Java Generics and Collections

6.10. Summing Up

We conclude by giving a checklist of places where reifiable types are required or recommended.

• An instance test must be against a reifiable type.

• A cast should usually be to a reifiable type. (A cast to a nonreifiable type usually issues an unchecked warning.)

• A class that extends THRowable must not be parameterized.

• An array instance creation must be at a reifiable type.

• The reified type of an array must be a subtype of the erasure of its static type (see the Principle of Truth in Advertising), and a publicly exposed array should be of a reifiable type (see the Principle of Indecent Exposure).

• Varargs should be of a reifiable type. (A vararg of a nonreifiable type will issue an unchecked warning.)

These restrictions arise from the fact that generics are implemented via erasure, and they should be regarded as the price one pays for the ease of evolution that we explored in the previous chapter.

For completeness, we also list restrictions connected with reflection:

• Class tokens correspond to reifiable types, and the type parameter in Class<T> should be a reifiable type. (See Section 7.2.)

These are discussed in the next chapter.