It is very common for a real$-$world organisation to keep track of a collection of things that are important to it$.$ For example, a bird$-$watching club keeps track of birds they have seen.

The software that supports this scenario is based on two classes. The first class models the society $($or club$),$ and the second class models the things they keep track of$,$ the objects of interest.

Objects of interest $($like any other objects$)$ have attributes representing data about those objects, modelled by fields.

The society object maintains a map consisting of objects of interest as keys and an associated list of information as values.

For example, a bird watching club might have as keys in the map, all the bird species that members had sighted, and as values lists of the dates of the sightings. Birds would need a species field.

These classes form a kind of simple database. To prevent the contents of this database being lost when the program stops running, we make it persistent, by providing methods that can write the data out to a file, and then read it back in again at a future time. You will learn how to do this when you reach Chapter $14$ and do not need to consider the details at this stage.