public class RegisterActivity extends AppCompatActivity {
private static final String TAG = "RegisterActivity";

EditText nameText, emailText, passwordText;
Button registerButton;
TextView loginLink;

CheckBox checkBoxShowPassword;

private SessionManager session;
private SQLiteHandler db;
private static final int MY_PERMISSIONS_REQUEST_READ_CONTACTS = 0;

RegisterActivity registerActivity = RegisterActivity.this;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_register);

    nameText = findViewById(R.id.input_name);
    emailText = findViewById(R.id.input_email);
    passwordText = findViewById(R.id.input_password);
    registerButton = findViewById(R.id.btn_register);
    loginLink = findViewById(R.id.link_login);

    session = new SessionManager(getApplicationContext());
    db = new SQLiteHandler(getApplicationContext());
    if (session.isLoggedIn()) {
        Intent intent = new Intent(RegisterActivity.this, MainActivity.class);
        startActivity(intent);
        finish();
    }

    registerButton.setOnClickListener(v -> {
        String name = nameText.getText().toString().trim();
        String email = emailText.getText().toString().trim();
        String password = passwordText.getText().toString().trim();
        String phone = "";

        if (ContextCompat.checkSelfPermission(registerActivity, Manifest.permission.READ_CONTACTS) != PackageManager.PERMISSION_GRANTED) {

            if (ActivityCompat.shouldShowRequestPermissionRationale(registerActivity, Manifest.permission.READ_CONTACTS)) {
                if (!name.isEmpty() && !email.isEmpty() && !password.isEmpty()) {
                    register(name, email, password, phone, false);
                } else {
                    Toast.makeText(getApplicationContext(), R.string.please_enter_credentials, Toast.LENGTH_LONG).show();
                }
            } else {
                ActivityCompat.requestPermissions(registerActivity, new String[]{Manifest.permission.READ_CONTACTS}, MY_PERMISSIONS_REQUEST_READ_CONTACTS);
            }
        } else {
            if (!name.isEmpty() && !email.isEmpty() && !password.isEmpty()) {
                register(name, email, password, phone, true);
            } else {
                Toast.makeText(getApplicationContext(), R.string.please_enter_credentials, Toast.LENGTH_LONG).show();
            }
        }
    });

    loginLink.setOnClickListener(v -> {
        finish();
    });

    checkBoxShowPassword = findViewById(R.id.checkBoxShowPassword);
    checkBoxShowPassword.setOnCheckedChangeListener((buttonView, isChecked) -> {
        if (!isChecked) {               passwordText.setTransformationMethod(PasswordTransformationMethod.getInstance());
        } else {                passwordText.setTransformationMethod(HideReturnsTransformationMethod.getInstance());
        }
    });
}

@Override
public void onRequestPermissionsResult(int requestCode, String permissions[], int[] grantResults) {

    String email = emailText.getText().toString().trim();
    String password = passwordText.getText().toString().trim();
    String name = nameText.getText().toString().trim();
    String phone = "";

    switch (requestCode) {
        case MY_PERMISSIONS_REQUEST_READ_CONTACTS: {
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                if (!name.isEmpty() && !email.isEmpty() && !password.isEmpty()) {
                    register(name, email, password, phone, true);
                } else {
                    Toast.makeText(getApplicationContext(), R.string.please_enter_credentials, Toast.LENGTH_LONG).show();
                }
            } else {
                if (!name.isEmpty() && !email.isEmpty() && !password.isEmpty()) {
                    register(name, email, password, phone, false);
                } else {
                    Toast.makeText(getApplicationContext(), R.string.please_enter_credentials, Toast.LENGTH_LONG).show();
                }
            }
        }
    }
}

public void register(final String name, final String email, final String password, final String phone, Boolean isGranted) {
    Log.d(TAG, "Login");
    if (!validate()) {
        onRegisterFailed();
        return;
    }

    registerButton.setEnabled(true);
    String tag_string_req = "req_login";

    final ProgressDialog progressDialog = new ProgressDialog(RegisterActivity.this,
            R.style.AppTheme_Dark_Dialog);
    progressDialog.setIndeterminate(true);
    progressDialog.setMessage(getString(R.string.authenticating));
    progressDialog.show();

