We have seen several ways in which a mapping can be implemented:

- A function such as
`sqrt`maps from its argument to the target value. - If the domain is a finite, compact set of integers, we can store
the target values in an array and look them up quickly.
- If the domain is a finite set, we can use a lookup table such
as an association list,
`TreeMap`or`HashMap`. - If the domain is a finite set represented as an array or linked list, we can create a corresponding array or list of target values.