The problem is that the documents are not very accurate in determining where exactly you will find the java file, which should be in the gen folder. I ran into the same problem and broke my head over it until I found that the .java file was spawned and it was on the way:
\ application \ assembly \ generated \ source \ aidl \ Debug \ com \ Android \ vending \ billing \ IInAppBillingService.java
This is not a headache problem, and your code runs smoothly without any intervention. Hope this helps!

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