Firebase RealTime Database Notes

 useful videos/Blogs :

https://youtu.be/T5P2jn75G6g

https://firebase.google.com/docs/database/android/read-and-write?authuser=0

Firebase database structure : https://youtu.be/T5P2jn75G6g





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IMPORTANT POINTS  for FIREBASE REALTIME DATABASE :


Firebase realtime database is a Key-Value based NO-SQL database.It stores data in the form of JSON tree , which means that the entire database is a single Json object.

This Json tree consists of Parent nodes and Child nodes,whenever we would be inserting data into the database we would be either adding a node to the database or updating an existing node. Each node has a Key and a Value associated.

The Keys are mostly strings whereas the Values associated with that Keys can be String,Int, etc or even a new Child node.

NOTE : Two keys at the same context cannot have same values.


Users is a parent node of User1 and firstname/lastname are child of User1 and User2.
The Top most node is called Root Node.

NOTE : You can also get all the json data by using the link provided by firebase & use it as a rest api. Just put ".json" at the end of the link. In our case it would be like this : https://exampleapp4-default-rtdb.firebaseio.com/.json

You can also get specific data from the json tree by modifiying the url link : Below is the link that will get us User1's json data.

https://exampleapp4-default-rtdb.firebaseio.com/Users/User1/.json


During Data fetching Data in Most cases is returned as HashMaps inside a DataSnapshot Object.


All Database operations are done on the Background thread by the Firebase itself.


Be Carreful with Event Listeners , make sure that one location has only one event listener attached on it ( For more Watch ) - https://youtu.be/tjX1TrHRuIs

( check addChildEventListner() Blog Below)



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SIMPLE INSERT OPERATION TO FIREBASE REALTIME DATABASE


After you have connected your app with your firebase & added all the required dependencies.

All you have to do is Create  object of database class & database reference class.



package com.deepesh.firebaselearningapp;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.text.Editable;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;

public class MainActivity extends AppCompatActivity {

    EditText editText;
    Button button1;
    TextView textview1;

    // Gets reference to entire Database.
    FirebaseDatabase firebaseDatabase;

    // Gets reference to specific Node , where to READ/WRITE data
    DatabaseReference databaseReference;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        editText=findViewById(R.id.editText1);

        button1=findViewById(R.id.button1);

        textview1=findViewById(R.id.textView1);

        firebaseDatabase = FirebaseDatabase.getInstance();
        databaseReference=firebaseDatabase.getReference();

        // getReference() - gives reference to root node of database.
        // getReference(String) - gives reference to specified node.

    }

    //button click function
    public void RunCode(View view) {

        String text = editText.getText().toString();
        databaseReference.setValue(text);

        Toast.makeText(this, "DATA INSERTED !", Toast.LENGTH_SHORT).show();
        Log.d("TAG", "RunCode: DATA INSERTED");

    }
}


<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <EditText
        android:id="@+id/editText1"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="73dp"
        android:ems="10"
        android:hint="Enter data..."
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="42dp"
        android:text="Ready to code"
        android:textSize="30sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText1" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="42dp"
        android:layout_marginBottom="35dp"
        android:onClick="RunCode"
        android:text="Cick"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>


SUCESS & FAILURE EVENT LISTENER 



//button click function
public void InsertData(View view) {

    String text = editText.getText().toString();

    // Gets a reference to any specified child node inside the
    // Parent node reference specified inside getReference()
    // Creates one if not present & sets value.
    databaseReference.child("user1").setValue(text)

            // EXECUTED ON SUCESSFUL INSERTION

            .addOnSuccessListener(new OnSuccessListener<Void>() {
                @Override
                public void onSuccess(Void aVoid) {

                    Log.d("TAG", "RunCode: DATA INSERTED");

                }
            })

            // EXECUTED ON UN-SUCESSFUL INSERTION

            .addOnFailureListener(new OnFailureListener() {
                @Override
                public void onFailure(@NonNull Exception e) {

                    Log.d("TAG", "FAILURE : " + e.getMessage());

                }
            });


}




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SIMPLE READ OPERATION TO FIREBASE REALTIME DATABASE


We use the Event Listeners to read the data.

Two Common listner are -  addValueEventListener() & addListenerForSingleValueEvent( ).





package com.deepesh.firebaselearningapp;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

public class MainActivity extends AppCompatActivity {

    EditText editText;
    Button insertbutton,readbutton;
    TextView textview1;

    // Gets reference to entire Database.
    FirebaseDatabase firebaseDatabase;

    // Gets reference to specific Node.
    DatabaseReference databaseReference;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        editText=findViewById(R.id.editText1);

        insertbutton=findViewById(R.id.insert_button);
        readbutton=findViewById(R.id.read_button);

        textview1=findViewById(R.id.textView1);

        firebaseDatabase = FirebaseDatabase.getInstance();
        databaseReference=firebaseDatabase.getReference();

        // getReference() - gives reference to root node of database.
        // getReference(String) - gives reference to specified node.

