Cannot infer type arguments for hashmap<>

Question

I am getting

   Cannot infer type arguments for java.util.HashMap<>

for the following code

class Test {

    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<>();
        map.put(1, "x");
        map.put(2, "y");
        map.put(3, "x");
        map.put(4, "z");

  //the following line has error
        Map<String, ArrayList<Integer>> reverseMap = new java.util.HashMap<>(map.entrySet().stream()
                .collect(Collectors.groupingBy(Map.Entry::getValue)).values().stream()
                .collect(Collectors.toMap(item -> item.get(0).getValue(),
                        item -> new ArrayList<>(item.stream().map(Map.Entry::getKey).collect(Collectors.toList())))); 
        System.out.println(reverseMap);

    }

}

What went wrong and Can anyone Explain me this ? I have checked for proper imports and found out that I am importing java.util.hashmap and none other. Still the pesky error is buging me.

Answer 1

That's a bug in ecj (eclipse compiler), you can work around it and add more type information :

item -> new ArrayList<Integer>(item.stream().map(Entry::getKey)

See how I've added ArrayList<Integer>.

It compiles fine with javac-8 and 9.

And btw seems like there is a simpler way to do things:

map.entrySet()
            .stream()
            .collect(Collectors.groupingBy(
                    Entry::getValue,
                    HashMap::new,
                    Collectors.mapping(Entry::getKey, Collectors.toList())));