FireBase Auth

public class MainActivity extends AppCompatActivity {

    private final static String TAG = "MainActivity";
    private FirebaseAuth mAuth;

    private TextView text;
    private EditText login;
    private EditText pass;
    private Button save;

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

        text = findViewById(R.id.text);
        login = findViewById(R.id.login);
        pass = findViewById(R.id.pass);
        save = findViewById(R.id.save);

        save.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                performLoginOrAccountCreation(login.getText().toString(), pass.getText().toString());
            }
        });

        FirebaseApp.initializeApp(this);
        mAuth = FirebaseAuth.getInstance();

    }

    @Override
    protected void onStart() {
        super.onStart();
        FirebaseUser user = mAuth.getCurrentUser();
        updateUI(user);
    }

    private void performLoginOrAccountCreation(final String email, final String password){
        mAuth.fetchSignInMethodsForEmail(email).addOnCompleteListener(
                this, new OnCompleteListener<SignInMethodQueryResult>() {
                    @Override
                    public void onComplete(@NonNull Task<SignInMethodQueryResult> task) {
                        if (task.isSuccessful()) {

                            SignInMethodQueryResult result = task.getResult();
                            if(result != null && result.getSignInMethods() != null && result.getSignInMethods().size() > 0){
                                signIn(email, password);
                            }else{ createAccount(email, password); }

                        } else {
                            Log.w(TAG, "User check failed", task.getException());
                            Toast.makeText(MainActivity.this, "There is a problem, please try again later.", Toast.LENGTH_SHORT).show();
                        }
                    }
                });
    }

    private void createAccount(String email, String password) {
        mAuth.createUserWithEmailAndPassword(email, password)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {

                        FirebaseUser user = null;
                        if (task.isSuccessful()) { user = mAuth.getCurrentUser(); }
                        else {
                            Log.w(TAG, "createUserWithEmail:failure", task.getException());
                            Toast.makeText(MainActivity.this, "Authentication failed.", Toast.LENGTH_SHORT).show();
                        }
                        updateUI(user);

                    }
                });
    }

    private void signIn(String email, String password) {
        mAuth.signInWithEmailAndPassword(email, password)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {

                        FirebaseUser user = null;
                        if (task.isSuccessful()) { user = mAuth.getCurrentUser(); }
                        else {
                            Log.w(TAG, "signInWithEmailAndPassword:failure", task.getException());
                            Toast.makeText(MainActivity.this, "Authentication failed.", Toast.LENGTH_SHORT).show();
                        }
                        updateUI(user);

                    }
                });
    }

    private void updateUI(FirebaseUser user) {
        if(user == null) {
            text.setText("NO USER");
        }else{
            text.setText(user.getEmail() + " authed");
        }
    }

}