    }

    //button click function
    public void InsertData(View view) {

        String text = editText.getText().toString();
        databaseReference.setValue(text);

        Toast.makeText(this, "DATA INSERTED !", Toast.LENGTH_SHORT).show();
        Log.d("TAG", "RunCode: DATA INSERTED");

    }

    //button click function
    public void ReadData(View view) {

        // addValueEventListener is called whenever value at given reference node is changed.
        // onDataChange() is passed the latest data.
        databaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {

                // we are taking string so pass a "String" class.
                String data = snapshot.getValue(String.class);
                textview1.setText(data);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });

    }
}


<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <EditText
        android:id="@+id/editText1"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="73dp"
        android:ems="10"
        android:hint="Enter data..."
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="42dp"
        android:text="Ready to code"
        android:textSize="30sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText1" />

    <Button
        android:id="@+id/insert_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="42dp"
        android:layout_marginBottom="36dp"
        android:onClick="InsertData"
        android:text="Insert"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/read_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="54dp"
        android:layout_marginBottom="36dp"
        android:text="Read"
        android:onClick="ReadData"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>


EXAMPLE 2 : 


<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <EditText
        android:id="@+id/firstname_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="210dp"
        android:layout_marginBottom="207dp"
        android:hint="Enter Firstname"
        app:layout_constraintBottom_toBottomOf="@+id/button1"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/lastname_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="51dp"
        android:hint="Enter lastname"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/firstname_editText" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="108dp"
        android:text="Hello World !"
        android:textSize="20sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="120dp"
        android:layout_marginBottom="46dp"
        android:onClick="InsertData"
        android:text="Insert Data"
        app:layout_constraintBottom_toTopOf="@+id/button2"
        app:layout_constraintStart_toStartOf="@+id/lastname_editText" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="7dp"
        android:layout_marginBottom="101dp"
        android:onClick="ReadData"
        android:text="Read Data"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="@+id/button1" />

</androidx.constraintlayout.widget.ConstraintLayout>



package com.deepesh.exampleapp4;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.Map;

public class MainActivity extends AppCompatActivity {

    TextView textview1;
    EditText firstname_edittext;
    EditText lastname_edittext;
    DatabaseReference databaseReference;

    final String ROOT_NODE = "Users";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        textview1 = findViewById(R.id.textView1);
        firstname_edittext = findViewById(R.id.firstname_editText);
        lastname_edittext = findViewById(R.id.lastname_editText);

        databaseReference = FirebaseDatabase.getInstance().getReference(ROOT_NODE);

    }

    // Button onClick() function
    public void InsertData(View view) {

        String fname = firstname_edittext.getText().toString();
        String lname = lastname_edittext.getText().toString();
        // Creating a new User
        databaseReference.child("User3").child("firstname").setValue(fname);
        databaseReference.child("User3").child("lastname").setValue(lname);

        Log.d("TAG", "InsertData: DATA INSERTED SUCESSFULLY !");

    }


    // Button onClick() function
    public void ReadData(View view) {

        databaseReference.child("User3").addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {

                // retusn data as hashmap
                Map<String, Object> data = (Map<String, Object>) snapshot.getValue();

                String fname = (String) data.get("firstname");
                String lname = (String) data.get("lastname");

                textview1.setText("FIRSTNAME : " + fname + " LASTNAME : " + lname);

            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });

    }
}


// Unlike addValueEventListner() which is called everytime the value changes,
        // addListenerForSingleValueEvent() is called only when we call it.





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useful video : https://youtu.be/r_LREdNw6p4


SIMPLE CREATE  &  WRITE / READ  A CHILD NODE 


(UI  same as above)


package com.deepesh.firebaselearningapp;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

public class MainActivity extends AppCompatActivity {

    EditText editText;
    Button insertbutton,readbutton;
    TextView textview1;

    // Gets reference to entire Database.
    FirebaseDatabase firebaseDatabase;

    // Gets reference to specific Node.
    DatabaseReference databaseReference;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        editText=findViewById(R.id.editText1);

        insertbutton=findViewById(R.id.insert_button);
        readbutton=findViewById(R.id.read_button);

        textview1=findViewById(R.id.textView1);

        firebaseDatabase = FirebaseDatabase.getInstance();
        databaseReference=firebaseDatabase.getReference();

        // getReference() - gives reference to root node of database.
        // getReference(String) - gives reference to specified node.

