RFR: 8317993: Add capturing factories to classes in java.util.function package

[Raffaello Giulietti]

On Tue, 17 Oct 2023 08:05:09 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> This PR proposes to add a number of "capturing factories" in classes in the `java.util.function` package.
> 
> The PR additionally (an optionally) proposes to add a new function `UnaryOperator::andThenUnary` to allow composition while retaining the `UnaryOperator` type.
> 
> With the new changes, it is possible to write code like this (example from `java.util.function.Function`):
> 
> 
> // Resolve ambiguity
> var function = Function.of(String::isEmpty); // Function<String, Boolean>
> var predicate = Predicate.of(String::isEmpty); // Predicate<String>
> 
> // Fluent composition
> var chained = Function.of(String::length)       // Function<String, Integer>
>                    .andThen(Integer::byteValue);      // Function<String, Byte>
> 
> 
> Please see the original bug report for a comprehensive description of these proposed changes.
> 
> Note: It is not the objective to promote `var` declaration or to prevent previous ways of capturing lambdas and method references. The comments in the code above is for explaining the binding and once that becomes obvious, such comments are likely to not appear in real code. Users that prefers having a normal type declaration can still do that.
> 
> Note: Functional interfaces of primitives have not been considered (in this round). Otherwise, functional interfaces that might be ambiguous or that supports composition have been included. Hence, `Supplier` did not get a factory method.

src/java.base/share/classes/java/util/function/BiConsumer.java line 78:

> 76:     /**
> 77:      * {@return a representation of the provided {@code uncaptured} lambda or method reference
> 78:      * in the form of a {@code BiConsumer}}

There's no need for the provided arguments to this and the other "of" methods to be a lambda or a method reference expression, although it is true all that the methods are useful especially for such cases.

src/java.base/share/classes/java/util/function/BiConsumer.java line 105:

> 103:      */
> 104:     static <T, U> BiConsumer<T, U> of(BiConsumer<T, U> uncaptured) {
> 105:         return Objects.requireNonNull(uncaptured);

This does more than just capturing: not sure if the `null` check is generally useful here and elsewhere.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16213#discussion_r1362091242
PR Review Comment: https://git.openjdk.org/jdk/pull/16213#discussion_r1362105477