In this paper we present a concept for automated testing of object-oriented applications and a tool called SeDiTeC that implements these concepts for Java applications. SeDiTeC uses UML sequence diagrams, that are complemented by test case data sets consisting of parameters and return values for the method calls, as test specification and therefore can easily be integrated into the development process as soon as the design phase starts. SeDiTeC supports specification of several test case data sets for each sequence diagram as well as to combine several sequence diagrams to so-called combined sequence diagrams thus reducing the number of diagrams needed. For classes and their methods whose behavior is specified in sequence diagrams and the corresponding test case data sets SeDiTeC can automatically generate test stubs thus enabling testing right from the beginning of the implementation phase. Validation is not restricted to comparing the test case data sets with the observed data, but can also include validation of pre- and postconditions.