Java 8 streams: find items from one list that match conditions calculated based on values from another list

Andrey Yaskulsky :

Have two classes and two corresponding lists:

class Click {
   long campaignId;
   Date date;
}

class Campaign {
   long campaignId;
   Date start;
   Date end;
   String type;
}

List<Click> clicks = ..;
List<Campaign> campaigns = ..;

And want to find all Clicks in clicks that:

Have a corresponding Campaign in campaigns list, i.e., Campaign with the same campaignId AND
This Campaign has type = "prospective" AND
This Campaigns.start < click.date < Campaigns.end

So far I have the following implementation (which seems confusing and complex to me):

clicks.
        stream().
        filter(click -> campaigns.stream().anyMatch(
                campaign -> campaign.getCampaignType().equals("prospecting") &&
                        campaign.getCampaignId().equals(click.getCampaignId()) &&
                        campaign.getStart().after(click.getDate()) &&
                        campaign.getEnd().before(click.getDate()))).
        collect(toList());

I wonder if there is simpler solution for the problem.

user1803551 :

One thing that stands out is that your 2nd requirement has nothing to do with the matching, it's a condition on campaigns only. You'd have to test if this is any better for you:

clicks.stream()
    .filter(click -> campaigns.stream()
        .filter(camp -> "prospecting".equals(camp.type))
        .anyMatch(camp -> 
            camp.campaignId == click.campaignId &&
            camp.end.after(click.date) &&
            camp.start.before(click.date)
        )
    )
    .collect(Collectors.toList());

Otherwise, I have never seen a streams solution which does not involve streaming the 2nd collection inside the predicate of the 1st, so you can't do much better than what you did. In terms of readability, if it looks that confusing to you then create a method that test for the boolean condition and call it:

clicks.stream()
    .filter(click -> campaigns.stream()
        .filter(camp -> "pre".equals(camp.type))
        .anyMatch(camp -> accept(camp, click))
    )
    .collect(Collectors.toList());

static boolean accept(Campaign camp, Click click) {
    return camp.campaignId == click.campaignId &&
            camp.end.after(click.date) &&
            camp.start.before(click.date);
}

Finally, 2 unrelated suggestions:

Don't use the old Date class, instead use the new java.time API's LocalDate.
If Campaign's type can only have some predefined values (like "submitted", "prospecting", "accepted"...) then an enum would be a better fit than a general String.