This chapter will continue the introduction to classes by developing and explaining a second, rather trivial, application which is intended to present and explain the concepts of data attributes, class wide actions, instance actions, class constants and constructors, which were mentioned or briefly introduced in Chapter 1. The application is described as trivial only in the sense that it does not do anything useful, its conceptual understanding is anything but trivial.

The classes presented in the previous chapter contained only a single action which always returned the same information. By adding data attributes to a class declaration it is possible for each instance to maintain information about its own state and to change this information as its actions are used. The main() action from the client class in Chapter 1 was described as a class wide action, which means that it is associated with the entire class and can be used without having first to declare an instance of the class. The alternative possibility is to have instance actions where the action can only be used by first declaring an instance of the class.

The distinction between class wide and instance actions is related to the distinction between class wide data attributes and instance data attributes. There is only one copy of a class wide attribute for the entire class, and it can be accessed by any instance or class wide action. Instance attributes have one copy per instance of the class and can only be accessed by instance actions. A class wide attribute can also be declared to have a constant value, in which case it is known as a class constant, and unlike a class or instance variable attribute its value cannot be changed by any actions.

All the advice which was given at the start of Chapter 1 concerning how to approach that chapter can be repeated for this chapter. A full understanding of the concepts in this chapter is unlikely to be obtained when it is followed for the first time. It will take time and effort to assimilate and accommodate these concepts. It will also be of benefit to revisit this chapter after Chapter 3 has been completed, in order to consolidate the initial understanding.

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