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Some commonly used methods have nondeterministicspecifications, e.g., iterating through a set canreturn the elements in any order. However, non-deterministicspecifications typically have deterministic implementations, e.g.,iterating through two sets constructed in the same way mayreturn their elements in the same order. We use the termADINS code to refer to code that Assumes a DeterministicImplementation of a method with a Non-deterministic Specification. Such ADINS code can behave unexpectedly whenthe implementation changes, even if the specification remainsthe same. Further, ADINS code can lead to flaky tests -- teststhat pass or fail seemingly non-deterministically. We present a simple technique, called NONDEX, for detectingflaky tests due to ADINS code. We implemented NONDEX forJava: we found 31 methods with non-deterministic specificationsin the Java Standard Library, manually built non …