计算Map中每个类别的产品总和 - Java

Question

我有一个地图，我正在尝试为每个类别名称计算产品的总和值，然后获取总和最大的类别，并显示此总和。

我按类别对地图进行了分组，现在我正在努力计算这个总和，有什么解决方案吗？

那是我的 class:

public class ProductCategories {
        private String categoryName;
        private int productId;
        private double price;

        public ProductCategories(String categoryName, int productId, double price) {
            this.categoryName = categoryName;
            this.productId = productId;
            this.price = price;
        }

        public String getCategoryName() {
            return categoryName;
        }

        public int getProductId() {
            return productId;
        }

        public double getPrice() {
        return price;
    }



        public void setCategoryName(String categoryName) {
            this.categoryName = categoryName;
        }

        public void setProductId(int productId) {
            this.productId = productId;
        }

        public void setProductId(double price) {
        this.price = price;
    }


    @Override
        public String toString() {
            return "ProductCategories{" + "Category name='" + categoryName + '\'' + ", productID=" + productId + ", price='" + price + '\'' + '}';
        }


        private static Map<String, Integer> productIdToNameMap(List<ProductCategories> categoryList) {
            return categoryList.stream()
                    .collect(Collectors.toMap(ProductCategories::getCategoryName, ProductCategories::getProductId, (first, second) -> first + second, TreeMap::new
                    ));
        }

        private static Map<String, List<ProductCategories>> multiValueMap(List<ProductCategories> categoryList) {
            return categoryList.stream()
                    .collect(Collectors.groupingBy(ProductCategories::getCategoryName));
        }

        public static void main(String[] args) {
            List<ProductCategories> categoryList = new ArrayList<>();
            categoryList.add(new ProductCategories("The Fellowship of the Ring", 1954, 12));
            categoryList.add(new ProductCategories("The Return of the King", 1954, 12));
            categoryList.add(new ProductCategories("The Return of the King", 1955, 12));


            //System.out.println(productIdToNameMap(categoryList));
            System.out.println(multiValueMap(categoryList));
        }
}

输出：

{The Return of the King=[ProductCategories{Category name='The Return of the King', productID=1954, price='12.0'}, ProductCategories{Category name='The Return of the King', productID=1955, price='12.0'}], The Fellowship of the Ring=[ProductCategories{Category name='The Fellowship of the Ring', productID=1954, price='12.0'}]}

Answer 1

首先，您的方法名称有些误导。你应该考虑一下，改成比multiValueMap.

更有意义的东西

您可以通过简单地应用下游收集器来为各个类别创建总和，例如 Collectors.summingDouble:

private static Map<String, Double> multiValueMap(List<ProductCategories> categoryList) {
    return categoryList.stream()
            .collect(Collectors.groupingBy(ProductCategories::getCategoryName,
                    Collectors.summingDouble(ProductCategories::getPrice)));
}

Answer 2

I am trying to calculate the total sum value of products for each CategoryName, then get the category where the total sum is the greatest, and display this sum.

要找到最高总价，首先您需要找到每个类别的总价。

您可以将收集器 Collectors.groupingBy 和 Collectors.summingDouble 的组合用作下游，或者通过 Collectors.toMap 提供 合并函数 作为这个收集器的第三个参数。

然后在中间映射的值上创建一个流并检索最大值。

如果您同时需要最高总价和类别名称，则需要在条目集上创建一个流并查看具有最大值的条目，提供 Map.Entry.comparingByValue() 作为比较器。

    private static Map.Entry<String, Double> getMaxByCategoryName(List<ProductCategories> categoryList) {
        return categoryList.stream()
            .collect(Collectors.toMap(ProductCategories::getCategoryName,
                        ProductCategories::getPrice,
                        Double::sum))
            .entrySet().stream()
            .max(Map.Entry.comparingByValue())
            .orElse(Map.entry("no data evalable", 0d));
    }

计算Map中每个类别的产品总和 - Java

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