How to get array from Real-Time Database in Firebase

Question

I want to get data from this firebase-database (the picture below) into an ArrayListOf< Product> when Product class is :

data class Product(
val title:String,
val photoURL:String,
val description:String,
val price:Double
)
//and i want to make an array like this 
val r = arrayListOf<Product>()

so basicly i want to make array list of Firebase_Database_products any help is appreciated :)

Firebase official docs has outlined the methods to query lists in Firebase realtime-database: firebase.google.com/docs/database/android/lists-of-data. Also, here is a useful article outlining the storing & querying: medium.com/a-practical-guide-to-firebase-on-android/… — Neelavar
– Neelavar, Commented Sep 23, 2018 at 2:09

evals · Accepted Answer · 2018-09-23 12:04:13Z

just for readers in future here is the required code in Kotlin:

val products = arrayListOf<Product>()
        val ref = FirebaseDatabase.getInstance().getReference("products")
        ref.addValueEventListener(object : ValueEventListener {
            override fun onDataChange(dataSnapshot: DataSnapshot) {
                for (productSnapshot in dataSnapshot.children) {
                    val product = productSnapshot.getValue(Product::class.java)
                    products.add(product!!)
                }
                System.out.println(products)
            }

            override fun onCancelled(databaseError: DatabaseError) {
                throw databaseError.toException()
            }
        })
    }

and you have to initialize variables in Product class like this :

data class Product(
val title:String = "",
val photo:String = "",
val description:String = "",
val price:Double = -1.0
)

if you leave it without initializing you will get class does not define a no-argument constructor error

Frank van Puffelen · Accepted Answer · 2018-09-23 02:47:30Z

Something like this should do the trick:

DatabaseReference ref = FirebaseDatabase.getInstance().getReference("products");
ref.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        ArrayList<Product> products = new ArrayList<Product>();
        for (DataSnapshot productSnapshot: dataSnapshot.getChildren()) {
            Product product = productSnapshot.getValue(Product.class);
            products.add(product);
        }
        System.out.println(products);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        throw databaseError.toException();
    }
}

A few things to note:

This code throws away the key of each product (p1 and p2 in your JSON). You may need these keys if you later want to modify or delete the product, so should consider storing them in your Product class. Also see: Is there a way to store Key in class which I cast from Firebase object?
You really owe it to yourself and to us to spend some time in the documentation reading up on listeners and queries.

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