Java流对列进行分组并获取多个聚合值

Question

如果我有一个对象列表，例如产品，这些产品将按产品类型分组。我需要汇总的价格和数量。

{{name:a1,type:normal,price:23,quantity:4}, 
{name:a2,type:normal,price:3,quantity:3},
{name:a3,type:luxury,price:233,quantity:1},
{name:a4,type:luxury,price:123,quantity:2}}

我需要一个看起来像这样的结果列表

{{type:normal,price:26,quantity:7},{type:luxury,price:356,quantity:3}}

有没有办法使用Java流来实现这一目标？

Answer 1

怎么样，

Map<String, Product> result = products.stream()
    .collect(Collectors.groupingBy(Product::getType,
        Collectors.reducing(new Product(null, null, 0, 0), (p1, p2) -> new Product(null, p1.getType(),
            p1.getPrice() + p2.getPrice(), p1.getQuantity() + p2.getQuantity()))));

其中Product类应该看起来像这样。

public class Product {
    private String name;
    private String type;
    private double price;
    private int quantity;

    public Product(String name, String type, double price, int quantity) {
        super();
        this.name = name;
        this.type = type;
        this.price = price;
        this.quantity = quantity;
    }
    // ...
}