R.Java in Android.

It is auto generated file by android asset packaging tool(aapt) , r.java contains different id's for all resource files.

If u create any component in activity_main.xml that will automatically creates a resource id for that component.That id will traverse through out the program.

R.java for Hello World Example:

 /* AUTO-GENERATED FILE.  DO NOT MODIFY.
 *
 * This class was automatically generated by the
 * aapt tool from the resource data it found.  It
 * should not be modified by hand.
 */

package com.siddhu.helloworld;

public final class R {
    public static final class attr {
    }
    public static final class drawable {
        public static final int ic_launcher=0x7f020000;
    }
    public static final class id {
        public static final int menu_settings=0x7f070000;
    }
    public static final class layout {
        public static final int activity_main=0x7f030000;
    }
    public static final class menu {
        public static final int activity_main=0x7f060000;
    }
    public static final class string {
        public static final int app_name=0x7f040000;
        public static final int hello_world=0x7f040001;
        public static final int menu_settings=0x7f040002;
    }
    public static final class style {
        /** 
        Base application theme, dependent on API level. This theme is 
  replaced by AppBaseTheme from res/values-vXX/styles.xml on 
  newer devices.
    

            Theme customizations available in newer API levels can go in
            res/values-vXX/styles.xml, while customizations related to
            backward-compatibility can go here.
        
         */
        public static final int AppBaseTheme=0x7f050000;
        /**  Application theme. 
 All customizations that are NOT specific to a particular 
 API-level can go here. 
         */
        public static final int AppTheme=0x7f050001;
    }
}