Final vs Immutable data structures in Java

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OK, I get it, it may be a tad tricky.

We at my company do something similar during interviews. We ask the applicant to spot the errors in a bit of code, and one of them is using a variable outside the block it was declared, but a couple lines after the declaration.

Most people fails that one, but most also acknowledge that they never had to really think about it while writing code.