A constructivism-based approach to teach the object-oriented (OO) programming paradigm in introductory computer courses was developed and used for several years. A multi-entity system from every-day life was adopted, to exploit the novice programmer's existing knowledge and build on it the OO conceptual framework. A sequence of assignments has been designed and developed to allow students exposed to this approach to experiment with Java programming and see how the OO conceptual framework is implemented. In this paper, this sequence of assignments is presented, discussed and evaluated in the context of the defined approach. The set of assignments that is based on a software-engineering-centered view and more precisely on a design-first approach, comes with the description of the strategy and graded hints that lead students to the final solution. Although it was first implemented as supplementary material, it quickly became the core component of the course.