Problem with collecting to stream after filter to map

Question

I have two lists and I have to create a map from them. One, I am iterating in for loop and second I wanted to go threw by a stream and than collect to map, but I have no idea how to use Collectors.toMap in that specific case. Is it possible?

I already made a solution, but not using stream, however I am very curious is it possible to do it and if yes, how to do it?

public void findMatch(List<ObjectA> objectAList, List<ObjectB> objectBList) {
Map<ObjectB, ObjectA> objectBObjectAMap = new HashMap<>();
for (ObjectB objectB : objectBList) {
    if (isNull(objectB.getHandoverTime())) {
        objectBObjectAMap.putAll(
        objectAList
                .stream()
                .filter(objectA -> {
                    ObjectC objectC = objectB.getObjectC();
                    return objectA.getNumber().equals(objectC.getNumber())
                            && objectA.getQuality().equals(objectC.getQuality());
                })
                .collect(Collectors.toMap(???)));
        }
    }
}

What do you plan to do if multiple ObjectA instances are associated with the same ObjectB instance? — Eran, Dec 20 '18 at 07:16
Possible duplicate of [Creating Map composed of 2 Lists using stream().collect in Java](https://stackoverflow.com/questions/39962796/creating-map-composed-of-2-lists-using-stream-collect-in-java) — xyz, Dec 20 '18 at 07:36

score 2 · Answer 1 · edited Dec 20 '18 at 08:10

You can try with flatMap:

Map<ObjectB, ObjectA> objectBObjectAMap =
    objectBList.stream()
               .filter(b -> isNull(b.getHandoverTime()))
               .flatMap(b -> objectAList.stream()
                                        .filter(a -> {
                                            ObjectC c = b.getObjectC();
                                            return a.getNumber().equals(c.getNumber()) &&
                                                   a.getQuality().equals(c.getQuality());
                                        })
                                        .map(a -> new SimpleEntry<>(b,a)))
               .collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue));

This is assuming each ObjectB instance will not be associated with more than one ObjectA instance.

There is still problem in collector: Non-static method cannot be referenced from a static context. — rudkul, Dec 20 '18 at 07:36
@rudkul must be some typo. I located a missing ) (see edit). Hopefully it's the only typo. — Eran, Dec 20 '18 at 07:43

score 0 · Answer 2 · answered Dec 20 '18 at 09:32

I tried to restore your problem and I create three simple clasess

public class ObjectA {
    Long number;
    String quality;
    ...
}

public class ObjectB {
    Long number;
    String quality;
    Date handoverTime;
    ObjectC objectC;
    ...
}

public class ObjectC {
    Long number;
    String quality;
    ...
}

and main function with I hope your processing.

    ObjectA a1 = new ObjectA();
    a1.setNumber(1L);
    a1.setQuality("aaa1");

    ObjectA a2 = new ObjectA();
    a2.setNumber(2L);
    a2.setQuality("aaa2");

    List<ObjectA> aList = new ArrayList<>();
    aList.add(a1);
    aList.add(a2);

    ObjectB b1 = new ObjectB();
    b1.setNumber(3L);
    b1.setQuality("bbb1");
    //b1.setHandoverTime(new Date());
    ObjectC c1 = new ObjectC();
    c1.setNumber(1L);
    c1.setQuality("aaa1");
    b1.setObjectC(c1);

    ObjectB b2 = new ObjectB();
    b2.setNumber(4L);
    b2.setQuality("bbb2");
    //b2.setHandoverTime(new Date());
    ObjectC c2 = new ObjectC();
    c2.setNumber(2L);
    c2.setQuality("aaa2");
    b2.setObjectC(c2);

    List<ObjectB> bList = new ArrayList<>();
    bList.add(b1);
    bList.add(b2);

    Map<ObjectB, ObjectA> mapzz = findMatch(aList, bList);

    System.out.println(mapzz);

and following method

public static Map<ObjectB, ObjectA> findMatch(List<ObjectA> objectAList, List<ObjectB> objectBList) {
    List<ObjectA> checkPoint1 = new ArrayList<>();
    Map<ObjectB, ObjectA> mapzz = new HashMap<>();

    mapzz.putAll(
        objectBList.stream()
            .filter(objB -> isNull(objB.getHandoverTime()))
            .collect(Collectors.toMap(Function.identity(), 
                    objB -> objectAList
                        .stream()
                        .filter((a) -> a.getNumber().equals(objB.getObjectC().getNumber()) && a.getQuality().equals(objB.getObjectC().getQuality()))
                        .peek(checkPoint1::add)
                        .findAny().get()
                    ))
    );
    return mapzz;
}

Result is as follow:

{ObjectB{number=3, quality=bbb1, handoverTime=null, objectC=ObjectC{number=1, quality=aaa1}}=ObjectA{number=1, quality=aaa1}, ObjectB{number=4, quality=bbb2, handoverTime=null, objectC=ObjectC{number=2, quality=aaa2}}=ObjectA{number=2, quality=aaa2}}

I hope that it helps.

Problem with collecting to stream after filter to map

2 Answers2