    }

    //button click function
    public void InsertData(View view) {

        String text = editText.getText().toString();

       // Gets a reference to any specified child node inside the
        // Parent node reference specified inside getReference()
        // Creates one if not present & sets value.
        databaseReference.child("user1").setValue(text);

        Toast.makeText(this, "DATA INSERTED !", Toast.LENGTH_SHORT).show();
        Log.d("TAG", "RunCode: DATA INSERTED");

    }

    //button click function
    public void ReadData(View view) {

        // Unlike addValueEventListner() which is called everytime the value changes,
        // addListenerForSingleValueEvent() is called only when we call it.

        databaseReference.child("user1").addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {

                // we are taking string so pass a "String" class.
                String data = snapshot.getValue(String.class);
                textview1.setText(data);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });

    }
}





@Override
protected void onDestroy() {
    super.onDestroy();

    // IMPORTANT - other wise may cause Memory Leaks !
    databaseReference.child("user1").removeEventListener(listener);
}



--------------------------------------------------------------------------------------------


INSERT & READ MULTIPLE CHILD NODES

   







package com.deepesh.firebaselearningapp;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.Map;


public class MainActivity extends AppCompatActivity {

    EditText age_editText;
    EditText name_editText;
    TextView textview1;
    DatabaseReference databaseReference;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        age_editText = findViewById(R.id.age_editText);
        name_editText = findViewById(R.id.name_editText);
        textview1 = findViewById(R.id.textView1);

        //Gets reference to the "users" Node
        databaseReference = FirebaseDatabase.getInstance().getReference("users");

    }

    //button click function
    public void InsertData(View view) {

        String name = name_editText.getText().toString();
        int age = Integer.parseInt(age_editText.getText().toString());


        // Gets reference to specified child node under "users" node
        // Creates one if not found.
        databaseReference.child("user1").child("Name").setValue(name);
        databaseReference.child("user1").child("Age").setValue(age);


        Log.d("TAG", "DATA INSERTED !");
    }

    //button click function
    public void ReadData(View view) {


        databaseReference.child("user1").addListenerForSingleValueEvent(new ValueEventListener() {

            // RETURNS A OBJECT CONTAINING A SINGLE HASHMAP
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                Map<String, Object> data = (Map<String, Object>) snapshot.getValue();

                Log.d("TAG", "NAME : " + data.get("Name"));
                Log.d("TAG", "AGE : " + data.get("Age"));
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });
    }


}



<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <EditText
        android:id="@+id/age_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="25dp"
        android:ems="10"
        android:hint="Enter age..."
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/name_editText" />

    <EditText
        android:id="@+id/name_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="36dp"
        android:ems="10"
        android:hint="Enter name..."
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="72dp"
        android:text="Ready to code"
        android:textSize="30sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/age_editText" />

    <Button
        android:id="@+id/insert_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="42dp"
        android:layout_marginBottom="38dp"
        android:onClick="InsertData"
        android:text="Insert"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/read_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="54dp"
        android:layout_marginBottom="38dp"
        android:onClick="ReadData"
        android:text="Read"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>



-------------------------------------------------------------------------------------


useful video : 

https://youtu.be/f-eSSDYK9cA



UPDATE DATA  & DELETE DATA.



package com.deepesh.firebaselearningapp;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.OnSuccessListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import java.util.HashMap;
import java.util.Map;


public class MainActivity extends AppCompatActivity {

    EditText age_editText;
    EditText name_editText;
    TextView textview1;
    DatabaseReference databaseReference;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        age_editText = findViewById(R.id.age_editText);
        name_editText = findViewById(R.id.name_editText);
        textview1 = findViewById(R.id.textView1);

        //Gets reference to the Required Node inside Database.
        // if No parameter pass , then points to the Highest-Level parent Node.
        databaseReference = FirebaseDatabase.getInstance().getReference("users");

    }

    //button click function
    public void UpdateData(View view) {

        String name = name_editText.getText().toString();
        int age = Integer.parseInt(age_editText.getText().toString());

        // Create a hashmap , key should be the location
        // path of data you want to update/change
        // Path is Case-Sensitive.
        // Map Value should be the new VALUE.

        // The first "/" points to the Node , you
        // Specified inside the getReference()

        // If no data is present or no such node present at
        // given Path , firebase creates one.

        // Then pass the Hashmap inside the
        // updateChildren() function.

