import android.content.Intent;
            import android.os.Bundle;
            import android.view.View;
            import android.widget.Button;
            import android.widget.EditText;

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

            import com.google.android.gms.tasks.OnCompleteListener;
            import com.google.android.gms.tasks.Task;
            import com.google.firebase.FirebaseApp;
            import com.google.firebase.FirebaseException;
            import com.google.firebase.auth.AuthResult;
            import com.google.firebase.auth.FirebaseAuth;
            import com.google.firebase.auth.FirebaseUser;
            import com.google.firebase.auth.PhoneAuthCredential;
            import com.google.firebase.auth.PhoneAuthProvider;
            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;
            import java.util.concurrent.TimeUnit;


            public class LoginActivity extends AppCompatActivity {

                private EditText mPhoneNumber, mCode;
                private Button mSend;

                private PhoneAuthProvider.OnVerificationStateChangedCallbacks mCallbacks;

                String mVerificationId;

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

                    FirebaseApp.initializeApp(this);

                    userIsLoggedIn();

                    mPhoneNumber = findViewById(R.id.phoneNumber);
                    mCode = findViewById(R.id.code);

                    mSend = findViewById(R.id.verify);

                    mSend.setOnClickListener(new View.OnClickListener() {
                        @Override
                        public void onClick(View v) {
                            if(mVerificationId != null)
                                verifyPhoneNumberWithCode();
                            else
                                startPhoneNumberVerification();
                        }
                    });


                    mCallbacks = new PhoneAuthProvider.OnVerificationStateChangedCallbacks() {
                        @Override
                        public void onVerificationCompleted(PhoneAuthCredential phoneAuthCredential) {
                            signInWithPhoneAuthCredential(phoneAuthCredential);
                        }

                        @Override
                        public void onVerificationFailed(FirebaseException e) {
                        }


                        @Override
                        public void onCodeSent(String verificationId, PhoneAuthProvider.ForceResendingToken forceResendingToken) {
                            super.onCodeSent(verificationId, forceResendingToken);

                            mVerificationId = verificationId;
                            mSend.setText("Verify Code");
                        }
                    };

                }



                private void verifyPhoneNumberWithCode(){
                    PhoneAuthCredential credential = PhoneAuthProvider.getCredential(mVerificationId, mCode.getText().toString());
                    signInWithPhoneAuthCredential(credential);
                }

                private void signInWithPhoneAuthCredential(PhoneAuthCredential phoneAuthCredential) {
                    FirebaseAuth.getInstance().signInWithCredential(phoneAuthCredential).addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                        @Override
                        public void onComplete(@NonNull Task<AuthResult> task) {
                            if(task.isSuccessful()){

                                final FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();

                                if(user != null){
                                            final DatabaseReference mUserDB = FirebaseDatabase.getInstance().getReference().child("user").child(user.getUid());
                                            mUserDB.addListenerForSingleValueEvent(new ValueEventListener() {
                                                @Override
                                                public void onDataChange(DataSnapshot dataSnapshot) {
                                                    if(!dataSnapshot.exists()){
                                                        Map<String,Object> userMap = new HashMap<>();
                                                        userMap.put("phone",user.getPhoneNumber());
                                                        userMap.put("name",user.getPhoneNumber());
                                                        mUserDB.updateChildren(userMap);

                                                    }
                                                    userIsLoggedIn();
                                                }

                                                @Override
                                                public void onCancelled(DatabaseError databaseError) {

                                                }
                                            });
                                            //userIsLoggedIn();
                                        }

                                    }
                                }




                    });
                }

                private void userIsLoggedIn() {
                    FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
                    if(user != null){
                        startActivity(new Intent(getApplicationContext(), MainPageActivity.class));
                        finish();
                        return;
                    }
                }

                private void startPhoneNumberVerification() {
                    PhoneAuthProvider.getInstance().verifyPhoneNumber(
                            mPhoneNumber.getText().toString(),
                            60,
                            TimeUnit.SECONDS,
                            this,
                            mCallbacks);
                }
            }

import androidx.appcompat.app.AppCompatActivity;

            import android.Manifest;
            import android.content.Intent;
            import android.os.Build;
            import android.os.Bundle;
            import android.view.View;
            import android.widget.Button;
            import com.google.firebase.auth.FirebaseAuth;

            public class MainPageActivity extends AppCompatActivity {

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

                    Button mLogout = findViewById(R.id.logout);
                Button mFindUser = findViewById(R.id.findUser);

                    mFindUser.setOnClickListener(new View.OnClickListener(){
                        @Override
                        public void onClick(View v){
                            startActivity(new Intent(getApplicationContext(),FindUserActivity.class));

                        }
                    });



                    mLogout.setOnClickListener(new View.OnClickListener(){
                        @Override
                        public void onClick(View v){
                            FirebaseAuth.getInstance().signOut();
                            Intent intent = new Intent(getApplicationContext(), LoginActivity.class);
                            intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                            startActivity(intent);
                            finish();
                            return;
                        }
                    });
            getPermissions();
                }

                private void getPermissions() {
                    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                        requestPermissions(new String[]{Manifest.permission.WRITE_CONTACTS,Manifest.permission.READ_CONTACTS},1);
                    }
                }


