This paper presents a method to build software-prototypes of Computer-Based Systems (CBS) That describe and simulate parts of software and parts of hardware using common concurrent software Behavioral Object Oriented Model (BOOM). For the hardware objects this behavioral prototyping can be viewed as a pre-stage of specification and analysis before the real physical design. The CBS prototypes are built in Java using a method called Vowels2. This method permits the development of the prototype as a concurrent application, from the user-requirements to the software programming in Java. The process of prototype description is made step by step. These steps use a subset of the UML graphical diagrams. The Java coding is deduced from these commented diagrams, using general patterns of thread-active-objects linked by customized objects: synchronous stimuli, asynchronous FIFO-queues etc. The method is illustrated by a simplified example containing parallelism and synchronization: the blood-test machine.