ChildEventListener, if possible. However, I am not pretty sure that this is the right way of trying to detect value change. If there is any other way that will work properly, I'd like to know about it.

There is nothing built to the Firebase Realtime Database to tell you what specific data under the snapshot has changed in the onDataChange method.
If you want to know what specific property has change, you'll need to:
Keep the snapshot that you get in onDataChange in a field.
When onDataChange gets called, compare the data in the new snapshot with the data in the field.
Say that you have a reference on a node in your JSON, and under that node is a status property, and you want to both listen to the entire node, and detect if the status has changed.
You'd do that with something like:
// Add a field to your class to keep the latest snapshot
DataSnapshot previousSnapshot;
// Then add your listener
databaseReference.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
bool wasActive = false;
if (previousSnapshot != null && previousSnapshot.child("status").exists()) {
wasActive = dataSnapshot.child("status").getValue(Boolean.class);
}
boolean isActive = dataSnapshot.child("status").getValue(Boolean.class);
if (isActive <> wasActive) {
... the user's status changed
}
previousSnapshot = dataSnapshot;
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
throw databaseError.toException(); // never ignore errors
}
});

Retrieve the children of a parent node and perform a method on each value

I am building an app that needs to retrieve all child from a certain parent node and would like to perform a method call on each child retrieved.
Here is my current database:
Click Me
I would like to call all child of conradjr and on retrieval of a child for example "ranniecardino15", it should store it to a variable then perform a method call. How can I do this?
I have made a code like this but the method call "getCurrentLocation" is only performed on the last child retrieved.
public void getCurrentChildUser() {
DatabaseReference getuser = FirebaseDatabase.getInstance().getReference().child("Children");
getuser.child(user).orderByChild("CurrentLocation").addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
for (DataSnapshot childSnapshot : dataSnapshot.getChildren()) {
childuser = childSnapshot.getKey();
if (childuser != null){
getCurrentLocation();
}
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
}
public void getCurrentLocation(){
DatabaseReference getuser = FirebaseDatabase.getInstance().getReference().child("Children");
getuser.child(user).child(childuser).child("CurrentLocation").addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
if(dataSnapshot != null){
currentloc = dataSnapshot.getValue(String.class);
System.out.println("Current Location: "+currentloc);
String split[] = currentloc.split(",");
lat1 = Double.parseDouble(split[0]);
lng1 = Double.parseDouble(split[1]);
System.out.println("Current Latitude: "+lat1+" and Current Longitude: "+lng1);
getSavedLocation();
}
}
#Override
public void onCancelled(DatabaseError databaseError) {}
});
}
Please help me :) Thanks in advance :)

To solve this, you need to change the declaration of your getCurrentLocation() method from:
public void getCurrentLocation(){}
to
public void getCurrentLocation(String childuser){}
Now you need to call this method from inside the onDataChange() method like this:
getCurrentLocation(childuser);
See, I have passed the childuser as an argument to the getCurrentLocation() so it can be used in a proper way.

Getting list of items/children from Firebase Database?

I'm running into some trouble figuring out how to get the number of items/children in my Firebase real-time database for my app to use as an int value.
Check the image below:
So as you can see above, I have 2 children of images_list.
What I want to do is: get the number of items/children of images_list returned to my app, which would obviously equal 2 initially, and have this number expand whenever I add more items/children.
So my question is, what code do I implement to grab the number of children in this database? And how could I then convert this number into an int value? I've already experimented with differing methods, but I haven't found a solution as of yet.
I hope that made sense. Thanks so much for any of your assistance!
Code solution down here; I had to move most of this code from my RecyclerView Adapter into my MainActivity for it to work
Interface
public interface FirebaseCallback {
void onCallback(List<String> list);
}
readData method
private void readData(final FirebaseCallback firebaseCallback) {
mDatabaseImagesRef = FirebaseDatabase.getInstance().getReference("images_list");
ValueEventListener valueEventListener = new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
for (DataSnapshot ds : dataSnapshot.getChildren()) {
String imageItems = ds.getValue(String.class);
imageList.add(imageItems);
}
firebaseCallback.onCallback(imageList);
Log.i("imageList.size() is: ", String.valueOf(imageList.size()));
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
Log.i(LOG_TAG, databaseError.getMessage());
}
};
mDatabaseImagesRef.addListenerForSingleValueEvent(valueEventListener);
}
called in onCreate()
readData(new FirebaseCallback() {
#Override
public void onCallback(List<String> list) {
mImageAdapter.notifyDataSetChanged();
}
});
getItemCount() in RecyclerViewAdapter
#Override
public int getItemCount() {
return imagesList.size();
}

