type combinations

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Stefan Ram - 29 Apr 2006 22:34 GMT

I have a class whose instances are both an A and a B:

interface A {}
interface B {}
class C implements A, B {}

The method »m« needs to declare that it returns somthing that
is also both an A and a B (but not necessarily a C).
I try to express this via the type paramter »T«:

<T extends A & B> T m(){ return new C(); }

I was hoping that the compiler (JDK 1.6 beta) would sse that
»new C()« satisfies this requirement. But the compiler
reports:

incompatible types
found : C
required: T
{ return new C(); }
1 error ^

How could the code be modified to express that:

- m returns an object that implements (or »extends«) both A and B, and
- »return new C()« is accepted within m's declaration for this purpose?

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Thomas Hawtin - 29 Apr 2006 22:00 GMT

> I have a class whose instances are both an A and a B:
>
[quoted text clipped - 7 lines]
>
> <T extends A & B> T m(){ return new C(); }

It's the caller that determines exactly what T is. T need not be a
subtype of C. What you appear to be trying to do is:

A&B m() { return new C(); } // Illegal.

or

? extends A&B m() { return new C(); } // Still illegal.

> I was hoping that the compiler (JDK 1.6 beta) would sse that
> »new C()« satisfies this requirement. But the compiler
> reports:

C does not always satisfy the requirements. Consider:

class D extends C {}
...
D d = YourClass.<D>m();

> How could the code be modified to express that:
>
> - m returns an object that implements (or »extends«) both A and B, and
> - »return new C()« is accepted within m's declaration for this purpose?

You cannot use intersection types other than for generic parameters (and
implicitly as the result of the ?: operator).

Tom Hawtin

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Unemployed English Java programmer
http://jroller.com/page/tackline/

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Stefan Ram - 29 Apr 2006 23:17 GMT

>What you appear to be trying to do is:
> A&B m() { return new C(); } // Illegal.

That is indeed what I'd like to express.

>C does not always satisfy the requirements. Consider:
>class D extends C {}
> D d = YourClass.<D>m();

I see.

Thanks for both the answers to my post!

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s.Siddharth.Sharma@gmail.com - 29 Apr 2006 23:07 GMT

hi Stefan,

You can only return something that is both an A and B by returning an
object of a class which implements these both interfaces given that
both A and B are non related.
The compiler is giving you error because T can be given any data type
same or different than C. Hence, the declared returning data type (T)
might not fall in the same type heirarchy of C.

Regards,
Sidd

> I have a class whose instances are both an A and a B:
>
[quoted text clipped - 22 lines]
> - m returns an object that implements (or »extends«) both A and B, and
> - »return new C()« is accepted within m's declaration for this purpose?

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