       Map<String,Object> updatedData = new HashMap<>();
       updatedData.put("/user2/Name",name);
       updatedData.put("/user2/Age",age);

       databaseReference.updateChildren(updatedData);

        Log.d("TAG", "DATA UPDATED");
    }




    //button click function
    public void DeleteData(View view) {
        
        
        //USE ANY ONE ONLY !

        // 1st WAY TO DELETE DATA
        // Setting it null , automatically deletes the Data.
       databaseReference.child("user2").child("Name").setValue(null);

       
       
       // 2nd WAY TO DELETE DATA
        // Returns a Task object & we can set result listeners too.
      Task<Void> task = databaseReference.child("user2").child("Name").removeValue();

      task.addOnSuccessListener(new OnSuccessListener<Void>() {
          @Override
          public void onSuccess(Void aVoid) {

              Log.d("TAG", "DATA DELETED SUCESSFULLY !");
          }
      });

      task.addOnFailureListener(new OnFailureListener() {
          @Override
          public void onFailure(@NonNull Exception e) {

              Log.d("TAG", "DATA DELETION FAIL !");
          }
      });
      
    }


}



-------------------------------------------------------------------------------------


addChildEventListener( )









package com.deepesh.firebaselearningapp;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;
import java.util.HashMap;
import java.util.Map;

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "TAG";
    EditText age_editText;
    EditText name_editText;
    EditText userid_editText;
    TextView textview1;
    DatabaseReference databaseReference;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        age_editText = findViewById(R.id.age_editText);
        name_editText = findViewById(R.id.name_editText);
        userid_editText = findViewById(R.id.userid_editText);
        textview1 = findViewById(R.id.textView1);


        databaseReference = FirebaseDatabase.getInstance().getReference("users");

        // EVENT LSITENER - GETS INVOKED EVERYTIME THERE IS A CHANGE
        // IN ANY CHILD NODES UNDER IT.
        databaseReference.addChildEventListener(new ChildEventListener() {

            // CALLED WHEN A NEW NODE IS INSERTED UNDER
            // PARENT NODE SPECIFIED IN getReference().
            @Override
            public void onChildAdded(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
                Log.d(TAG, "onChildAdded: CHILD ADDED ");
                Map<String, Object> changedData = (Map<String, Object>) snapshot.getValue();
                Log.d(TAG, "NAME : " + changedData.get("Name"));
                Log.d(TAG, "AGE : " + changedData.get("Age"));
            }

            //CALLED WHEN ANY CHILD NODE VALUE IS CHANGED
            @Override
            public void onChildChanged(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
                Log.d(TAG, "onChildChanged: CHILD CHANGED ");
                // The snapshot Contains the Child node (Hashmap)
                Map<String, Object> changedData = (Map<String, Object>) snapshot.getValue();
                Log.d(TAG, "NAME : " + changedData.get("Name"));
                Log.d(TAG, "AGE : " + changedData.get("Age"));
            }

            //CALLED WHEN ANY CHILD NODE VALUE IS REMOVED
            @Override
            public void onChildRemoved(@NonNull DataSnapshot snapshot) {
                Log.d(TAG, "onChildRemoved: CHILD REMOVED ");
                // The snapshot Contains the Child node (Hashmap)
                Map<String, Object> changedData = (Map<String, Object>) snapshot.getValue();
                Log.d(TAG, "NAME : " + changedData.get("Name"));
                Log.d(TAG, "AGE : " + changedData.get("Age"));
            }

            @Override
            public void onChildMoved(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {

            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });


    }

    //button click function
    public void UpdateData(View view) {

        String name = name_editText.getText().toString();
        int age = Integer.parseInt(age_editText.getText().toString());

        //RECOMMENDED WAY TO UPDATE/INSERT MULTIPLE DATA
        Map<String, Object> data = new HashMap<>();
        data.put("Name", name);
        data.put("Age", age);

        String userid = userid_editText.getText().toString();
        databaseReference.child(userid).setValue(data);

        Log.d("TAG", "DATA UPDATED");

    }


    //button click function
    public void ReadData(View view) {

        String userid = userid_editText.getText().toString();

        databaseReference.child(userid).addListenerForSingleValueEvent(new ValueEventListener() {

            // RETURNS A OBJECT CONTAINING A SINGLE HASHMAP
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                Map<String, Object> data = (Map<String, Object>) snapshot.getValue();

                Log.d("TAG", "NAME : " + data.get("Name"));
                Log.d("TAG", "AGE : " + data.get("Age"));
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });

    }

    //button click function
    public void InsertData(View view) {

        String name = name_editText.getText().toString();
        int age = Integer.parseInt(age_editText.getText().toString());

