一句话来说，R.java就是你的资源的别名，这个别名可以在java中直接使用，你不必理会它是图片，文件。它就是一个int值。

Once you provide a resource in your application (discussed in Providing
Resources),

you can apply it by referencing its resource ID. All resource IDs are defined
in your project’s R class, which the aapt tool automatically generates.
http://developer.android.com/guide/topics/resources/accessing-resources.html

http://www.satyakomatineni.com/akc/display?url=displaynoteimpurl&ownerUserId=satya&reportId=2883
http://www.yugandroid.in/android-tutorials/r-java-file.html