  Automated random testing has shown to be an effective approach to finding
faults but still faces a major unsolved issue: how to generate test inputs
diverse enough to find many faults and find them quickly. Stateful testing, the
automated testing technique introduced in this article, generates new test
cases that improve an existing test suite. The generated test cases are
designed to violate the dynamically inferred contracts (invariants)
characterizing the existing test suite. As a consequence, they are in a good
position to detect new errors, and also to improve the accuracy of the inferred
contracts by discovering those that are unsound. Experiments on 13 data
structure classes totalling over 28,000 lines of code demonstrate the
effectiveness of stateful testing in improving over the results of long
sessions of random testing: stateful testing found 68.4% new errors and
improved the accuracy of automatically inferred contracts to over 99%, with
just a 7% time overhead.
