Your Button Tag not closed into activity_main.xml
<Button
android:id="@+id/btn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Display"
Close that
<Button
android:id="@+id/btn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Display" />
and clean project. Your R.java
class will be generated.