public class AuthenticationActivity extends AppCompatActivity implements View.OnClickListener {

    EditText username, password;
    Button signInButton, registerButton;

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

        username = (EditText) findViewById(R.id.username);
        password = (EditText) findViewById(R.id.password);
        signInButton = (Button) findViewById(R.id.signInButton);
        registerButton = (Button) findViewById(R.id.registerButton);

        signInButton.setOnClickListener(this);
        registerButton.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.signInButton:
                handleLogin();
                break;
            case R.id.registerButton:
                handleRegistration();
                break;
        }
    }

    private void handleLogin() {}
    private void handleRegistration() {}
}