java - JDK8 lambda 过滤器从列表到其他列表

Question

我想要一个 List<Pojo>至其他List<Pojo>
输入如下数据:

Pojo -> type:A money:20.00
Pojo -> type:A money:10.00
Pojo -> type:B money:23.00
Pojo -> type:A money:11.00
Pojo -> type:C money:38.00
Pojo -> type:B money:233.00

并输出如下数据(每种类型最大的钱):

Pojo -> type:A money:20.00
Pojo -> type:B money:233.00
Pojo -> type:C money:38.00

那么，我如何使用 lambda 来完成这项工作？
提前致谢!

Answer 1

这是一种使用 max by 的方法

public class Test4 {
    public static void main(String[] args) {
        List<Pojo> pojoList = new ArrayList<>();
        pojoList.add(new Pojo("A", 20.00d));
        pojoList.add(new Pojo("A", 10.00d));
        pojoList.add(new Pojo("B", 23.00d));
        pojoList.add(new Pojo("A", 11.00d));
        pojoList.add(new Pojo("C", 38.00d));
        pojoList.add(new Pojo("B", 233.00d));
        Map<String, Optional<Pojo>> map = pojoList.stream()
                .collect(Collectors.groupingBy(Pojo::getType, Collectors.maxBy(Comparator.comparing(Pojo::getMoney))));
        List<Pojo> pojos = map.values().stream().map(Optional::get).collect(Collectors.toList());
        System.out.println(pojos);

    }

    static class Pojo {
        public String type;
        public double money;

        public Pojo(String type, double money) {
            this.type = type;
            this.money = money;
        }

        public double getMoney() {
            return money;
        }

        public String getType() {
            return type;
        }

        @Override
        public String toString() {
            return "Pojo [type=" + type + ", money=" + money + "]";
        }

    }
}

输出

[Pojo [type=A, money=20.0], Pojo [type=B, money=233.0], Pojo [type=C, money=38.0]]

更新

有 Collectors.collectingAndThen 可用于对结果应用函数，以获取由 Optional 包装的值

导入

import static java.util.Comparator.comparing;
import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.maxBy;

收集然后然后

Map<String, Pojo> result = pojoList.stream()
        .collect(groupingBy(Pojo::getType, collectingAndThen(maxBy(comparing(Pojo::getMoney)), Optional::get)));

输出

{A=Pojo [type=A, money=20.0], B=Pojo [type=B, money=233.0], C=Pojo [type=C, money=38.0]}