Java generics of generics of

Question

I have a generic class which represents a fragment of text. That fragment of text may have any of a number of different modes (different types of highlighting). Those mode

Answer 1

Without testing, I'd guess:

public class TextFragment<E extends TextFragmentMode<E>> {

Hmm, short test shows it doesn't seem to work either...

Answer 2

TextFragment<E> needs to say two things about E.

• It "extends" TextFragmentMode<E>.
• In order to do that, you must also constrain it to extend Enum<E>.

Because of Java inheritance wonkiness, you need to write that the other way around:

public class TextFragment<E extends Enum<E> & TextFragmentMode<E>> {

Answer 3

You need to introduce a new type that accounts for the bound of Enum

public class TextFragment<T extends Enum<T>, E extends TextFragmentMode<T>> {

Answer 4

The problem is that you're trying to make E extend TextFragmentMode and Enum, which aren't related types. What type E would satisfy both constraints?

I suspect you want two type parameters, something like this:

public class TextFragment<E extends Enum<E>, M extends TextFragmentMode<E>>

Now you have each constraint expressed on a different type parameter, and they both make sense - you can definitely find an E which is an enum, and an M which is a TextFragmentMode<E>. However, it's pretty complicated...

... do you definitely need it to be this generic? What will you be doing with M in the class? Could you not just take a TextFragmentMode<E> as a constructor parameter (or whatever) and make it generic in one type parameter again?