android:id="@+id/button1" add button1`s id to R.java

In common the xml is mark language like html.
the configure info is in first mark label

<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="1" />
and end with "/>" or with end label "</Button>"

there are some mark:
id: id
layout_width="warp_content" or "match_parent"
every label has it`s attributes ,we can set these
attributes in first label.