I’ve been doing a little development generify some Java classes to make them reusable across a couple of web applications and wanted to share this neat trick I learned along the way.
Method that returns an object of the same class and type arguments as the callee
I think this is a fairly common requirement for example when you have an object that replaces another. A contrived example
class Magazine {
Magazine nextEdition;
Magazine nextEdition() {
return this.nextEdition;
}
}
For some reason you want to make this class generic by genre.
interface Genre {
}
class Magazine<G extends Genre> {
}
How do you ensure nextEdition() returns the same genre? The answer is to add an additional generic type parameter that references itself
class Magazine<M extends Magazine<M, G>, G extends Genre> {
M nextEdition;
M nextEdition() {
return this.nextEdition;
}
}
Not obvious, a little verbose, but it works!