Sample login Firebase Android.

LoginActivity.java

public class LoginActivity extends AppCompatActivity {
  
    private Firebase ref;
    private Firebase.AuthResultHandler authResultHandler;

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

        authResultHandler = new Firebase.AuthResultHandler() {

            @Override
            public void onAuthenticated(AuthData authData) {
                System.out.println("authData:" + authData.getUid());

                // login ok, open new Activity
            }

            @Override
            public void onAuthenticationError(FirebaseError firebaseError) {
                System.out.println("onAuthenticationError:" + firebaseError.getCode() + ":" + firebaseError.getMessage());
                
            }
        };

    }
    
    public void login(View view) {
        email = emailEditText.getText().toString();
        pass = passEditText.getText().toString();

        ref.authWithPassword(email, pass, authResultHandler);
    }
}