To count the all the children beneath images_list node, please use the following lines of code:
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference imagesListRef = rootRef.child("images_list");
ValueEventListener valueEventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
int count = (int) dataSnapshot.getChildrenCount(); //Cast long to int
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
Log.d(TAG, databaseError.getMessage());
}
};
imagesListRef.addListenerForSingleValueEvent(valueEventListener);
how could I then convert this number into an int value?
According to the official documentation, getChildrenCount() method returns a long and not an int. So you need to cast that primitive long to an int.
how to extract count from this block of code so that it can be referenced and used outside this code as well?
You cannot simply create the count variable as a global variable and use it outside the onDataChange() method because it will always be 0 due the asynchronous behaviour of this method. This means that if try to use its result outside this method, the data hasn't finished loading yet from the database and that's why is not accessible. With other words, your count will always be 0.
A quick solve for this problem would be to use the value of your count variable only inside the onDataChange() method, otherwise I recommend you see the last part of my anwser from this post in which I have explained how it can be done using a custom callback. You can also take a look at this video for a better understanding.

try this
userReference = FirebaseDatabase.getInstance().getReference("images_list");
userReference.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
num = dataSnapshot.getChildrenCount();
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});

mRef= FirebaseDatabase.getInstance().getReference("images_list");
on your "onDataChange" method inside the listener , use getChildrenCount() to get the number of images, you can iterate those children using i loop , Exmeple :
mRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
int count = dataSnapshot.getChildrenCount();
// here you get your images number
for(DataSnapshot ds : dataSnapshot.getChildren()) {
String key = ds.getKey()
// and here you get the key
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});

Try this code:
public void onDataChange(#NonNull DataSnapshot snap) {
int count = (int) snap.getChildrenCount();
//shows the number of items
}

retrieve some data from the firebase database with some nodes being null

cBankRef=myRef.child("user_id").child("ASSETS");
cBankRef.addChildEventListener(new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
String value = dataSnapshot.child("1").child("description").getValue(String.class);
Log.d(TAG, "onChildAdded: the value is"+value);
if(dataSnapshot.child("2").exists()) {
String valuess = dataSnapshot.child("3").child("description").getValue(String.class);
Log.d(TAG, "onChildAdded: the value is: " + valuess);
}
if (dataSnapshot.child("3").exists()) {
String values = dataSnapshot.child("1").child("description").getValue(String.class);
Log.d(TAG, "onChildAdded: the cash at bank is: "+values);
}
}
#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) {
}
});
first i accessed the user_id child and the ASSETS then through the nodes ("1"),("2") i had to add other nodes in order to get the value, the("1") and ("2") were ("Cash at Bank") ("stock") in the beginning but i decided to change them to numbers but it's giving null #makrand pawar

You have multiple issues in your example:
Your cBankRef is pointing to \user_id\ASSETS\Cash at bank\stock\Cash in Hand, which does not exist (Cash at bank node does not exist under ASSETS).
You are then attaching a ChildEventListener to this reference, which won't return any values, because there aren't any children under it (as it doesn't exist).
Inside onChildAdded(), you are trying to access the houses node which also doesn't exist in your example (house does though).
To access the children under the Stock node, you'll need something like:
ref = FirebaseDatabase.getInstance().getReference().child("user_id").child("ASSETS").child("Stock");
ref.addChildEventListener(new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
String value = dataSnapshot.child("house").child("values").getValue(String.class);
Log.d(TAG, "onChildAdded: the value is: "+value);
}
// ...
});
The DatabaseReference#child() method is not a SELECT statement. To access everything underneath a node, you only need to create a reference to the highest node required, then attach a listener to that.
For example, if you need access to everything underneath the ASSETS node, you could attach a listener there instead:
ref = FirebaseDatabase.getInstance().getReference().child("user_id").child("ASSETS");
ref.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
String value = dataSnapshot.child("Stock").child("house").child("values").getValue(String.class);
Log.d(TAG, "onChildAdded: the value is: "+value);
if (dataSnapshot.child("Cash at bank").exists()) {
String value = dataSnapshot.child("Cash at bank").getValue(String.class);
Log.d(TAG, "onChildAdded: the cash at bank is: "+value);
}
}
// ...
});
Then when the listener returns you can use DataSnapshot#child() instead to access the individual values retrieved from that location. Also, using DataSnapshot#exists() is a simple way to check that a node exists before using it.
If you plan to have additional values or nodes under ASSETS that don't currently exist, they will be returned by the above listener as soon as they are created.

To achieve this, i suggest you using ValueEventListener like this:
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference stockRef = rootRef.child("user_id").child("ASSETS").child("Stock");
ValueEventListener eventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot ds : dataSnapshot.getChildren()) {
String values = ds.child("house").child("values").getValue(String.class);
Log.d("TAG", values);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {}
};
stockRef.addListenerForSingleValueEvent(eventListener);