无法在groupBy Java 8中推断出功能接口类型

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.

Answer 2

There are two things wrong with code you have shown us. Firstly,you have passed wrong object with second argument of Collectors.groupingBy . You have passed EnumMap , not Supplier<EnumMap> .

Secondly, you cannot instantiate EnumMap with TestGroupBy.class , because TestGrouBy is not an enum. In your case it should be new EnumMap<>(LifeCycle.class) :

garden.stream()
        .collect(Collectors.groupingBy(
              e -> e.getLifeCycle(),   // Can not resolve method getLifeCycle()
              () -> new EnumMap<>(LifeCycle.class),
              Collectors.toSet()));

Also change implementation of mySupplier() method, because it's not correct. It should be:

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

Answer 3

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

This code above will not work. Instead you should try:

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

FInally, the reason for not working the below code:

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

is:

new EnumMap(TestGroupBy.class),

Because groupingBy accepts Supplier, not the kind of object which has been provided.

Also note that new EnumMap(TestGroupBy.LifeCycle.class) should have been used.