private void checkExistingEmail(){
    firebaseAuth.fetchSignInMethodsForEmail(userEmail.getText().toString()).addOnCompleteListener(new OnCompleteListener<SignInMethodQueryResult>() {
        @Override
        public void onComplete(@NonNull Task<SignInMethodQueryResult> task) {

            boolean check = !task.getResult().getSignInMethods().isEmpty();

            if(check){
                userEmail.setError("Account with same email address already exist!");
            }
        }
    });
}

regButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if(validate()){
                String user_email = userEmail.getText().toString().trim();
                String user_password = userPassword.getEditText().getText().toString().trim();

                firebaseAuth.createUserWithEmailAndPassword(user_email, user_password).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {

                        checkExistingEmail();

                        if(task.isSuccessful()){
                            sendEmailVerification();
                        }else{
                            Toast.makeText(RegistrationActivity.this, "Registration is Unsuccessful!", Toast.LENGTH_SHORT).show();
                        }
                    }
                });
            }
        }
    });

private void sendEmailVerification(){
    FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();
    if(firebaseUser != null){

        progressDialog.setMessage("Please hold on. We are testing your patience!");
        progressDialog.setProgressStyle(progressDialog.STYLE_SPINNER);
        progressDialog.show();

        firebaseUser.sendEmailVerification().addOnCompleteListener(new OnCompleteListener<Void>() {
            @Override
            public void onComplete(@NonNull Task<Void> task) {

                if(task.isSuccessful()){
                    sendUserData();
                    progressDialog.dismiss();
                    Toast.makeText(RegistrationActivity.this, "Registration Successful! Verification mail has been sent!", Toast.LENGTH_SHORT).show();
                    //after account has been created, by right will straight open next activity. but force signout so user can sign in again
                    firebaseAuth .signOut();
                    finish();
                    startActivity(new Intent(RegistrationActivity.this, MainActivity.class));
                }else{
                    progressDialog.dismiss();
                    Toast.makeText(RegistrationActivity.this, "Registration Failed!", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }
}