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A software system is composed of interconnected components that are software objects. As with all systems a component may be a sub-system, i.e. a software system in its own right. Most of us have to deal with complex systems all the time, for example modern operating systems. They are complex, respond in many ways to many unpredictably ordered inputs and should, in principle, continue to run once started. Consider the Windows 95 (or Windows 98 or NT) software systems. One software system is the operating system itself and any application programs that are also running – such as word processors and web browsers. Each application program is itself a sub-system containing its own set of objects. One form of behaviour exhibited by operating systems is that they can create new and destroy existing instances of application programs. We consider software systems, and even simple computer programs, to be a network of software objects. Humans express software using a programming language, so to A note on spelling: In British English computer programs are spelt with a single ‘m’ and no ‘e’, whereas all other kinds of programme, such as television programmes and programmes of study or events etc. are all spelt with ‘mme’. A computer program can also be though of as an object itself. For example, a compiled Windows 98 program (i.e. a file with a ‘.exe’ extension to its name) can be thought of as a component object of of the system that is the Window operating system. This ‘exe-file’ object responds to messages sent to it, such as keypresses, mouse clicks and menu item selections. It has a set of internal data it stores and operates on, and it sends replies to messages received. In fact, for many parts of Windows, this is quite an accurate description.
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