New method on java.util.function.Function -- ternary method

[David Alayachew]

Hello all,

I have an idea that I want to run by you all -- a new method on
java.util.function.Function to capture ternary operations.

Here is a mockup of what I wanted to do.

>
> import java.util.function.Function;
>
> @FunctionalInterface
> public interface Function2<I, O> extends Function<I, O>
> {
>
>    public static <I, O> Function<I, O> ternary
>    (
>        Predicate<I> test,
>        Function<I, O> trueOutput,
>        Function<I, O> falseOutput
>    )
>    {
>
>       return
>           (I input) ->
>               test.test(input)
>               ? trueOutput.apply(input)
>               : falseOutput.apply(input)
>               ;
>
>    }
>
> }
>

I think this is useful for a few reasons.

 * This composes, just like the ternary operator itself.

 * It pairs well with Function.identity() and method references to clearly
(but concisely) communicate intent.

 * Ternary operations are common, so this will find great use by developers
of all sorts.

There is at least one part I don't quite like about this design - what if
one (or both!) of your outputs is not a functional transformation of the
input?

For example, String username = id.isBlank() ? "UNKNOWN" : lookup(id);

Obviously, this is easy to work around - simply ignore the input of the
function. But you lose clarity and simplicity that way. I would put a note
in the javadoc that says that this method should only be used in instances
where both outputs are a functional transformation of the input. That way,
intent is clarified. But if we go that route, maybe this function should
get a better name to capture that? testThenApply? ternaryTransform?
ternaryApply?

Thank you for your time and help!
David Alayachew
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