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

[forax at univ-mlv.fr]

> From: "David Alayachew" <davidalayachew at gmail.com>
> To: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "ІП-24 Олександр Ротань" <rotan.olexandr at gmail.com>, "core-libs-dev"
> <core-libs-dev at openjdk.org>
> Sent: Friday, April 19, 2024 11:02:12 PM
> Subject: Re: Addition of Predicate-based findIndex and findLastIndex methods to
> java.util.List

> No Rémi, I don't think your idea is the right approach. You are working on the
> wrong level of abstraction.

Hello David, 

> Many users ask requests like this all the time, and what you are suggesting
> would be even more error-prone than the equivalent for loop or the IntStream
> suggestion that the user above requested.

Could you give an example of why it is more error prone ? 

> Not to mention that getting it to parallelize would be a task many users are
> likely to mess up -- either in correctness or in performance.

Correctness is not an issue athat's why i've used ofSequential() when creating the Gatherer. 
You are right that performance can be an issue, the Gatherer API is more limited compared to what a Spliterator can do. 

> I think you would get far more mileage from adding 2 methods on the list
> interface streamWithIndex() and parallelStreamWithIndex() that would return a
> Stream<WithIndex<T>>, as opposed to just Stream<T>.

Any stream can be "indexed", having an index is not an intrisinc property of a stream of List, so adding an index is more an intermediate operation than a source operation. 
So more like 
list.stream().withIndex() 
than like 
list.streamWithIndex() 

Now, we (the lambda-util expert group) talked 10 years ago to add such design but decide to postpone it because WithIndex<T> should be a value type and the Stream API should be specialized to avoid unecessary allocations. 
So you are right that withIndex() is a more general design but because of the performance issue we can not yet offer such design. 

As a counter argument, you can say that we have the same issue with the Collector API and the Gatherer API which are both not specialized but at least for a Collector, the fact that the accumulator does a side effect which is usualyl a write in memory is slow anyway. And this is also true for a Gatherer, it works well when the Gatherer is itself followed by a Collector. 

> That way, users are not writing a custom Gatherer each time they want to work
> with the index. They just have the index be a field in the object. They can
> work with it the same way they would any other object field.

This custom Gatherer can be declared in Gatherers (with an 's' at the end). 

> Furthermore, doing it this way makes the correct answer obvious. If I need to do
> something with an index, stream with the index.

Technically, here, you want the result to be an index, so you may not want to pay the price of creating an index (your WithIndex objetcs) for every elements but just one when the predicate is true. 
This point is moot if the WithIndex is a value type and Stream is a specialized generics but as I said above, where are not yet at that point. 

> On top of that, it significantly enhances readability by making it clear to the
> reader that, whatever this stream is doing will require use of the index, so
> watch out for that.

With your proposal, you still need to write something like 

list.withIndexStream().filter(withIndex -> predicate(withIndex.element())).mapToInt(WithIndex::index).findFirst().orElse(-1) 

I do not see the enhancement in readability. 

regards, 
Rémi 

> On Fri, Apr 19, 2024, 1:47 PM Remi Forax < [ mailto:forax at univ-mlv.fr |
> forax at univ-mlv.fr ] > wrote:

>> 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" < [ mailto:rotan.olexandr at gmail.com |
>>> rotan.olexandr at gmail.com ] >
>>> To: "core-libs-dev" < [ mailto:core-libs-dev at openjdk.org |
>>> 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 |
>>> 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
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