  Developers use Question and Answer (Q&A) websites to exchange knowledge and
expertise. Stack Overflow is a popular Q&A website where developers discuss
coding problems and share code examples. Although all Stack Overflow posts are
free to access, code examples on Stack Overflow are governed by the Creative
Commons Attribute-ShareAlike 3.0 Unported license that developers should obey
when reusing code from Stack Overflow or posting code to Stack Overflow. In
this paper, we conduct a case study with 399 Android apps, to investigate
whether developers respect license terms when reusing code from Stack Overflow
posts (and the other way around). We found 232 code snippets in 62 Android apps
from our dataset that were potentially reused from Stack Overflow, and 1,226
Stack Overflow posts containing code examples that are clones of code released
in 68 Android apps, suggesting that developers may have copied the code of
these apps to answer Stack Overflow questions. We investigated the licenses of
these pieces of code and observed 1,279 cases of potential license violations
(related to code posting to Stack overflow or code reuse from Stack overflow).
This paper aims to raise the awareness of the software engineering community
about potential unethical code reuse activities taking place on Q&A websites
like Stack Overflow.
