RadioGroup checkedButton property

Use android:checkedButton="@+id/rdb_positive" ,i think you add + sign then its works

try this......

<RadioGroup    android:layout_width="wrap_content"    android:layout_height="wrap_content"    android:orientation="vertical" >    <RadioButton        android:id="@+id/rdb_positive"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:checked="true"        android:text="answer_positive" />    <RadioButton        android:id="@+id/rdb_negative"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="answer_negative" /></RadioGroup>

You can get rid of that error by declaring id rdb_positive inside ids.xml and then referencing the id from both RadioGroup and RadioButton elements.

<RadioGroup    style="@style/FormInputField"    android:orientation="vertical"    android:checkedButton="@id/rdb_positive"> <!-- REFERENCE TO ids.xml -->    <RadioButton        android:id="@id/rdb_positive"         android:text="@string/answer_positive" /> <!-- REFERENCE TO ids.xml -->    <RadioButton        android:id="@+id/rdb_negative"        android:text="@string/answer_negative" /></RadioGroup>

ids.xml:

<resources>    <item type="id" name="rdb_positive" /></resources>