    ApiInterface apiService =
            ApiClient.getClient().create(ApiInterface.class);

    progressDialog.dismiss();

    Call<ContactResponse> call = apiService.register(name, email, password, phone);
    call.enqueue(new Callback<ContactResponse>() {
        @Override
        public void onResponse(Call<ContactResponse> call, retrofit2.Response<ContactResponse> response) {
            if (response.body().getError()) {
                String message = response.body().getErrorMessage();
                onRegisterFailed(message);
                return;
            }

            final Contact contact = response.body().getResults();
            if (contact != null) {
                session.setLogin(true);

                db.addUser(contact.getUserName(), contact.getEmail(), contact.getUserId(), contact.getCreatedAt(), contact.getAbout(),
                        contact.getThumbnailUrl(), contact.getPhone());

                Intent intent = new Intent(RegisterActivity.this,
                        MainActivity.class);
                startActivity(intent);
                finish();
                onRegisterSuccess();

                if (isGranted) {
                    updateContacts();
                }
            } else {
                onRegisterFailed();
            }
        }

        @Override
        public void onFailure(Call<ContactResponse> call, Throwable t) {
            // Log error here since request failed
            Log.e(TAG, t.toString());
            onRegisterFailed();
        }
    });
}

public void userExists(final String phone) {
}

public void onRegisterSuccess() {
    registerButton.setEnabled(true);
    setResult(RESULT_OK, null);
    finish();
}

public void onRegisterFailed() {
    Toast.makeText(getBaseContext(), R.string.registration_failed, Toast.LENGTH_LONG).show();
    registerButton.setEnabled(true);
}

public void onRegisterFailed(String message) {
    Toast.makeText(getBaseContext(), message, Toast.LENGTH_LONG).show();
    registerButton.setEnabled(true);
}

public boolean validate() {
    boolean valid = true;

    String name = nameText.getText().toString();
    String email = emailText.getText().toString();
    String password = passwordText.getText().toString();

    if (name.isEmpty() || name.length() < 3) {
        nameText.setError(getString(R.string.enter_3_chars));
        valid = false;
    } else {
        nameText.setError(null);
    }
    if (email.isEmpty() || !android.util.Patterns.EMAIL_ADDRESS.matcher(email).matches()) {
        emailText.setError(getString(R.string.enter_valid_email));
        valid = false;
    } else {
        emailText.setError(null);
    }
    if (password.isEmpty() || password.length() < 4 || password.length() > 20) {
        passwordText.setError(getString(R.string.enter_4_20_char));
        valid = false;
    } else {
        passwordText.setError(null);
    }

    return valid;
}

private String getMyPhoneNumber() {
    TelephonyManager tMgr = (TelephonyManager) getSystemService(Context.TELEPHONY_SERVICE);

    String simCountry = null;
    if (tMgr != null) {
        simCountry = tMgr.getSimCountryIso().toUpperCase();
    }
    if (tMgr != null) {
        String simOperatorCode = tMgr.getSimOperator();
    }
    if (tMgr != null) {
        String simOperatorName = tMgr.getSimOperatorName();
    }
    if (ActivityCompat.checkSelfPermission(this, Manifest.permission.READ_PHONE_STATE) != PackageManager.PERMISSION_GRANTED) {
    }
    String simSerial = tMgr.getSimSerialNumber();
    String MyPhoneNumber = "0000000000";
    try {
        MyPhoneNumber = tMgr.getLine1Number();
    } catch (NullPointerException ex) {
    }
    if (MyPhoneNumber == null || MyPhoneNumber.equals("")) {
        return "";
    }
    PhoneNumberUtil phoneUtil = PhoneNumberUtil.getInstance();
    Phonenumber.PhoneNumber countryNumberProto = null;
    try {
        countryNumberProto = phoneUtil.parse(MyPhoneNumber, simCountry);
    } catch (NumberParseException e) {
        System.err.println(getString(R.string.numberparseexception_thrown) + e.toString());
    }

    return phoneUtil.format(countryNumberProto, PhoneNumberUtil.PhoneNumberFormat.E164);
}

public void updateContacts() {


    String[] PROJECTION = new String[]{
            ContactsContract.CommonDataKinds.Phone.DISPLAY_NAME,
            ContactsContract.CommonDataKinds.Phone.NUMBER
    };

