函数式编程

package tech.ityoung.o2o;

import java.util.ArrayList;
import java.util.DoubleSummaryStatistics;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class lambdaStream {

    public static void main(String[] args) {
        List<Orders> orders = new ArrayList<>();
        orders.add(new Orders(1, "张三", 100.00));
        orders.add(new Orders(2, "李四", 100.00));
        orders.add(new Orders(3, "王五", 990.00));
        orders.add(new Orders(4, "赵六", 20.00));
        orders.add(new Orders(5, "李四", 770.00));
        orders.add(new Orders(6, "张三", 220.00));
        orders.add(new Orders(7, "赵六", 565.00));

        Map<String, DoubleSummaryStatistics> collect =
                orders.stream().collect
                        (Collectors.groupingBy
                                (Orders::getUsername, Collectors.summarizingDouble(Orders::getTotal)));
        System.out.println(collect);
    }


}

class Orders {
    private int id;
    private String username;
    private double total;

    public Orders(int id, String username, double total) {
        this.id = id;
        this.username = username;
        this.total = total;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public double getTotal() {
        return total;
    }

    public void setTotal(double total) {
        this.total = total;
    }
}