eclipse R.java Unable to open class file

I'm getting this error more and more, and it's getting stickier, harder to flush out. Often, refreshing the project in the explorer view (but not doing the trick now), or poking around with some other things, clears it out. B开发者_如何学Pythonut, I would like to have some definitive way to fix this or to know why this happening and fix the fundamental problem.

The message from the console in red:

[2011-05-15 00:25:13 - ] ERROR: Unable to open class file \R.java: No such file or directory

Thanks,

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Not sure if this is the right way to do this, but this question is essentially a duplicate of this one, with a chock full of helpful suggestions.

Strange... thought I set that correctly a while ago... but here it is again

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Not sure if this is your problem, but sometimes eclipse freaks on me and Project -> Clean... does the trick.