Android RadioGroup Cannot resolve symbol getCheckedRadioButtonId

I am new to android. At the moment I am working on some examples in my starter book "Android 5"

In the example I am working there is some code which is not working.

XML:

<RadioGroup
        android:id="@+id/rg_art"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">

        <RadioButton
            android:id="@+id/rb_art_netto"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/txt_netto"
            android:textSize="16dp"
            android:checked="true" />

        <RadioButton
            android:id="@+id/rb_art_brutto"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/txt_brutto"
            android:textSize="16dp" />
    </RadioGroup>

Activity:

package com.example.raven.tax_calc;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.RadioGroup;

public class FormularActivity extends Activity {

    public static final String BETRAG_KEY = "betrag";
    public static final String BETRAG_ART = "art";
    public static final String UST_PROZENT = "ust";

    // Betrag
    public void onClickBerechnen(View button) {
        final EditText txtBetrag = (EditText) findViewById(R.id.edt_betrag);
        final String tmpBetrag = txtBetrag.getText().toString();
        float betrag = 0.0f;
        if(tmpBetrag.length() > 0 ){
            betrag = Float.parseFloat(tmpBetrag);
        }
    }

    // Art des Betrages (Brutto, Netto)
    boolean isNetto = true;
    final RadioGroup rg = (RadioGroup) findViewById(R.id.rg_art);
    switch (rg.getCheckedRadioButtonId()) {
        case R.id.rb_art_netto:
            isNetto = "true";
            break;
        case R.id.rb_art_brutto:
            isNetto = false;
            break;
        default:
    }

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

rg.getCheckedRadioButtonId is red highlighted and mouse-over says "Cannot resolve symbol"

What am I doing wrong? Can't find an mistake :-(