您可以使用函子/函数式编程在 Java 7 中对列表进行分组(并计算每组的元素)吗?

问题描述：

你可以分组List吗?types = new ArrayList(Arrays.asList(TypeEnum.A, TypeEnum.B, TypeEnum.A)); 到 Map;countPerType;，使用函子(例如 Google 的 Guava，Apache 的 Commons Functor) pre Java 8?

Can you group List<TypeEnum> types = new ArrayList(Arrays.asList(TypeEnum.A, TypeEnum.B, TypeEnum.A)); into a Map<TypeEnum, Integer> countPerType;, using functors (e.g. Google's Guava, Apache's Commons Functor) pre Java 8?

我正在尝试了解函数式编程，但不确定这种事情是否真的可行(因为我不仅要映射集合值，还要尝试聚合)?

I'm trying to get my head around functional programming, but am unsure if that sort of thing would actually be possible (as I'm not just mapping a collections value, but trying to aggregate)?

在命令式风格中，我会做这样的事情:

In imperative style, I would have done something like this:

public Map<TypeEnum, Integer> countByGroup(List<TypeEnum> types) {
    Map<TypeEnum, Integer> countedTypes = new HashMap<>();
    for(TypeEnum type : types) {
        if(countedTypes.containsKey(type)) {
            countedTypes.put(type, countedTypes.get(type) + 1);
        } else {
            countedTypes.put(type, 1);
        }
    }
    return countedTypes;
}

依赖副作用似乎有些不合适 - 或者它是如何完成的......?

Relying on side effects seems somewhat inappropriate - or is that how it's done...?

Procedure<TypeEnum> count = new Procedure<TypeEnum>() {
public Map<TypeEnum, Integer> countPerType = null;

    @Override
    public void run(TypeEnum type) {
        if(countPerType.containsKey(type)) {
            countPerType.put(type, countPerType.get(type) + 1);
        } else {
            countPerType.put(type, 1);
        }
    }

    public Procedure<TypeEnum> init(Map<TypeEnum, Integer> countPerType) {
        this.countPerType = countPerType;
        return this;
    }
}.init(countPerType); // kudos http://stackoverflow.com/a/12206542/2018047

答

Olivier Grégoire 评论正确答案:

使用 Guava，您需要一个简单的 Multiset，更具体地说是它的 EnumMultiset 实现.Multiset 是一种用于跟踪计数元素的数据结构.

Using Guava, you want a simple Multiset, and more specifically its EnumMultiset implementation. Multiset is a data structure made to keep track of counted elements.

给定你的 List类型 你可以使用 create:

Given your List<TypeEnum> types you can create an EnumMultiset using create:

Multiset<TypeEnum> multiset = EnumMultiset.create(types);

您可以使用 count:

And you can query for the count of an element in the Multiset using count:

multiset.count(TypeEnum.A); // 2
multiset.count(TypeEnum.B); // 1

您可以使用函子/函数式编程在 Java 7 中对列表进行分组(并计算每组的元素)吗?

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