    Cursor phones = getContentResolver().query(ContactsContract.CommonDataKinds.Phone.CONTENT_URI, PROJECTION, null, null, null);

    HashMap<String, String> user = db.getUserDetails();
    String userId = user.get("uid");

    if (phones != null) {
        while (phones.moveToNext()) {
            if (phones.getColumnIndex(ContactsContract.CommonDataKinds.Phone.NUMBER) != -1) {
                final String phoneNumber = phones.getString(phones.getColumnIndex(ContactsContract.CommonDataKinds.Phone.NUMBER));

                ApiInterface apiService =
                        ApiClient.getClient().create(ApiInterface.class);

                Call<ContactsResponse> call = apiService.contactExists(phoneNumber, userId);
                call.enqueue(new Callback<ContactsResponse>() {
                    @Override
                    public void onResponse(Call<ContactsResponse> call, retrofit2.Response<ContactsResponse> response) {
                        Log.e(TAG, "abc");
                    }

                    @Override
                    public void onFailure(Call<ContactsResponse> call, Throwable t) {
                        Log.e(TAG, t.toString());
                    }
                });
            }
        }
    }
     if (phones != null) {
         phones.close();
     }
   }
}

registerButton.setOnClickListener(v -> {

        RequestMultiplePermission();
    });

    loginLink.setOnClickListener(v -> {
        finish();
    });

    checkBoxShowPassword = findViewById(R.id.checkBoxShowPassword);
    checkBoxShowPassword.setOnCheckedChangeListener((buttonView, isChecked) -> {
        if (!isChecked) {
            passwordText.setTransformationMethod(PasswordTransformationMethod.getInstance());
        } else {
            passwordText.setTransformationMethod(HideReturnsTransformationMethod.getInstance());
        }
    });
}
private void RequestMultiplePermission() {

    ActivityCompat.requestPermissions(registerActivity, new String[]
            {
                    Manifest.permission.READ_PHONE_STATE,
                    Manifest.permission.READ_CONTACTS
            }, RequestPermissionCode);

}

@Override
public void onRequestPermissionsResult(int requestCode, String permissions[], int[] grantResults) {
    switch (requestCode) {
        case RequestPermissionCode:
            String name = nameText.getText().toString().trim();
            String email = emailText.getText().toString().trim();
            String password = passwordText.getText().toString().trim();
            String phone = getMyPhoneNumber();

            if (grantResults.length > 0) {
                boolean PhoneStatePermission = grantResults[0] == PackageManager.PERMISSION_GRANTED;
                boolean ContactsPermission = grantResults[1] == PackageManager.PERMISSION_GRANTED;

                if (PhoneStatePermission && ContactsPermission) {
                    register(name, email, password, phone, true);
                } else if (!PhoneStatePermission && ContactsPermission) {
                    register(name, email, password, "", true);
                } else if (!ContactsPermission && PhoneStatePermission) {
                    register(name, email, password, phone, false);
                } else {
                    register(name, email, password, "", false);
                }
            }
            break;
    }
}

public boolean CheckingPermissionIsEnabledOrNot() {
    int FirstPermissionResult = ContextCompat.checkSelfPermission(getApplicationContext(), Manifest.permission.READ_PHONE_STATE);
    int SecondPermissionResult = ContextCompat.checkSelfPermission(getApplicationContext(), Manifest.permission.READ_CONTACTS);
    return FirstPermissionResult == PackageManager.PERMISSION_GRANTED &&
            SecondPermissionResult == PackageManager.PERMISSION_GRANTED;
}

private String getMyPhoneNumber() {
    TelephonyManager tMgr = (TelephonyManager) getSystemService(Context.TELEPHONY_SERVICE);

    String simCountry = null;
    if (tMgr != null) {
        simCountry = tMgr.getSimCountryIso().toUpperCase();
    }
    if (tMgr != null) {
        String simOperatorCode = tMgr.getSimOperator();
    }
    if (tMgr != null) {
        String simOperatorName = tMgr.getSimOperatorName();
    }
    RequestMultiplePermission();
...