public class MainActivity extends AppCompatActivity {

    SignInButton button;
    FirebaseAuth mAuth;
    private static final int RC_SIGN_IN = 3;
    GoogleSignInClient mGoogleSignInClient;

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

        mAuth = FirebaseAuth.getInstance();
        button = findViewById(R.id.googleBtn);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

            }
        });
    }
 }