        //RECOMMENDED WAY TO UPDATE/INSERT MULTIPLE DATA
        Map<String, Object> data = new HashMap<>();
        data.put("Name", name);
        data.put("Age", age);

        String userid = userid_editText.getText().toString();
        databaseReference.child(userid).setValue(data);

        Log.d("TAG", "DATA INSERTED");


    }

    //button click function
    public void DeleteData(View view) {

        String userid = userid_editText.getText().toString();
        databaseReference.child(userid).removeValue();

        Log.d("TAG", "DATA REMOVED ");

    }


}



<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <EditText
        android:id="@+id/age_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="25dp"
        android:ems="10"
        android:hint="Enter age..."
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/name_editText" />

    <EditText
        android:id="@+id/name_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="36dp"
        android:ems="10"
        android:hint="Enter name..."
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="24dp"
        android:layout_marginTop="51dp"
        android:text="Ready to code"
        android:textSize="30sp"
        app:layout_constraintStart_toStartOf="@+id/userid_editText"
        app:layout_constraintTop_toBottomOf="@+id/userid_editText" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="42dp"
        android:layout_marginBottom="39dp"
        android:onClick="UpdateData"
        android:text="Update"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/button4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="54dp"
        android:layout_marginBottom="39dp"
        android:onClick="ReadData"
        android:text="Read"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

    <Button
        android:id="@+id/button5"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="30dp"
        android:layout_marginTop="104dp"
        android:layout_marginBottom="103dp"
        android:onClick="InsertData"
        android:text="Insert"
        app:layout_constraintBottom_toBottomOf="@+id/button3"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView1" />

    <Button
        android:id="@+id/button6"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="70dp"
        android:layout_marginBottom="53dp"
        android:onClick="DeleteData"
        android:text="Delete"
        app:layout_constraintBottom_toTopOf="@+id/button4"
        app:layout_constraintEnd_toEndOf="parent" />

    <EditText
        android:id="@+id/userid_editText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="86dp"
        android:layout_marginTop="22dp"
        android:ems="10"
        android:hint="Enter UserId"
        android:inputType="textPersonName"
        app:layout_constraintStart_toStartOf="@+id/age_editText"
        app:layout_constraintTop_toBottomOf="@+id/age_editText" />
</androidx.constraintlayout.widget.ConstraintLayout>



-------------------------------------------------------------------------------------


INSERT JAVA OBJECTS (POJO) IN FIREBASE.








package com.deepesh.firebaselearningapp;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;


public class MainActivity extends AppCompatActivity {

    EditText age_editText;
    EditText name_editText;
    TextView textview1;
    DatabaseReference databaseReference;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        age_editText = findViewById(R.id.age_editText);
        name_editText = findViewById(R.id.name_editText);
        textview1 = findViewById(R.id.textView1);

        databaseReference = FirebaseDatabase.getInstance().getReference("users");

        databaseReference.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {

                // GIVES ERROR.
                //Person person1 =  (Person)snapshot.getValue();

                //WORKS FINE
                //Person person1 =  snapshot.getValue(Person.class);

                Person person1 = snapshot.getValue(Person.class);

                textview1.setText("NAME : " + person1.getName() + "\n");
                textview1.append("AGE : " + person1.getAge());

            }

            @Override
            public void onChildChanged(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {

            }

            @Override
            public void onChildRemoved(@NonNull DataSnapshot snapshot) {

            }

            @Override
            public void onChildMoved(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {

            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });

    }


    //button click function
    public void ReadData(View view) {

        databaseReference.child("user2").addListenerForSingleValueEvent(new ValueEventListener() {

            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {

            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });

    }

    //button click function
    public void InsertData(View view) {

        String name = name_editText.getText().toString();
        int age = Integer.parseInt(age_editText.getText().toString());

        Person person1 = new Person(name, age);

        databaseReference.child("user2").setValue(person1);

        Toast.makeText(this, "DATA INSERTED", Toast.LENGTH_SHORT).show();

    }

}



package com.deepesh.firebaselearningapp;

// RULES TO FOLLOW :
// 1] The field names should Match the names in the Firebase Nodes.
// 2] Must contain getters & setters -  will be used by firebase
// 3] Must Contain a Empty constructor too - will be used by firebase

public class Person {

    int Age;
    String Name;

    Person(String name, int age) {
        this.Age = age;
        this.Name = name;
    }

