Converting between structurally compatible function types

[Jed Wesley-Smith]

You are asking javac to know that the second type parameter is only used
covariantly (ie. only as a return type), but the compiler cannot know this
unless it goes and inspects every place it is used. This is because Java
has use-site variance, rather than declaration-site variance.

Consider that while a Function<String, Integer>is absolutely acceptable
where you need a Function<String, Object>, a Function<Integer, String> is
not acceptable when a Function<Object, String> is required. This is because
the first parameter is exclusively used as an input parameter. Inputs are
contravariant, and again you can only know this by inspecting the
use-sites, and nothing would prevent someone from casually using the first
type parameter in return position, or vice versa.

Contrast this with Scala that has declaration site variance, using '+' for
covariance and '-' for contravariance. The function definition is actually
Function1[-A, +B], and the compiler bars you from using these incorrectly.

Java can only really support what you want via helper methods that hide the
ugliness, eg:

  public <A, B> Function <A, B> covariant(Function<A, ? extends B> f) {
    @SuppressWarnings("unchecked")
    Function <A, B> fb = (Function <A, B>) f;
    return fb;
  }

  public <A, B> Function <A, B> contravariant(Function<? super A, B> f) {
    @SuppressWarnings("unchecked")
    Function <A, B> fb = (Function <A, B>) f;
    return fb;
  }

cheers,
jed

On 24 February 2015 at 04:31, Millies, Sebastian <
Sebastian.Millies at softwareag.com> wrote:

> Hello there,
>
> I have a very basic question: why does the following not compile:
>
> Function<String,Integer> f = s -> 1;
> Function<String,Object> g = f;
>
> After all, I would expect that anywhere where a function from String to
> arbitrary objects is required, I can supply a function to integers in
> particular.
> I guess Java just sees the names are different, and that’s it?
>
> How do I best convert between the two types?
>
>
> n  Sebastian
>
>
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