The GEANT4 experience - The object oriented design and production of a large program

GEANT4 is a detector simulation "toolkit", currently being implemented by an international group of about 100 physicists and programmers, under the auspices of CERN's Research and Development programme RD44. It is the first major piece of particle physics software to be written using object oriented techniques; the chosen language is C++. It will supersede GEANT3, a Fortran package which was becoming very difficult to maintain. GEANT4 will be much more flexible, and will allow the user to implement his or her own physics processes and geometrical shapes in an object oriented way. This paper describes our experience, half way into a 4 year project, of using these new methods and languages.