Predefining IDs via a separate file

Android allows that you define ID of user interface components dynamically in the layout files, via the@+id/your_id notation.

To control your IDs you can also create a file called ids.xml in your /res/values folder and define all IDs in this file.

<?xml version="1.0" encoding="utf-8"?>

<resources>

    <item name="button1" type="id"/>

</resources> 

This allow you to use the ID directly in your layout file.

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    tools:context=".MainActivity" >

   <Button


         android:id="@id/button1"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:layout_centerHorizontal="true"

        android:layout_centerVertical="true"

        android:layout_marginRight="27dp"

        android:text="Button" />

/RelativeLayout>