Some Examples of Generics in Java 1.5

The forthcoming releases of Java JDK v1.5 and C# v2.0 support generic classes (classes with generic type parameters) and generic methods. Furthermore, each also supports constrained generic types. Much has been written about generic types. In " A Comparative Study of Language Support for Generic Programming " by Garcia et al details of generic types and programming in C++, Haskell, Standard ML, Eiffel, Java and C# are compared. Useful details and generic coding examples from the soon to be released Java JDK 1.5 (now in beta) are presented in the paper " Generics in the Java Programming Language " by Gilad Bracha. Some of the examples used in this column were inspired by this paper. Generic types in Java and C# introduce more expressiveness at the source code level and move type checking from run-time to compile-time when inserting objects into generic collections. In the current pre-generic versions of Java and C#, genericity in collections is obtained through the backdoor of using the universal super type Object as a polymorphic placeholder for the actual reference type that defines the objects to be inserted into the collection. Other than programmer comments, there is little in the source code that reveals the type of object that the programmer intends to hold in the collection. Only by carefully reverse-engineering the source code does this become apparent if in fact only a single type is inserted into the non-generic collection. Generic collections in both Java and C# provide useful self-documentation in the source code in addition to strengthening compile-time type checking. Java and C# use invariant generic typing in contrast with Eiffel which uses covariant typing. In Java and C# a List is not a subtype of a List. These are considered separate stand-alone classes. In Eiffel, a List would be considered a subtype of a List if B conforms to A (B is a descendent of A or equal to A). Although covariant typing generally leads to more flexibility in the use of generics, it has been shown to allow code that is statically correct but fails at runtime (see " Type-Save