Android R.java file
Android R.java is an auto-generated file by aapt (Android Asset Packaging Tool) that contains resource IDs for all the resources of res/ directory.
If you create any component in the activity_main.xml file, id for the corresponding component is automatically created in this file. This id can be used in the activity source file to perform any action 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.