Android: Center position of the button (fill_parent, wrap_content)

<Button android:id="@+id/btClick" android:gravity="center_vertical|center_horizontal"
android:layout_gravity="center_vertical|center_horizontal|center" 
android:layout_height="wrap_content" 
android:layout_width="100px" 
android:text=""/>

android:layout_width="fill_parent" - object have width like parent
android:layout_height="wrap_content" - parent height wants to be just big enough to enclose its content