The Java object-oriented programming language has been the subject of an important involvement from programmers and industry. Especially for applications related to the Web. The problem of such rapid penetration of Java programs into commercial product is that software engineers does not have any methodology and have to develop complex parallel applications. In this paper, we present, using a real (http://lglsun.epfl.ch/Team/GDM/DSGamma.html) but simple Web parallel application, a possible formal development methodology based on the stepwise refinement of CO-OPN/2 formal specifications. The application chosen is based on the Gamma paradigm, in which additions are realized across distributed multisets of integers maintained by local Java applets. CO-OPN/2 is an object-oriented specification formalism allowing the formal specification of abstract data types based on algebraic specification and of concurrent objects using a Petri net like approach. An interesting feature of CO-OPN/2 is that it gives a coherent solution for the expression of complex properties combining inheritance, concurrency and synchronizations. The refinement of formal descriptions for building progressively the Java program is presented in several refinement steps. This refinement process focuses on the distribution of data and treatments, and validation and verification are studied through the refinement of system properties.