Type-directed programming is an important and widely used paradigm in the design of software. With this form of programming, an application may analyze type information to determine its behavior. By analyzing the structure of data, many operations, such as serialization, cloning, adaptors and iterators may be defined once, for all types of data. That way, as the program evolves, these operations need not be updated—they will automatically adapt to new data forms. Otherwise, each of these operations must be individually redefined for each type of data, forcing programmers to revisit the same program logic many times during a program’s lifetime.The Java language supports type directed programming with the instanceof operator and the Java Reflection API. These mechanisms allow Java programs to depend on the name and structure of the run-time classes of objects. However, the Java mechanisms for type-directed programming are difficult to use. They also do not integrate well with generics, an important new feature of the Java language.In this paper, we describe the design of several expressive new mechanisms for type-directed programming in Java, and show that these mechanisms are sound when included in a language similar to Featherweight Java. Basically, these new mechanisms pattern-match the name and structure of the type parameters of generic code, instead of the run-time classes of objects. Therefore, they naturally integrate with generics and provide strong guarantess about program correctness. As these mechanisms are based on pattern matching, they naturally and succinctly express many operations that depend on type information. Finally, they provide programmers with some degree of protection for their abstractions. Whereas instanceof and reflection can determine the exact run-time type of an object, our mechanisms allow any supertype to be supplied for analysis, hiding its precise structure from others.
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