MainActivity.java

package com.example.pinkesh.firebasephoneauthentication;

import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Toast;

import com.firebase.ui.auth.AuthUI;
import com.firebase.ui.auth.ErrorCodes;
import com.firebase.ui.auth.IdpResponse;
import com.firebase.ui.auth.ResultCodes;
import com.google.firebase.auth.FirebaseAuth;

import java.util.Arrays;

public class MainActivity extends AppCompatActivity {

    private static final int RC_SIGN_IN = 123;
    private  FirebaseAuth auth;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        auth = FirebaseAuth.getInstance();

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

//                if (auth.getCurrentUser() != null) {
//                    // already signed in
//                    Toast.makeText(MainActivity.this, "Users already logged in", Toast.LENGTH_SHORT).show();
//                }else {
                    // not signed in
                    startActivityForResult(
                            AuthUI.getInstance()
                                    .createSignInIntentBuilder()
                                    .setAvailableProviders(
                                            Arrays.asList(
                                                    new AuthUI.IdpConfig.Builder(AuthUI.PHONE_VERIFICATION_PROVIDER).build()
                                            ))
                                    .build(),
                            RC_SIGN_IN);
               // }


//                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
//                        .setAction("Action", null).show();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        // RC_SIGN_IN is the request code you passed into startActivityForResult(...) when starting the sign in flow.
        if (requestCode == RC_SIGN_IN) {
            IdpResponse response = IdpResponse.fromResultIntent(data);
            // Successfully signed in
            if (resultCode == ResultCodes.OK) {
                Toast.makeText(this, "OTP verification success", Toast.LENGTH_SHORT).show();
                return;
            } else {
                // Sign in failed
                if (response == null) {
                    // User pressed back button
                    Log.e("Login","Login canceled by User");
                    return;
                }
                if (response.getErrorCode() == ErrorCodes.NO_NETWORK) {
                    Log.e("Login","No Internet Connection");
                    return;
                }
                if (response.getErrorCode() == ErrorCodes.UNKNOWN_ERROR) {
                    Log.e("Login","Unknown Error");
                    return;
                }
            }
            Log.e("Login","Unknown sign in response");
        }
    }

}