    Person() {

    }

    public int getAge() {
        return Age;
    }

    public void setAge(int age) {
        Age = age;
    }

    public String getName() {
        return Name;
    }

    public void setName(String name) {
        Name = name;
    }

}



<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <EditText
        android:id="@+id/age_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="25dp"
        android:ems="10"
        android:hint="Enter age..."
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/name_editText" />

    <EditText
        android:id="@+id/name_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="36dp"
        android:ems="10"
        android:hint="Enter name..."
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="269dp"
        android:text="Ready to code"
        android:textSize="30sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="42dp"
        android:layout_marginBottom="39dp"
        android:onClick="InsertData"
        android:text="Insert"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/button4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="54dp"
        android:layout_marginBottom="39dp"
        android:onClick="ReadData"
        android:text="Read"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>




-------------------------------------------------------------------------------------


INSERT & DISPLAY DATA ON RECYCLERVIEW.





activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <EditText
        android:id="@+id/age_editText"
        android:layout_width="0dp"
        android:layout_height="44dp"
        android:layout_marginTop="63dp"
        android:layout_marginBottom="60dp"
        android:ems="10"
        android:hint="Enter age..."
        android:inputType="textPersonName"
        app:layout_constraintBottom_toBottomOf="@+id/userid_editText"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@+id/name_editText" />

    <EditText
        android:id="@+id/name_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:ems="10"
        android:hint="Enter name..."
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="44dp"
        android:layout_marginBottom="17dp"
        android:onClick="InsertData"
        android:text="Insert"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/button4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="56dp"
        android:layout_marginBottom="17dp"
        android:onClick="ReadData"
        android:text="Read"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerview1"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginTop="66dp"
        android:layout_marginBottom="59dp"
        app:layout_constraintBottom_toBottomOf="@+id/button3"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@+id/userid_editText" />

    <EditText
        android:id="@+id/userid_editText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="138dp"
        android:ems="10"
        android:hint="Enter UserId"
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>


MainActivity.java

package com.deepesh.firebaselearningapp;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.List;


public class MainActivity extends AppCompatActivity {

    EditText age_editText;
    EditText name_editText;
    EditText userId_editText;
    RecyclerView recyclerview1;
    MyAdapter myAdapter;
    DatabaseReference databaseReference;
    List<User> users_list;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        age_editText = findViewById(R.id.age_editText);
        name_editText = findViewById(R.id.name_editText);
        userId_editText = findViewById(R.id.userid_editText);

        users_list = new ArrayList<>();
        myAdapter= new MyAdapter(this,users_list);
        recyclerview1=findViewById(R.id.recyclerview1);
        recyclerview1.setLayoutManager(new LinearLayoutManager(this));
        recyclerview1.setAdapter(myAdapter);


        databaseReference = FirebaseDatabase.getInstance().getReference("users");
        databaseReference.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {

                User myuser = snapshot.getValue(User.class);
                users_list.add(myuser);

                // Notifies the System about Data change inside the Recyclerview adapter
                myAdapter.notifyDataSetChanged();

            }

            @Override
            public void onChildChanged(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {

            }

            @Override
            public void onChildRemoved(@NonNull DataSnapshot snapshot) {

            }

            @Override
            public void onChildMoved(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {

            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });

    }


    //button click function
    public void ReadData(View view) {

        databaseReference.child("users").addListenerForSingleValueEvent(new ValueEventListener() {

            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {

            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });

    }

    //button click function
    public void InsertData(View view) {

        String name = name_editText.getText().toString();
        int age = Integer.parseInt(age_editText.getText().toString());
        String userID = userId_editText.getText().toString();

        User myuser = new User(name,age);
        databaseReference.child(userID).setValue(myuser);

    }

}


User.java

package com.deepesh.firebaselearningapp;

// RULES TO FOLLOW :
// 1] The field names should Match the names in the Firebase Nodes.
// 2] Must contain getters & setters -  will be used by firebase
// 3] Must Contain a Empty constructor too - will be used by firebase

public class User {

    int Age;
    String Name;

    User(String name, int age) {
        this.Age = age;
        this.Name = name;
    }

    User() {

    }

    public int getAge() {
        return Age;
    }

    public void setAge(int age) {
        Age = age;
    }

    public String getName() {
        return Name;
    }

    public void setName(String name) {
        Name = name;
    }

}


MyAdapter.java

package com.deepesh.firebaselearningapp;


import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
import java.util.List;

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {

    List<User> users_list;

    MyAdapter(Context context, List<User> userlist){
        this.users_list=userlist;
    }


    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.single_row,parent,false);

        return new MyViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {

        User user = users_list.get(position);

        holder.textview1.setText(user.getName() + "  | "+user.getAge());
    }

    @Override
    public int getItemCount() {
        return users_list.size();
    }

    static class MyViewHolder extends RecyclerView.ViewHolder {

        TextView textview1;

