How to Read Firebase Database

how can i read firebase realtime database?

You're importing the new v9 functions, but in your code you are then trying to use the older syntax. You'll have to pick one or the other.

In v9 syntax, the ref.on("value" would be:

import { onValue } from "firebase/database";
...
onValue(ref, (pixel) => {
  console.log("read")
}

Also see the Firebase documentation on reading from the Realtime Database with the v9 SDK.

Read Data From Firebase database

Ok, I got it sorted out. the above code in the question is perfectly fine and should work .
I am explaining for later users who might have the same issue due to mistakes which I made.

the singleSnapshot.getValue(User.class); was unable to cast the result to user class because when uploading the data i.e. in setValue I had my
photoUrl equal to null and as a result you can see the photoUrl is not present in my child node. So what I believe is (and maybe someone want correct me on this) that my singleSnapshot.getValue method was unable to cast the result.

So for now I have omitted photoUrl from my User class and will get back to it when I will start working withe relevant module.

Thanks again everyone for all the suggestions and help.

public class User {
String name;
Uri photoURL;
String bloodGroup;
String city;
String country;
double latitude;
double longitude;
boolean availableToDonate;

// Get, set and constructors are obvious here I am just saving some space in the post
}

How to read, write and query data in Firebase Realtime Database using Firebase SDK v9 (Modular)

Disclaimer: The v9 modular SDK is a beta release. The syntax may (or may not) change but I'll try to update the answer if any asap. Benefits of this upgrade can be found here.

The modular SDK cannot be used over CDN and you need to use npm or yarn for the time being. I've tested this in a Vue web-app.

1. Initialize the Firebase App:

import { initializeApp } from "firebase/app";

const firebaseConfig = { 
  apiKey: "<api-key>",
  authDomain: "<project-id>.firebaseapp.com",
  databaseURL: "https://<project-id>.firebaseio.com",
  projectId: "<project-id>",
  storageBucket: "<project-id>.appspot.com",
  messagingSenderId: "0000000000",
  appId: "<app-id>",
}

const app = initializeApp(firebaseConfig)

2. Create a Database Instance and Reference:

import { getDatabase, ref } from "firebase/database"

const db = getDatabase(app) // <-- Pass in the initialized app

//Creating the reference (The path in db you are trying to read/write/update)
const dbRef = ref("/users")

3. Read Operation and Listeners:

You need to use get() method to retrieve data which accepts a query as parameter. If you are simply reading the data without any sorting or filtering, just pass the reference created above in the query(). (Note: Passing the reference directly in get() works in this case)

import { get, query, onValue } from "firebase/database"

const usersSnapshot = await get(query(dbRef)) //This should get the whole users node from db.

//To add a listener you can use the onValue() method like,
onValue(query(dbRef), snapshot => {
  console.log(snapshot.val())
})

//

In v8, the syntax for events looks like .on("event_name"). In v9, all events are a method which can be used this way:

import { onChildAdded, onChildChanged, onChildRemoved } from "firebase/database"

onChildAdded(query(dbRef), (snapshot) => {
  console.log("child added");
  console.log(snapshot.val()); // Logs newly added child
});

onChildChanged(query(dbRef), (snapshot) => {
  console.log("child changed")
  console.log(snapshot.val()); // Logs updated value of changed child
});
 
onChildRemoved(query(dbRef), (snapshot) => {
  console.log("child removed");
  console.log(snapshot.val()); // Logs value of removed child
});

4. Writing Data:

// You maybe have figured out the upgrade trend ¯\_(ツ)_/¯

import { set, update } from "firebase/database"

const newUserName = "user1122"
const userRef = ref(`users/${newUserName}`)
 
await set(userRef, {name: newUserName, age: 11})
await update(userRef, {age: 12})

5. Working with lists:

This is best part and it now kind of feels similar to adding stages in a MongoDB Aggregation.

import { query, orderByChild, orderByValue, orderByKey, limitToLast } from "firebase/database"

//Example: getting 3 youngest users from the db
const usersSnapshot = await get(query(dbRef, ...[orderByChild("age"), limitToFirst(3)]))

I've found no issues using the modular SDK so far. Just make sure you've imported all things which are needed.

PS: Just in-case if you use Vue, React or any framework with hot-reload when developing, you might encounter the "Firebase App named '[DEFAULT]' already exists (app/duplicate-app)" error. Checking if firebase.apps.length is 0 helps and that's done this way in modular SDK:

import { getApps, initializeApp } from "firebase/app"

if (!getApps().length) initializeApp(firebaseConfig)

How to read data from Firebase Database? - Java

To be able to read the data under the jjjj node, please use the following lines of code:

DatabaseReference db = FirebaseDatabase.getInstance().getReference();
DatabaseReference nameRef = db.child("players").child("jjjj");
nameRef.get().addOnCompleteListener(new OnCompleteListener<DataSnapshot>() {
    @Override
    public void onComplete(@NonNull Task<DataSnapshot> task) {
        if (task.isSuccessful()) {
            DataSnapshot snapshot = task.getResult();
            String loses = snapshot.child("loses").getValue(Long.class);
            String name = snapshot.child("name").getValue(String.class);
            String wins = snapshot.child("wins").getValue(Long.class);
            Log.d("TAG", loses + "/" + name + "/" + wins);
        } else {
            Log.d("TAG", task.getException().getMessage()); //Never ignore potential errors!
        }
    }
});

The result in the logcat will be:

3/jjjj/4

Things to notice:

Always create a reference that points to the node that you want to read.
If your database is located in another lcoation than the default, check this answer out.

firebase realtime database, how to read only own data?

If the keys directly under notifications are the Firebase Authentication UIDs (as your rules suggest), you can limit access to only the user's node with:

{
  "rules": {
    //".read": "auth != null", // Remove this line
    ".write": false,
    "notifications":{
      "$user_id":{
        ".read": "auth.uid === $user_id", // add this line
        ".indexOn":"createdAt"
      }
    }
  }
}

Also see the Firebase documentation on securing content-owner only access.

How to Read Data From Firebase Realtime Database?

The error seems pretty clear. You're calling var keys = snap.value.keys; and there is no keys on the snap.value. From this, it seems that you think snap.value is a Map object, which doesn't seem to be the case.

From looking at your data structure, it is more likely that snap.value is actually a List object.

This is because Firebase interprets objects with sequential, numeric keys are arrays. If this is indeed the case (something you can easily verify by running the code in a debugger), you can iterate over the list with:

for (var child in snap.value){

Also see:

Best Practices: Arrays in Firebase.