Can not infer functional interface type in groupBy Java 8

Question

I am not sure this has been answered earlier or not. Can anyone please tell me what is the problem with third groupBy which I've written ? Why it can not infer ?

class TestGroupBy
{
    enum LifeCycle {
        ANNUAL, PERENNIAL
    }

    private String name;
    private LifeCycle lifeCycle;

    TestGroupBy(String name, LifeCycle lifeCycle) {
        this.name = name;
        this.lifeCycle = lifeCycle;
    }

    LifeCycle getLifeCycle() {
        return this.lifeCycle;
    }

    static EnumMap mySupplier() {
        return new EnumMap(TestGroupBy.class);
    }

    public static void main(String[] args) {
        List<TestGroupBy> garden = new ArrayList<>();
        garden.add(new TestGroupBy("Test1", TestGroupBy.LifeCycle.ANNUAL));
        garden.add(new TestGroupBy("Test2", TestGroupBy.LifeCycle.PERENNIAL));
        garden.add(new TestGroupBy("Test4", TestGroupBy.LifeCycle.ANNUAL));
        garden.add(new TestGroupBy("Test5", TestGroupBy.LifeCycle.PERENNIAL));

        // This works
        garden.stream()
            .collect(Collectors.groupingBy(e -> e.getLifeCycle()));

        // This works
        garden.stream()
            .collect(Collectors.groupingBy(
                e -> e.getLifeCycle(),
                TestGroupBy::mySupplier,
                Collectors.toSet()
            ));
        // This does not work
        garden.stream()
            .collect(Collectors.groupingBy(
                e -> e.getLifeCycle(),   // Can not resolve method getLifeCycle()
                new EnumMap(TestGroupBy.class),
                Collectors.toSet()));
    }
}

Answer 1

Stop using raw types!

This is mySupplier without raw types:

static EnumMap<LifeCycle, Set<TestGroupBy>> mySupplier() {
    return new EnumMap<>(LifeCycle.class);
}

The key type of an EnumMap must be an enum type, so you should use LifeCycle as the first argument. The second argument is what the collector you use at the end returns. You used toSet here, so I suppose you want a set of TestGroupBy.

That's how your supplier should look like, with proper generic arguments and LifeCycle.class as the key type of EnumMap!

Now, you can do this:

    garden.stream()
            .collect(Collectors.groupingBy(
                    e -> e.getLifeCycle(),
                    () -> new EnumMap<>(LifeCycle.class),
                    Collectors.toSet()));

Note that your have to add () -> to make it a supplier.