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);
            textview1 = itemView.findViewById(R.id.textview_single_row);
        }
    }

}




single_row.xml 

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/textview_single_row"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:paddingStart="15dp"
        android:paddingEnd="5dp"
        android:paddingBottom="5dp"
        android:textSize="28sp"
        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:text="Hello" />

</androidx.constraintlayout.widget.ConstraintLayout>



------------------------------------------------------------------------------------------------------------


SORTING & FILTERING 








PERFORM SQL LIKE QUERIES  : 









<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <EditText
        android:id="@+id/age_editText"
        android:layout_width="0dp"
        android:layout_height="44dp"
        android:layout_marginTop="61dp"
        android:layout_marginBottom="53dp"
        android:ems="10"
        android:hint="Enter age..."
        android:inputType="textPersonName"
        app:layout_constraintBottom_toBottomOf="@+id/profession_editText"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/name_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:ems="10"
        android:hint="Enter name..."
        android:inputType="textPersonName"
        app:layout_constraintBottom_toTopOf="@+id/age_editText"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="13dp"
        android:layout_marginBottom="17dp"
        android:onClick="InsertData"
        android:text="Insert"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/button4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="30dp"
        android:layout_marginBottom="17dp"
        android:onClick="ReadData"
        android:text="Read"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="17dp"
        android:onClick="ShowData"
        android:text="Show"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerview1"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginTop="10dp"
        android:layout_marginBottom="11dp"
        app:layout_constraintBottom_toTopOf="@+id/button3"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/userid_editText" />

    <EditText
        android:id="@+id/userid_editText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:hint="Enter UserId"
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/city_editText" />

    <EditText
        android:id="@+id/profession_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="113dp"
        android:ems="10"
        android:hint="Enter Profession"
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/city_editText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="44dp"
        android:ems="10"
        android:hint="Enter city"
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@+id/profession_editText" />


</androidx.constraintlayout.widget.ConstraintLayout>



package com.deepesh.firebaselearningapp;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.TextView;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.Query;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity{

    private static final String TAG = "TAG";
    TextView name_editText,age_editText,profession_editText,city_editText,userId_editText;
    DatabaseReference databaseReference;
    List<User> users_list;

    RecyclerView recyclerView1;
    MyAdapter myAdapter;

    ValueEventListener valueEvent_listener;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // initialises all fields
        init();

        databaseReference = FirebaseDatabase.getInstance().getReference("users");

    }



    private void init(){

        name_editText = findViewById(R.id.name_editText);
        age_editText=findViewById(R.id.age_editText);
        profession_editText=findViewById(R.id.profession_editText);
        city_editText=findViewById(R.id.city_editText);
        userId_editText = findViewById(R.id.userid_editText);
        users_list= new ArrayList<>();

        recyclerView1=findViewById(R.id.recyclerview1);
        myAdapter= new MyAdapter(this,users_list);
        recyclerView1.setLayoutManager(new LinearLayoutManager(this));
        recyclerView1.setAdapter(myAdapter);

        valueEvent_listener = new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {

                for (DataSnapshot dataSnapshot : snapshot.getChildren()){

                    User myuser = dataSnapshot.getValue(User.class);
                    users_list.add(myuser);
                }
                // Notifies adapter of change in dataset
                myAdapter.notifyDataSetChanged();
                Log.d(TAG, "onDataChange: DATA SHOWED !");

            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        };


    }


    // user clicks button
    public void InsertData(View view) {

        String name = name_editText.getText().toString();
        String profession = profession_editText.getText().toString();
        String city = city_editText.getText().toString();
        int age = Integer.parseInt(age_editText.getText().toString());
        String uid = userId_editText.getText().toString();

        User new_user = new User(uid,name,age,profession,city);

        databaseReference.child(uid).setValue(new_user);

        Toast.makeText(this, "DATA INSERTED !", Toast.LENGTH_SHORT).show();

    }


    // user clicks button
    public void ReadData(View view) {

        String uid = userId_editText.getText().toString();

        databaseReference.child(uid).addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {

                User user1 = snapshot.getValue(User.class);
                Log.d(TAG, "NAME : " + user1.getName());
                Log.d(TAG, "AGE : " + user1.getAge());
                Log.d(TAG, "CITY: " + user1.getCity());

