java8 simplify transformations to map

CCC :

I have this class structure:

public class A {
    private List<B> bs;
...//getters
}

public class C {
    private Long id;
...//getters
}

public class B {

    private Long idOfC;
...//more stuff
}

B::getIdOfC matches C::getId

In a better design B would just contain a reference to C, rather than its id (I cannot change that), so that's why now I need to create a map, so my method signature looks like this

public Map<A, List<C>> convert(Collection<A> collection)

Inside this convert method, there is a

List<C> getCsByIds(List<Long> id)

that's later used to match it against B.idOfC, but there should only be one call to this method as it's pretty expensive.

So If I go like this:

 List<B> bs = Arrays.asList(new B(10L), new B(11L)); //10L and 11L are the values of idOfC
   List<A> as = Arrays.asList(bs);
   //And assuming getCsByIds returns Arrays.asList(new C(10L), new C(11L), new C(12L));

then

    Map<A, List<C>> map = convert(as);
    map.values().get(0)

returns something like Arrays.asList(new C(10L), new C(11L))

The method that does this is pretty massive in my view:

    public Map<A, List<C>> convert(Collection<A> as) {
    List<Long> cIds = as.stream()
            .flatMap(a -> a.getBs().stream())
            .map(B::getCId)
            .collect(Collectors.toList());

    //single call to gsCsByIds
    Map<Long, C> csMap = getCsByIds(cIds)
            .stream()
            .collect(Collectors.toMap(C::getId, Function.identity()));

    //a whole new map is created by iterating over the list called "as"
    Map<A, List<C>> csByAs = new HashMap<>();
    if (!csMap.isEmpty()) {
        for (A a : as) {
            Set<C> cs = getCsFromMap(csMap, a.getBs());
            if (!cs.isEmpty()) {
                csByAs.put(a, new ArrayList<>(cs));
            }
        }
    }

    return csByAs;
}

private Set<B> getCsFromMap(Map<Long, C> cMap, List<B> bs) {
    return bs.stream()
            .map(b -> cMap.get(b.getIdOfc()))
            .collect(Collectors.toSet());
}

Is there a way to make this simpler???

Naman :

If the call to getCsByIds is expensive, your initial idea is pretty decent to execute itself. It can further be cut short to :

public Map<A, List<C>> convert(Collection<A> as) {
    List<Long> cIds = as.stream()
            .flatMap(a -> a.getBs().stream())
            .map(B::getIdOfC)
            .collect(Collectors.toList());
    Map<Long, C> csMap = getCsByIds(cIds).stream()
            .collect(Collectors.toMap(C::getId, Function.identity()));

    return as.stream()
            .collect(Collectors.toMap(Function.identity(),
                    a -> a.getBs().stream().map(b -> csMap.get(b.getIdOfC()))
                            .collect(Collectors.toList()), (a, b) -> b));
}

where you can choose your merge function (a,b) -> b accordingly.

java8 simplify transformations to map

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