  Code cloning plays a very important role in open-source software engineering.
The presence of clones within a project may indicate a need for refactoring,
and clones between projects are even more interesting, since code migration
takes place and violations are possible. But how is code being copied? How
prevalent is the process and on what level does it happen?
  In this general study, we attempt to shed some light on these questions by
searching for clones in a large dataset of over 23 thousand Java projects on
the level of both files and methods, and by studying the code fragments
themselves and their clone pairs. We study the size and the age of code
fragments, the prevalence of their clones, relationships between exact and
non-exact clones, as well as between method-level and file-level clones. We
also discover and describe various anomalies in the code clones that were
detected in the dataset.
  Our research shows that the copying occurs all through the years of the Java
code existence and that method-level copying is much more prevalent than
file-level copying, with only 35.4% of methods having no clones at all.
Additionally, some of the discovered anomalies can be useful for future
large-scale cloning research as they can be used for removing auto-generated
code.

Software Engineering