What is the concept behind R.java?

Question

In android R.java is used to provide access to resources defined in XML files. To access the resource we need to invoke findViewById() method passing in the id

Answer 1

android.R.java is not just where XML ids are stored. It also contains access to resources - such as drawables, layouts, strings, arrays, and basically anything you can declare in resources.

Personally I find that it is useful when using Eclipse. I can simply type findViewById(R.id. and Eclipse will show a tooltip with a list of options to choose from.

However at a platform level, I would say that the hardcoded id variables help prevent errors when using Strings to identify resources -- something that can be debuggable while programming (or during compilation, rather than runtime).