Java, Subclass return generics of generics

Question

Does anyone can tell me how to code the return in the function "getDirectHair()" ?
I want to create a method which is like a shortcut in the Human class to directly return the good type of Hair class (below named "h.getDirectHair()") instead of use "h.getPerson().getHair()".
I want to use the type <?> of Person<?> declared in Human class.

package test; public class Test { public static void main(String[] args) { Human<Bob> h = new Human<Bob>(); Blond blond = h.getPerson().getHair(); // no cast needed, because Human<Bob> is blond //how to do if I want to use directly this : blond = (!!!) h.getDirectHair(); //need cast !! Blond or Brown ? } } class Human<T extends Person<?>>{ private T person = null; public T getPerson() { return person; } public /* <?> */ Object getDirectHair(){ // => I want to return the type <?> of Person<?> // instead of Object, how to ?? return person.getHair(); } } class Person<T extends Hair> { T hair; public Person(T hairr) { hair = hairr; } public T getHair() { return hair; } } class Bob extends Person<Blond> { public Bob(Blond bean) { super(bean); } public Blond getHair() { return super.getHair(); } } class Barack extends Person<Brown> { public Barack(Brown bean) { super(bean); } public Brown getHair() { return super.getHair(); } } class Hair { } class Blond extends Hair { } class Brown extends Hair { }

Many thanks and best regards,
David.

@TheTerribleSwiftTomato I assume Hair is correct, as getHair return Blond and Brown — Jesper Fyhr Knudsen
– Jesper Fyhr Knudsen, Commented Jul 17, 2013 at 10:56
@user2590746 why do you want to use generics for this? it seems like the ideal case of inheritance, Bob extends Person, Person extends Human, Human has getHair — Jesper Fyhr Knudsen
– Jesper Fyhr Knudsen, Commented Jul 17, 2013 at 10:59
Ah, my bad, read too quickly and assumed getDirectHair() is about inheritance. — mikołak
– mikołak, Commented Jul 17, 2013 at 11:31

JB Nizet · Accepted Answer · 2013-07-17 11:08:34Z

4

Here's how to fix the problem:

public class Test { public static void main(String[] args) { Human<Blond, Bob> h = new Human<Blond, Bob>(); Blond blond = h.getPerson().getHair(); blond = h.getDirectHair(); } } class Human<H extends Hair, T extends Person<H>>{ private T person = null; public T getPerson() { return person; } public H getDirectHair(){ return person.getHair(); } }

answered Jul 17, 2013 at 11:08

JB Nizet

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5 Comments

Narendra Pathai Over a year ago

I guess in the existing heirarchy in the question it is not possible. Am I right?

JB Nizet Over a year ago

The hierarchy in the question is preserved by my solution. The difference is that the Human class has an additional generic type H.

JB Nizet Over a year ago

Then no, it's not possible. The type of the hair is unknown at compile time (?), so a cast is needed.

d3d3 Over a year ago

yes I think also this is the only solution, but it's like a double declaration: first Human<Blond, Bob> h = new Human<Blond, Bob>(); and second class Bob extends Person<Blond>. The link between Bob and Blond is wrote twice. And if you need to do this for many variables, not so good.

Louis Wasserman Over a year ago

+1 for the best answer you're going to get with Java generics.

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