Java 8 List<V> into Map<K, V> with Function

Question

I tried to follow Java 8 List into Map and try to change Set to Map in one list

Instead of looping (which works)

for (Type t : toSet()) {
    map.put(Pair.of(t, Boolean.TRUE), this::methodAcceptingMap);
}

I tried with solutions as:

toSet().stream()
       .collect(Collectors.toMap(Pair.of(Function.identity(), Boolean.TRUE), 
                                 this::methodAcceptingMap));

But got an error converting:

Type mismatch: cannot convert from Pair<Function<Object,Object>,Boolean> 
to Function<? super T,? extends K>

My map

private Map<Pair<Type, Boolean>, BiConsumer<Pair<Type, Boolean>, Parameters>> map =
      new HashMap<>();

Answer 1

Collectors.toMap takes two functions, and neither of your arguments fits.

You should use:

Map<Pair<Type, Boolean>, BiConsumer<Pair<Type, Boolean>, Parameters>> map =
    set.stream()
       .collect(Collectors.toMap(el -> Pair.of(el, Boolean.TRUE), 
                                 el -> this::methodAcceptingMap));

The expression Pair.of(t, Boolean.TRUE) is simply not of a Function type. And this::methodAcceptingMap could fit the signature of a BiConsumer , but the method requires a Function. So el -> this::methodAcceptingMap should be used as a function that takes a stream element and returns your BiConsumer .

Note that the assignment context ( map = ) is important in this case. Without it, the target type of these lambda expressions will be missing and compilation will fail.

Answer 2

I don't quite get your example. In for loop you're passing the same lambda for every value. I don't see sense in that. If you really want that, you need to pass obj -> (pair, param) -> this.methodAcceptingMap(pair, param) :

toSet().stream().collect(Collectors.toMap(
    obj -> Pair.of(obj, Boolean.TRUE), 
    obj -> (pair, param) -> this.methodAcceptingMap(pair, param)));