This question came up this morning on SO, I'm pretty conviced there was exactly the same question yesterday but I could not find the duplicate and gave up looking. What I DID see when searching was a huge number of questions, all pretty much on the same theme:
How do I do something in/from another class
Most of these questions have answers pointing out the specific problem in whatever code has been posted, but clearly the problem goes a bit deeper in most cases, since the asker doesn't understand OO programming, particularly the differences between classes and instances.
Is it enough to answer the specific, local question in these cases? Should the answerer be pointing to an OO programming primer? Do they count as duplicates?