<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>

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

    <!--EditText for adding employee name-->
    <EditText
        android:id="@+id/idEdtEmployeeName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_margin="10dp"
        android:hint="Enter Employee Name"
        android:importantForAutofill="no"
        android:inputType="textPersonName" />

    <!--EditText for adding employee phone-->
    <EditText
        android:id="@+id/idEdtEmployeePhoneNumber"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/idEdtEmployeeName"
        android:layout_margin="10dp"
        android:hint="Enter employee phone number"
        android:importantForAutofill="no"
        android:inputType="phone" />

    <!--EditText for adding employee address-->
    <EditText
        android:id="@+id/idEdtEmployeeAddress"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/idEdtEmployeePhoneNumber"
        android:layout_margin="10dp"
        android:hint="Enter employee address"
        android:inputType="textPostalAddress" />

    <!--Button for adding data to Firebase-->
    <Button
        android:id="@+id/idBtnSendData"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/idEdtEmployeeAddress"
        android:layout_margin="10dp"
        android:text="Add employee details"
        android:textAllCaps="false" />

</RelativeLayout>

public class EmployeeInfo {

    // string variable for 
    // storing employee name.
    private String employeeName;

    // string variable for storing
    // employee contact number
    private String employeeContactNumber;

    // string variable for storing
    // employee address.
    private String employeeAddress;

    // an empty constructor is 
    // required when using
    // Firebase Realtime Database.
    public EmployeeInfo() {

    }

    // created getter and setter methods
    // for all our variables.
    public String getEmployeeName() {
        return employeeName;
    }

    public void setEmployeeName(String employeeName) {
        this.employeeName = employeeName;
    }

    public String getEmployeeContactNumber() {
        return employeeContactNumber;
    }

    public void setEmployeeContactNumber(String employeeContactNumber) {
        this.employeeContactNumber = employeeContactNumber;
    }

    public String getEmployeeAddress() {
        return employeeAddress;
    }

    public void setEmployeeAddress(String employeeAddress) {
        this.employeeAddress = employeeAddress;
    }
}

import android.os.Bundle;
import android.text.TextUtils;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

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

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 {

    // creating variables for 
    // EditText and buttons.
    private EditText employeeNameEdt, employeePhoneEdt, employeeAddressEdt;
    private Button sendDatabtn;

    // creating a variable for our
    // Firebase Database.
    FirebaseDatabase firebaseDatabase;

    // creating a variable for our Database 
    // Reference for Firebase.
    DatabaseReference databaseReference;

    // creating a variable for 
    // our object class
    EmployeeInfo employeeInfo;

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

        // initializing our edittext and button
        employeeNameEdt = findViewById(R.id.idEdtEmployeeName);
        employeePhoneEdt = findViewById(R.id.idEdtEmployeePhoneNumber);
        employeeAddressEdt = findViewById(R.id.idEdtEmployeeAddress);

        // below line is used to get the 
        // instance of our FIrebase database.
        firebaseDatabase = FirebaseDatabase.getInstance();

        // below line is used to get reference for our database.
        databaseReference = firebaseDatabase.getReference("EmployeeInfo");

        // initializing our object 
        // class variable.
        employeeInfo = new EmployeeInfo();

        sendDatabtn = findViewById(R.id.idBtnSendData);

        // adding on click listener for our button.
        sendDatabtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                // getting text from our edittext fields.
                String name = employeeNameEdt.getText().toString();
                String phone = employeePhoneEdt.getText().toString();
                String address = employeeAddressEdt.getText().toString();

                // below line is for checking weather the 
                // edittext fields are empty or not.
                if (TextUtils.isEmpty(name) && TextUtils.isEmpty(phone) && TextUtils.isEmpty(address)) {
                    // if the text fields are empty 
                    // then show the below message.
                    Toast.makeText(MainActivity.this, "Please add some data.", Toast.LENGTH_SHORT).show();
                } else {
                    // else call the method to add 
                    // data to our database.
                    addDatatoFirebase(name, phone, address);
                }
            }
        });
    }

    private void addDatatoFirebase(String name, String phone, String address) {
        // below 3 lines of code is used to set
        // data in our object class.
        employeeInfo.setEmployeeName(name);
        employeeInfo.setEmployeeContactNumber(phone);
        employeeInfo.setEmployeeAddress(address);

        // we are use add value event listener method
        // which is called with database reference.
        databaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                // inside the method of on Data change we are setting 
                // our object class to our database reference.
                // data base reference will sends data to firebase.
                databaseReference.setValue(employeeInfo);

                // after adding this data we are showing toast message.
                Toast.makeText(MainActivity.this, "data added", Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {
                // if the data is not added or it is cancelled then
                // we are displaying a failure toast message.
                Toast.makeText(MainActivity.this, "Fail to add data " + error, Toast.LENGTH_SHORT).show();
            }
        });
    }
}