Addition of Predicate-based findIndex and findLastIndex methods to java.util.List

[Viktor Klang]

Sadly, there is no way to define a short-circuiting collector :(

That was my first course of exploration for JEP-461, and unfortunately I found no great way of retrofitting that behavior, especially not in a backwards-compatibility-safe way.

You can have a short-circuiting Gatherer like this
  <T> Gatherer<T, ?, Integer> findIndex(Predicate<? super T> predicate) {
    return Gatherer.ofSequential(
      () -> new Object() { int index; },
      Integrtor.ofGreedy((state, element, downstream) -> {
        var index = state.index++;
        if (predicate.test(element)) {
          return downstream.push(index);
        }
        return true;
      }));
  }

Since this would ne a short-circuiting Gatherer, it would not qualify as Greedy.

With that said, you could create something like:

public static <T, R> Gatherer<T, ?, Long> indiciesWhere(Predicate<T> p) {
       class Index { long at; }
       return Gatherer.ofSequential(
               Index::new,
               Integrator.ofGreedy((idx, e, d) -> {
                 var current = idx.at++;
                 return !p.test(e) || d.push(current);
               })
       );
}


So you could do: list.stream().gather(indiciesWhere(predicate)).findFirst()

Cheers,
√


Viktor Klang
Software Architect, Java Platform Group
Oracle

________________________________
From: core-libs-dev <core-libs-dev-retn at openjdk.org> on behalf of Remi Forax <forax at univ-mlv.fr>
Sent: Friday, 19 April 2024 19:47
To: ІП-24 Олександр Ротань <rotan.olexandr at gmail.com>
Cc: core-libs-dev <core-libs-dev at openjdk.org>
Subject: Re: Addition of Predicate-based findIndex and findLastIndex methods to java.util.List

Hello,
for me, it seems what you want is  Collector on Stream which is able to short-circuit,
so you can write
  list.stream().collect(Collectors.findFirst(s -> s.contains("o")))
and in reverse
  list.reversed().stream().collect(Collectors.findFirst(s -> s.contains("o")))

Using a Stream here is more general and will work with other collections like a LinkedHashSet for example.
Sadly, there is no way to define a short-circuiting collector :(

You can have a short-circuiting Gatherer like this
  <T> Gatherer<T, ?, Integer> findIndex(Predicate<? super T> predicate) {
    return Gatherer.ofSequential(
      () -> new Object() { int index; },
      Integrtor.ofGreedy((state, element, downstream) -> {
        var index = state.index++;
        if (predicate.test(element)) {
          return downstream.push(index);
        }
        return true;
      }));
  }

and use it like this:
  list.stream().gather(findIndex(s -> s.contains("o"))).findFirst().orElse(-1);

But it's more verbose.

I wonder if at the same time that the Gatherer API is introduced, the Collector API should be enhanced to support short-circuiting collectors ?

regards,
Rémi


________________________________
From: "ІП-24 Олександр Ротань" <rotan.olexandr at gmail.com>
To: "core-libs-dev" <core-libs-dev at openjdk.org>
Sent: Friday, April 19, 2024 5:59:39 PM
Subject: Addition of Predicate-based findIndex and findLastIndex methods to java.util.List
Subject
Addition of Predicate-based findIndex and findLastIndex methods to java.util.List

Motivation
The motivation behind this proposal is to enhance the functionality of the List interface by providing a more flexible way to find the index of an element. Currently, the indexOf and lastIndexOf methods only accept an object as a parameter. This limits the flexibility of these methods as they can only find the index of exact object matches.

Here I want to propose methods that would accept a Predicate as a parameter, allowing users to define a condition that the desired element must meet. This would provide a more flexible and powerful way to find the index of an element in a list.

The changes I am proposing are implemented in this PR: https://github.com/openjdk/jdk/pull/18639. Here is a brief overview of the changes made in this pull request:

Added the findIndex  (Predicate<? super E> filter) method to the List interface.
Added the findLastIndex  (Predicate<? super E> filter) method to the List interface.
Implemented these methods in all non-abstract classes that implement the List interface, as well as List itself (default impl).
The changes have been thoroughly tested to ensure they work as expected and do not introduce any regressions. The test cases cover a variety of scenarios to ensure the robustness of the implementation.

For example, consider the following test case:

List<String> list = new ArrayList<>();
list.add("Object one");
list.add("NotObject two");
list.add("NotObject three");

int index1 = list.findIndex(s -> s.contains("ct t"));
System.out.println(index1); // Expected output: 1
int index2 = list. findLastIndex(s -> s.startsWith("NotObject"));
System.out.println(index2); // Expected output: 2
Currently, to achieve the same result, we would have to use a more verbose approach:

int index1 = IntStream.range(0, list.size())
                     .filter(i -> list.get(i).contains("ct t"))
                     .findFirst()
                     .orElse(-1);
System.out.println(index1); // Output: 1
int index2 = IntStream.range(0, list.size())
                         .filter(i -> list.get(i).startsWith("NotObject"))
                         .reduce((first, second) -> second)
                         .orElse(-1);
System.out.println(index2); // Output: 2
Or other approaches that require additional instructions and, therefore, can`t be used in all scopes (like passing argument to method).

I believe these additions would greatly enhance the functionality and flexibility of the List interface, making it more powerful and user-friendly. I look forward to your feedback and am open to making any necessary changes based on your suggestions.

The main reason I am publishing this proposal in the mailing system is to gather feedback from the Java developers community, especially about possible caveats related to backward compatibility of your projects. Would appreciate every opinion!

Best regards

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/core-libs-dev/attachments/20240423/3c1603e7/attachment-0001.htm>