  In modern programming languages, exception handling is an effective mechanism
to avoid unexpected runtime errors. Thus, failing to catch and handle
exceptions could lead to serious issues like system crashing, resource leaking,
or negative end-user experiences. However, writing correct exception handling
code is often challenging in mobile app development due to the fast-changing
nature of API libraries for mobile apps and the insufficiency of their
documentation and source code examples. Our prior study shows that in practice
mobile app developers cause many exception-related bugs and still use bad
exception handling practices (e.g. catch an exception and do nothing). To
address such problems, in this paper, we introduce two novel techniques for
recommending correct exception handling code. One technique, XRank, recommends
code to catch an exception likely occurring in a code snippet. The other,
XHand, recommends correction code for such an occurring exception. We have
developed ExAssist, a code recommendation tool for exception handling using
XRank and XHand. The empirical evaluation shows that our techniques are highly
effective. For example, XRank has top-1 accuracy of 70% and top-3 accuracy of
87%. XHand's results are 89% and 96%, respectively.
