How to group elements of a List by elements of another in Java 8

Question

I have the following problem: Given these classes,

class Person {
    private String zip;
    ...
    public String getZip(){
        return zip;
    }
}

class          


        

Answer 1

The original answer does an unnecessary mapping with tuples, so you see there the final solution. You could remove the mapping, and simply filter directly the regions list:

//A Set is more appropriate, IMO
.stream()
.collect(toMap(p -> p, 
               p -> regions.stream()
                           .filter(r -> r.getZipCodes().contains(p.getZip()))
                           .collect(toSet())));

---

If I understand well, you could do something like this:

import java.util.AbstractMap.SimpleEntry;
import static java.util.stream.Collectors.toMap;
import static java.util.stream.Collectors.toList;

...

List persons = ...;
List regions = ...;

Map> map = 
    persons.stream()
           .map(p -> new SimpleEntry<>(p, regions))
           .collect(toMap(SimpleEntry::getKey, 
                          e -> e.getValue().stream()
                                           .filter(r -> r.getZipCodes().contains(e.getKey().getZip()))
                                           .collect(toList())));

From the List you get a Stream. Then you map each instance to a tuple > that contains all the regions. From there, you collect the data in a map with the toMap collector and, for each person, you build a List of Region that contains the zip code of that person.

For example, given the input:

List persons = Arrays.asList(new Person("A"), new Person("B"), new Person("C"));

List regions = 
     Arrays.asList(new Region(Arrays.asList("A", "B")), new Region(Arrays.asList("A")));

It outputs:

Person{zip='A'} => [Region{zipCodes=[A, B]}, Region{zipCodes=[A]}]
Person{zip='B'} => [Region{zipCodes=[A, B]}]
Person{zip='C'} => []

Also I guess the zipCodes for each Region could be a Set.