            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });
    }


    // user clicks button
    public void ShowData(View view) {

        // EXAMPLE QUERIES

        // select * from TABLENAME.
        databaseReference.addValueEventListener(valueEvent_listener);
//
//
//        // select * from TABLENAME where name ="Ali"
//        Query query1 = databaseReference.orderByChild("name").equalTo("Ali");
//        query1.addValueEventListener(valueEvent_listener);
//
//
//        // select * from TABLENAME order by name;
//        Query query2 = databaseReference.orderByChild("name");
//        query2.addValueEventListener(valueEvent_listener);
//
//
//         select * from TABLENAME where age > 30;
//        Query query3 = databaseReference.orderByChild("age").startAt(30);
//        query3.addValueEventListener(valueEvent_listener);
//
//
//        // select * from TABLENAME where age < 30;
//        Query query4 = databaseReference.orderByChild("age").endAt(30);
//        query3.addValueEventListener(valueEvent_listener);
//
//
//        // select * from TABLENAME limit 3;
//        databaseReference.limitToFirst(3).addValueEventListener(valueEvent_listener);

    }

}




package com.deepesh.firebaselearningapp;


import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
import java.util.List;

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {

    List<User> users_list;

    MyAdapter(Context context, List<User> userlist){
        this.users_list=userlist;
    }


    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.single_row,parent,false);

        return new MyViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {

        User myuser = users_list.get(position);

        holder.name_textview.setText(myuser.getName());
        holder.profession_textview.setText(myuser.getProfession());
        holder.city_textview.setText(myuser.getCity());
        holder.age_textview.setText(String.valueOf(myuser.getAge()));
    }

    @Override
    public int getItemCount() {
        return users_list.size();
    }

    static class MyViewHolder extends RecyclerView.ViewHolder {

        TextView name_textview;
        TextView age_textview;
        TextView city_textview;
        TextView profession_textview;


        public MyViewHolder(@NonNull View itemView) {
            super(itemView);

            name_textview=itemView.findViewById(R.id.nameTextview_single_row);
            age_textview=itemView.findViewById(R.id.age_textview_singleRow);
            city_textview=itemView.findViewById(R.id.city_textview_singleRow);
            profession_textview=itemView.findViewById(R.id.profession_textview_singleRow);

        }
    }

}



package com.deepesh.firebaselearningapp;

// RULES TO FOLLOW :
// 1] The field names should Match the names in the Firebase Nodes.
// 2] Must contain getters & setters -  will be used by firebase
// 3] Must Contain a Empty constructor too - will be used by firebase

public class User {

    int age;
    String name;
    String uid;
    String city;
    String profession;


    User(String userid ,String name, int age,String profession,String city) {
        this.age = age;
        this.name = name;
        this.profession=profession;
        this.city=city;
        this.uid=userid;
    }

    User() {

    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getUid() {
        return uid;
    }

    public void setUid(String uid) {
        this.uid = uid;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    public String getProfession() {
        return profession;
    }

    public void setProfession(String profession) {
        this.profession = profession;
    }

}





<?xml version="1.0" encoding="utf-8"?>
<androidx.cardview.widget.CardView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:padding="15dp">

        <TextView
            android:id="@+id/nameTextview_single_row"
            android:layout_width="411dp"
            android:layout_height="wrap_content"
            android:layout_marginStart="46dp"
            android:layout_marginTop="20dp"
            android:paddingStart="15dp"
            android:paddingEnd="5dp"
            android:paddingBottom="5dp"
            android:textSize="28sp"
            app:layout_constraintBottom_toTopOf="@+id/age_textview_singleRow"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            tools:text="Ready to Code" />

        <TextView
            android:id="@+id/age_textview_singleRow"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginBottom="2dp"
            android:textSize="20sp"
            app:layout_constraintBottom_toTopOf="@+id/profession_textview_singleRow"
            app:layout_constraintStart_toStartOf="@+id/profession_textview_singleRow"
            app:layout_constraintTop_toBottomOf="@+id/nameTextview_single_row"
            tools:text="Age" />

        <TextView
            android:id="@+id/profession_textview_singleRow"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginBottom="6dp"
            android:textSize="20sp"
            app:layout_constraintBottom_toTopOf="@+id/city_textview_singleRow"
            app:layout_constraintStart_toStartOf="@+id/city_textview_singleRow"
            app:layout_constraintTop_toBottomOf="@+id/age_textview_singleRow"
            tools:text="Profession" />

        <TextView
            android:id="@+id/city_textview_singleRow"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginStart="91dp"
            android:layout_marginBottom="6dp"
            android:textSize="20sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/profession_textview_singleRow"
            tools:text="Country" />


    </androidx.constraintlayout.widget.ConstraintLayout>


</androidx.cardview.widget.CardView>



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