类型不匹配：无法从地图转换 <Object,Map<Object,List<ActorContents> &gt;&gt;到地图 <Actor ,Map<String,List<ActorContents> &gt;&gt;

Question

I am trying to create nested map from a list. With the below snippet I get compile time error

Type mismatch: cannot convert from Map<Object,Map<Object,List<ActorContents>>> to Map<Actor,Map<String,List<ActorContents>>>

 Map<Actor, List<String>> actorTypeOfContents = typeofContentforActor(genres, genreId);

            Map<Actor, Map<String, List<ActorContents>>> imageMap1=                 
                actorContents.stream()
                        .collect(Collectors.groupingBy(e -> e.getActor(), Collectors.groupingBy( p -> Utility.find(actorTypeOfContents.get(p.getActor()), i -> StringUtils.contains(p.getName(), "_" + i + "_"))
                                ))); 

Utility method used is as below

public static <T> T find(List<T> items, Predicate<T> matchFunction) {
        for (T possibleMatch : items) {
            if (matchFunction.test(possibleMatch)) {
                return possibleMatch;
            }
        }
        return null;
    }

When I change the code as below There is no error and code executes.

List<String> actorNames =actorTypeOfContents.get(Actor.Genre1);

Map<Actor, Map<String, List<ActorContents>>> imageMap1=                 
                actorContents.stream()
                        .collect(Collectors.groupingBy(e -> e.getActor(), Collectors.groupingBy( p -> Utility.find(actorNames, i -> StringUtils.contains(p.getName(), "_" + i + "_"))
                                ))); 

Could you help to figure out what is wrong with the snippet

Map<Actor, Map<String, List<ActorContents>>> imageMap1=                 
                actorContents.stream()
                        .collect(Collectors.groupingBy(e -> e.getActor(), Collectors.groupingBy( p -> Utility.find(actorTypeOfContents.get(p.getActor()), i -> StringUtils.contains(p.getName(), "_" + i + "_"))
                                ))); 

Your assistance is highly appreciated

Answer 1

Lets consider the inner map Map<Object,List<ActorContents>> only, since the outer has the same issue. Consider this:

Map<Object,List<ActorContents>> map = new HashMap<>();
map.put(1, Arrays.asList(new ActorContents()));
map.put("one", Arrays.asList(new ActorContents()));

Now, you've a map with 2 keys that are different data types. You're asking the compiler to convert it into a map with a specific type for the key ( Actor ). The compiler doesn't know how to convert an integer or a string to an Actor .

I deliberately didn't reference your code because after reading my explanation, you should be able to figure out the problem yourself. It'll also do you good to read the generics tutorial.