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# 9 Genericity

Building software components (classes) as implementations of abstract data types yields systems with a solid architecture but does not in itself ensure reusability and extendibility. Two key techniques address the problem: generosity (unconstrained or constrained) and inheritance. Let us look first at the unconstrained form.

To make a class generic is to give it formal generic parameters representing as unknown types, as in these examples from EiffelBase, an open-source library covering basic data structures and algorithms:

    ARRAY [G]
    LIST [G]
    LINKED_LIST [G]

These classes describe data structures -- arrays, lists without commitment to a specific representation, lists in linked representation -- containing objects of a certain type. The formal generic parameter G denotes this type.

A class such as these doesn't quite yet describe a type, but a type template, since G itself denotes an unknown type. To derive a directly usable list or array type, you must provide a type corresponding to G , called an actual generic parameter; this may be either an expanded type, including basic types such as INTEGER , or a reference type. Here are some possible generic derivations:

    il: LIST [INTEGER]
    aa: ARRAY [ACCOUNT]
    aal: LIST [ARRAY [ACCOUNT]]

As the last example indicates, an actual generic parameter may itself be generically derived.

It would not be possible, without genericity, to have static type checking in a realistic object-oriented language.

A variant of this mechanism, constrained genericity, will enable a class to place specific requirements on possible actual generic parameters. Constrained genericity will be described after inheritance.