Preliminary Design of JML: A Behavioral Interface Specification Language for Java by Gary T. Leavens, Albert L. Baker, and Clyde Ruby Abstract JML is a behavioral interface specification language tailored to Java(TM). Besides pre- and postconditions, it also allows assertions to be intermixed with Java code; these aid verification and debugging. JML is designed to be used by working software engineers; to do this it follows Eiffel in using Java expressions in assertions. JML combines this idea from Eiffel with the model-based approach to specifications, typified by VDM and Larch, which results in greater expressiveness. Other expressiveness advantages over Eiffel include quantifiers, specification-only variables, and frame conditions. This paper discusses the goals of JML, the overall approach, and describes the basic features of the language through examples. It is intended for readers who have some familiarity with both Java and behavioral specification using pre- and postconditions. Keywords: Behavioral interface specification, Java, JML, Eiffel, Larch, model-based specification, assertion, precondition, postcondition, frame. 2000 CR Categories: D.2.1 [Software Engineering] Requirements/Specifications --- languages, tools, theory, Larch, Eiffel, JML, ESC/Java; D.2.4 [Software Engineering] Software/Program Verification --- assertion checkers, class invariants, formal methods, programming by contract; D.2.7 [Software Engineering] Distribution and Maintenance --- documentation; F.3.1 [Logics and Meanings of Programs] Specifying and Verifying and Reasoning about Programs --- assertions, invariants, logics of programs, pre- and post-conditions, specification techniques.