With most type systems there's never a problem with being able to express nonexistent types. In some type systems, such "nonexistent types" itself are useful to

With most type systems there's never a problem with being able to express nonexistent types. In some type systems, such "nonexistent types" itself are useful tools. (Where by "nonexistent type" I mean "a type for which there cannot be an instance". "imaginary type" might be a better term, since the type itself exists in the type system.) The problem here lies in the fact that Java lets you create an instance of any type you can write the signature for via `null`. This lies in conflict with the ability to specify nonexistent types (which is a relatively common ability in type systems and might not even be possible to excise without causing significant complexity).