R.java file

Android R.java is an auto-generated file by an app (Android Asset Packaging Tool) that includes Resource IDs for all resources in the Race / Directory.

If you create an element in the activity_main.xml file, then the ID for this file is automatically created in this file. This ID can be used in the activity source file to do any work on the component.

Note: If you delete R.jar file, android creates it automatically.

Let's see the android R.java file. It includes a lot of static nested classes such as menu, id, layout, attr, drawable, string etc.

/* 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.example.helloandroid;  
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. 
        Base application theme for API 11+. This theme completely replaces 
        AppBaseTheme from res/values/styles.xml on API 11+ devices. 
 API 11 theme customizations can go here.  
        Base application theme for API 14+. This theme completely replaces 
        AppBaseTheme from BOTH res/values/styles.xml and 
        res/values-v11/styles.xml on API 14+ devices. 
 API 14 theme customizations 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;  
    }  
}