  A feature-oriented product line is a family of programs that share a common
set of features. A feature implements a stakeholder's requirement, represents a
design decision and configuration option and, when added to a program, involves
the introduction of new structures, such as classes and methods, and the
refinement of existing ones, such as extending methods. With feature-oriented
decomposition, programs can be generated, solely on the basis of a user's
selection of features, by the composition of the corresponding feature code. A
key challenge of feature-oriented product line engineering is how to guarantee
the correctness of an entire feature-oriented product line, i.e., of all of the
member programs generated from different combinations of features. As the
number of valid feature combinations grows progressively with the number of
features, it is not feasible to check all individual programs. The only
feasible approach is to have a type system check the entire code base of the
feature-oriented product line. We have developed such a type system on the
basis of a formal model of a feature-oriented Java-like language. We
demonstrate that the type system ensures that every valid program of a
feature-oriented product line is well-typed and that the type system is
complete.
