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From: =?UTF-8?Q?Arne_Vajh=c3=b8j?= <arne@vajhoej.dk>
Newsgroups: comp.lang.java.programmer
Subject: Re: stream filter twice
Date: Sat, 20 Feb 2016 22:20:51 -0500
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On 2/20/2016 7:28 AM, Philipp Kraus wrote:
> I'm using streams to filter some data. I just do this with
>
>
> List<myClass> x = myclasscollection.stream().filter( i -> i.filter() ).collect( Collectors.toList() );
> if (!x.isempty())
>     return x;
>
>
> List<myClass> y = myclasscollection.stream().filter( i -> i.anotherfilter() ).collect( Collectors.toList() );
> if (!y.isEmpty())
>     return y;
>
> return  myclasscollection.stream().collect( Collectors.toList() );
>
> So my question is, can optimize this with a stream operation. I need
> to check each object of the collection, but only if no object exist,
> I need to run the check with another filter. A change to filter( i ->
> i.filter() || i.anotherfilter() ) or filter( i-> i.filter() ^
> i.anotherfilter() ) is not possible, because I need the empty check
> of all results.
>
> In a best-case idea I need only one run with the stream and can build
> the result list in one shoot. Did you have got any idea to optimize
> this code?

What is your goal? To produce easily understandable code? To save
a few micro seconds?

If the first then I think you current code is fine.

If the second then you will need to be creative.

Here is my attempt to be creative:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class MultiProcessing {
	public static <T> List<T> original(List<T> lst, Predicate<? super T> 
p1, Predicate<? super T> p2) {
		List<T> x = lst.stream().filter(p1).collect(Collectors.toList());
		if(!x.isEmpty()) return x;
		List<T> y = lst.stream().filter(p2).collect(Collectors.toList());
		if(!y.isEmpty()) return y;
		return lst;
	}
	public static <T> List<T> alternative(List<T> lst, Predicate<? super T> 
p1, Predicate<? super T> p2) {
		List<T> x = new ArrayList<>();
		List<T> y = new ArrayList<>();
		lst.stream().forEach(v -> { if(p1.test(v)) x.add(v); if(x.isEmpty() && 
p2.test(v)) y.add(v); });
		if(!x.isEmpty()) return x;
		if(!y.isEmpty()) return y;
		return lst;
	}
	private static <T> void test(List<T> lst, Predicate<? super T> p1, 
Predicate<? super T> p2) {
		System.out.println(original(lst, p1, p2));
		System.out.println(alternative(lst, p1, p2));
	}
	public static void main(String[] args) {
		List<Integer> lst = Arrays.asList(1, 4, 9, 16);
		test(lst, v -> v > 2, v -> v < 2);
		test(lst, v -> v > 20, v -> v < 2);
		test(lst, v -> v > 20, v -> v < -2);
	}
}

Arne