            }

import androidx.appcompat.app.AppCompatActivity;
            import androidx.recyclerview.widget.LinearLayoutManager;
            import androidx.recyclerview.widget.RecyclerView;

            import android.database.Cursor;
            import android.os.Bundle;
            import android.provider.ContactsContract;
            import android.telephony.TelephonyManager;
            import android.widget.LinearLayout;

            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;

            public class FindUserActivity extends AppCompatActivity {
                private RecyclerView mUserList;
                private RecyclerView.Adapter mUserListAdapter;
                private RecyclerView.LayoutManager mUserListLayoutManager;
            ArrayList<UserObject> userList,contactList;
                @Override
                protected void onCreate(Bundle savedInstanceState) {
                    super.onCreate(savedInstanceState);
                    setContentView(R.layout.activity_find_user);
            contactList= new ArrayList<>();
                    userList= new ArrayList<>();

            initializeRecyclerView();

                    getContactsList();
                }

                private void getContactsList(){
                    String ISOPrefix = getCountryISO();
                    Cursor phones = getContentResolver().query(ContactsContract.CommonDataKinds.Phone.CONTENT_URI,null,null,null,null);
                    while(phones.moveToNext()){
                        String name = phones.getString(phones.getColumnIndex(ContactsContract.CommonDataKinds.Phone.DISPLAY_NAME));
                        String phone = phones.getString(phones.getColumnIndex(ContactsContract.CommonDataKinds.Phone.NUMBER));
                        phone = phone.replace("","");
                        phone = phone.replace("-","");
                        phone = phone.replace("(","");
                        phone = phone.replace(")","");
                        if(!String.valueOf(phone.charAt(0)).equals("+"))
                            phone = ISOPrefix + phone;



                        UserObject mContact = new UserObject(name,phone);
                        contactList.add(mContact);
                        getUserDeatails(mContact);
                    }
                }

                private void getUserDeatails(UserObject mContact) {
                    DatabaseReference mUserDB = FirebaseDatabase.getInstance().getReference().child("user");
                    Query query = mUserDB.orderByChild("phone").equalTo(mContact.getPhone());
                    query.addListenerForSingleValueEvent(new ValueEventListener() {
                        @Override
                        public void onDataChange(DataSnapshot dataSnapshot) {
                            if(dataSnapshot.exists()){
                                String phone = "";
                                String name = "";
                                for(DataSnapshot childSnapshot : dataSnapshot.getChildren()){
                                    if(childSnapshot.child("phone").getValue()!= null)
                                        phone = childSnapshot.child("phone").getValue().toString();
                                    if(childSnapshot.child("name").getValue()!= null)
                                        phone = childSnapshot.child("name").getValue().toString();
                                    UserObject mUser = new UserObject(name,phone);
                                    userList.add(mUser);
                                    mUserListAdapter.notifyDataSetChanged();
                                    return;

                                }
                            }
                        }

                        @Override
                        public void onCancelled(DatabaseError databaseError) {

                        }
                    });
                }

                private String getCountryISO(){
                    String iso =null;
                TelephonyManager telephonyManager = (TelephonyManager) getApplicationContext().getSystemService(getApplicationContext().TELEPHONY_SERVICE);
                    if(telephonyManager.getNetworkCountryIso() != null)
                        if(telephonyManager.getNetworkCountryIso().toString().equals(""))
                            iso=telephonyManager.getNetworkCountryIso().toString();
                    return CountryToPhonePrefix.getPhone(iso);
            }
                private void initializeRecyclerView(){
                    mUserList= findViewById(R.id.userList);
                    mUserList.setNestedScrollingEnabled(false);
                    mUserList.setHasFixedSize(false);
                    mUserListLayoutManager = new LinearLayoutManager(getApplicationContext(), RecyclerView.VERTICAL,false);
                    mUserList.setLayoutManager(mUserListLayoutManager);
                    mUserListAdapter = new UserListAdapter(userList);
                    mUserList.setAdapter(mUserListAdapter);
                }
            }

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.ArrayList;

            public class UserListAdapter extends RecyclerView.Adapter<UserListAdapter.UserListViewHolder> {
                ArrayList<UserObject> userList;
                public UserListAdapter(ArrayList<UserObject> userList){
                    this.userList=userList;
                }

                @NonNull
                @Override
                public UserListViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
                    View layoutView = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_user,null,false);
                    RecyclerView.LayoutParams lp = new RecyclerView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,ViewGroup.LayoutParams.WRAP_CONTENT);
                    layoutView.setLayoutParams(lp);
                    UserListViewHolder rcv = new UserListViewHolder(layoutView);
                    return rcv;
                }

                @Override
                public void onBindViewHolder(@NonNull UserListViewHolder holder, int position) {
                    holder.mName.setText(userList.get(position).getName());
                    holder.mPhone.setText(userList.get(position).getPhone());

                }

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


                public class UserListViewHolder extends RecyclerView.ViewHolder{
                    public TextView mName,mPhone;
                    public UserListViewHolder(View view){
                        super(view);
                        mName = view.findViewById(R.id.name);
                        mPhone = view.findViewById(R.id.phone);

                    }

                }
            }