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R cannot be resolved to a variable

I am running Ubuntu 12.04.

Last night I had the Java ADT working, and running the first part of the Android app track here. However, after leaving the ADT and re-entering it I now get a few error messages

R cannot be resolved to a variable MainActivity.java

R cannot be resolved to a variable MainActivity.java

These errors are preventing the app from playing.

What is going on?

1 Answer

In eclipse the R file (which is an automatically generated resource id file) is rebuilt every so often. If there is another error in your program R will not build and so eclipse will tell you it doesn't exist. A good way to refresh the project is to clean it. (In eclipse, project, clean, clean all)