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

Question

Have two classes and two corresponding lists:

class Click {
   long campaignId;
   Date date;
}

class Campaign {
   long campaignId;
   Date start;
   Date end;         


        

Answer 1

Well, there is a very neat way to solve your problem IMO, original idea coming from Holger (I'll find the question and link it here).

You could define your method that does the checks (I've simplified it just a bit):

static boolean checkClick(List campaigns, Click click) {
    return campaigns.stream().anyMatch(camp -> camp.getCampaignId() 
               == click.getCampaignId());
}

And define a function that binds the parameters:

public static  Predicate bind(BiFunction f, T t) {
    return u -> f.apply(t, u);
}

And the usage would be:

BiFunction, Click, Boolean> biFunction = YourClass::checkClick;
Predicate predicate = bind(biFunction, campaigns);

clicks.stream()
      .filter(predicate::test)
      .collect(Collectors.toList());