Filter by type

[nipafx]

Particularly when working with sealed types, I occasionally have the use case to only keep elements in the stream that are an instance of a specified type or of several specified types. The stream's generic type should then of course match that type or those types. This is essentially a filter(instanceof).map(cast).

My gatherer looks like this:

@SafeVarargs
public static <IN, OUT> Gatherer<IN, ?, OUT> only(Class<? extends OUT>... types) {
	return Gatherer.of((_, element, downstream) -> {
		for (var type : types)
			if (type.isInstance(element))
				return downstream.push(type.cast(element));
		return !downstream.isRejecting();
	});
}

Some uses:

Stream.of(1, 2.0, "3")
	.gather(only(Integer.class)) // Stream<Integer>
	.forEach(System.out::println); // 1

Stream.of(1, 2.0, "3")
	.gather(only(Number.class)) // Stream<Number>
	.forEach(System.out::println); // 1 2.0

Stream.of(1, 2.0, "3")
	.gather(only(Double.class, String.class)) // Stream<Serializable>
	.forEach(System.out::println); // 2.0 3

If you'd be interested, I can open a PR. (Or you implement it yourself if that's easier.)

PS: Great project! 😃👍🏾

[tginsberg]

Hi @nipafx! Thanks for the kind words, they mean a lot! I'm a big fan of your work to better the Java community!

Interesting you bring this concept up (with examples!!) - I have a whole list of gatherers that fit in the category of "possible with existing operations or gatherers but not super convenient" and this is on the list. Originally I had delayed doing that kind of implementation to see how I felt about it. But I think I'm at the point where I should review the list and see which ones would be useful.

I do like this one a lot, so I'll probably do this and deliver it in 0.9.0. I appreciate the offer of a PR, but I think I'd find it fun to write this myself (this whole project is a side project for me because I enjoy writing libraries, so...).

[nipafx]

Thanks for the kind words

Right back at you! 💙🧡

I have a whole list of gatherers that fit in the category of "possible with existing operations or gatherers but not super convenient" and this is on the list

Indeed. In the past I did this with a flatMap(only(...)), where only returned a suitable function. But gatherers are much more suitable. Great that you're looking into it.

this whole project is a side project for me because I enjoy writing libraries, so...

And writing gatherers is fun, too! 😃