The Object Constraint Language (OCL) is a formal specification language that could be used for constraining the modelling elements that occur in UML diagrams. The Java Modeling Language (JML) is a behavioural interface specification language designed for specifying Java classes and interfaces. This paper investigates the mapping of OCL constraints to JML constraints together with its properties. In particular the paper shows how those constructs such as iterators that are not directly supported by JML are transformed to an equivalent form that can be mapped directly to JML. The paper proposes a special library for OCL types implemented in Java that facilitates the mapping. Extracts from a case study will be used to illustrate the mapping.