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

停止使用原始类型！

这是mySupplier没有原始类型的：

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

an 的键类型EnumMap必须是枚举类型，因此您应该将其LifeCycle用作第一个参数。第二个参数是您在最后使用的收集器返回的内容。您toSet在这里使用过，所以我想您想要一组TestGroupBy.

这就是您的供应商应该是什么样子，具有适当的通用参数和LifeCycle.class作为EnumMap!

现在，您可以这样做：

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

请注意，您必须添加() ->才能使其成为供应商。