Using fixed type constraints inside an interface in Java?

Question

interface I1 { ... }
interface I2 { ... }
interface I3 { ... }
interface I4 { ... }

interface MyFactory {
  Object<? extends I1 & I2 & I3> createI1I2I3(); // doesn't work
  Object<? extends I2 & I3 & I4> createI2I3I4(); // doesn't work
}

Is there a trick to do it? I was thinking about things like

interface I1I2I3 extends I1, I2, I3 { ... }

But I1I2I3 != <? extends I1 & I2 & I3>. There's a reason I just can't use this approach - I1, I2 and I3 are foreign code.

Update

For those who curious why might someone need such a weird thing:

interface Clickable {}
interface Moveable {}
interface ThatHasText {}

interface Factory {
  Object<? extends Clickable> createButton(); // just a button with no text on it
  Object<? extends Clickable & ThatHasText> createButtonWithText();
  Object<? extends Moveable & ThatHasText> createAnnoyingBanner();
}

I got to learn the use-case for such a construct ...

Johan Sjöberg
– Johan Sjöberg

2011-09-26 18:55:47 +00:00
Commented Sep 26, 2011 at 18:55 — Johan Sjöberg
– Johan Sjöberg, Commented Sep 26, 2011 at 18:55

Johan Sjöberg · Accepted Answer · 2011-09-26 19:00:04Z

2

Object doesn't accept type parameter You can use the following construct instead:

interface I1 {  }
interface I2 {  }
interface I3 {  }
interface I4 {  }

interface MyFactory {
    public <T extends I1 & I2 & I3> T createI1I2I3(); 
    public <T extends I2 & I3 & I4> T createI2I3I4(); 
}

answered Sep 26, 2011 at 19:00

Johan Sjöberg

49.4k22 gold badges135 silver badges150 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Kal · Accepted Answer · 2011-09-26 19:00:58Z

2

Your return type should be parameterized, so you can do

interface MyFactory {

   <T extends I1 & I2 & I3> T createI1I2I3();

}

answered Sep 26, 2011 at 19:00

Kal

24.9k8 gold badges67 silver badges66 bronze badges

Collectives™ on Stack Overflow

Using fixed type constraints inside an interface in Java?

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related