Java 8流按3个字段分组，按sum和count聚合产生单行输出 [英] Java 8 streams group by 3 fields and aggregate by sum and count produce single line output

问题描述

我知道在论坛中提出了类似的问题，但他们似乎都没有完全解决我的问题。现在我对Java 8很新，所以请耐心等待。
我有一个产品清单，例如：

I know there a similar questions asked in the forum but none of them seem to be addressing my problem fully. Now I'm very new to Java 8, so please bear with me. I have a list of Products, for example:

Input:
name    category    type    cost
prod1       cat2     t1      100.23
prod2       cat1     t2      50.23
prod1       cat1     t3      200.23
prod3       cat2     t1      150.23
prod1       cat2     t1      100.23


Output:
Single line (name, category, type) summing the cost and count of products.




Product {
    public String name;
    public String category;
    public String type;
    public int id;
    public double cost;

}

我需要按名称，类别和类型对此进行分组产生一个结果，
汇总了这些数据，并产生了每种产品的总成本和数量。大多数示例显示按两个字段进行分组并使用单个条件进行聚合。

I need to group this by name, category and type and produce a single result that summarizes this data and produces the total cost and count of each product. Most examples show grouping by two fields and aggregating using single criteria.

根据forumn的建议，我想出了这个分组：

Following suggestions on the forumn, i came up with this for groupings:

    public class ObjectKeys {

    ArrayList<Object> keys;

    public ObjectKeys(Object...searchKeys) {

         keys = new ArrayList<Object>();

            for (int i = 0; i < searchKeys.length; i++) {
                keys.add( searchKeys[i] );
            }
    }

}

然后使用如下：

Map<String, Map<String, Map<String, List<Product>>>> productsByNameCategoryType =
    products.stream().collect(groupingBy(new ObjectKeys(l.name(), l.category(),l.type())))

但是如何将count和sum连接到上面的代码？特别是对于超过2个领域的团体。
有更好的方法吗？

But how do I chain count and sum to the the above code? Especially for group by with more than 2 fields. Is there a better way to do this?

就像我提到的，我的Java8不太好，请帮忙。

Like I mentioned my Java8 is not that good, please help.

推荐答案

前提条件

Precondition

class Product {
    public String name;
    public String category;
    public String type;
    public int id; 
    //todo:implement equals(), toString() and hashCode()
 }

class Item{
   public Product product;
   public double cost;
}

汇总方式

您可以使用收藏家#groupingBy & 收藏家＃summarizingDouble 。

List<Item> items = ...; 
Map<Product, DoubleSummaryStatistics> stat = items.stream().collect(groupingBy(
            it -> it.product,
            Collectors.summarizingDouble(it -> it.cost)
));

// get some product summarizing
long count = stat.get(product).getCount();
double sum = stat.get(product).getSum();

//list all product summarizing
stat.entrySet().forEach(it ->
  System.out.println(String.format("%s - count: %d, total cost: %.2f"
        , it.getKey(),it.getValue().getCount(), it.getValue().getSum()));
);

合并具有相同产品的商品

首先，您需要在 Item 类中添加 qty 字段：

class Item{
   public int qty;
   //other fields will be omitted

   public Item add(Item that) {
        if (!Objects.equals(this.product, that.product)) {
            throw new IllegalArgumentException("Can't be added items"
                     +" with diff products!");
        }
        return from(product, this.cost + that.cost, this.qty + that.qty);
    }

    private static Item from(Product product, double cost, int qty) {
        Item it = new Item();
        it.product = product;
        it.cost = cost;
        it.qty = qty;
        return it;
    }

}

那么你可以使用 Collectors＃toMap 合并具有相同产品的商品：

then you can using Collectors#toMap to merges items with same product:

Collection<Item> summarized = items.stream().collect(Collectors.toMap(
        it -> it.product,
        Function.identity(),
        Item::add
)).values();

最后

你可以看到两种方式做同样的事情，但第二种方法更容易在流上运行。以及我在github中检查过的两种方法的测试，你可以点击查看更多细节：汇总项目& 合并项目。

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