list.stream().mapToDouble(User::getHeight).sum()//和
list.stream().mapToDouble(User::getHeight).max()//最大
list.stream().mapToDouble(User::getHeight).min()//最小
list.stream().mapToDouble(User::getHeight).average()//平均值

當然，除了統計double型別，還有int和long

bigdecimal需要用到reduce求和

Double示例：

public class HelloWorld {
    private static final DecimalFormat df = new DecimalFormat("0.00");//保留兩位小數點
    public static void main(String[] args) {
        Random random = new Random();
        List<User> list = new ArrayList<>();
        for(int i=1;i<=5;i++) {
            double weight = random.nextDouble() * 100 + 100;//隨機身高：100-200
            User u = new User(i, "使用者-" + i, weight);
            list.add(u);
        }
        System.out.println("使用者：" + list);
        double sum = list.stream().mapToDouble(User::getHeight).sum();
        System.out.println("身高 總和：" + df.format(sum));
        double max = list.stream().mapToDouble(User::getHeight).max().getAsDouble();
        System.out.println("身高 最大：" + df.format(max));
        double min = list.stream().mapToDouble(User::getHeight).min().getAsDouble();
        System.out.println("身高 最小：" + df.format(min));
        double average = list.stream().mapToDouble(User::getHeight).average().getAsDouble();
        System.out.println("身高 平均：" + df.format(average));

    }
    private static class User{
        Integer id;
        String name;
        double height;//身高

        public User(Integer id, String name, double height) {
            this.id = id;
            this.name = name;
            this.height = height;
        }

        public Integer getId() {
            return id;
        }

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

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public double getHeight() {
            return height;
        }

        public void setHeight(double height) {
            this.height = height;
        }

        @Override
        public String toString() {
            return "User{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", height=" + height +
                    '}';
        }
    }

}

執行結果：

使用者：
    [User{id=1, name='使用者-1', height=192.15677342306662}, 
     User{id=2, name='使用者-2', height=196.35056058694772}, 
     User{id=3, name='使用者-3', height=101.96271958293853}, 
     User{id=4, name='使用者-4', height=110.83134063008366}, 
     User{id=5, name='使用者-5', height=106.27720636757154}]
身高 總和：707.58
身高 最大：196.35
身高 最小：101.96
身高 平均：141.52

BigDecimal示例：

public class HelloWorld {
    private static final DecimalFormat df = new DecimalFormat("0.00");//保留兩位小數點
    public static void main(String[] args) {
        Random random = new Random();
        List<User> list = new ArrayList<>();
        for(int i=1;i<=5;i++) {
            double weight = random.nextDouble() * 100 + 100;//隨機身高：100-200
            list.add(new User(i, new BigDecimal(weight).setScale(BigDecimal.ROUND_HALF_UP, 2)));
        }
        System.out.println("list：" + list);
        BigDecimal add = list.stream().map(User::getHeight).reduce(BigDecimal.ZERO, BigDecimal::add);
        System.out.println("身高 總和：" + df.format(add));
        Optional<User> max = list.stream().max((u1, u2) -> u1.getHeight().compareTo(u2.getHeight()));
        System.out.println("身高 最大：" + df.format(max.get().getHeight()));
        Optional<User> min = list.stream().min((u1, u2) -> u1.getHeight().compareTo(u2.getHeight()));
        System.out.println("身高 最小：" + df.format(min.get().getHeight()));

    }
    private static class User{
        Integer id;
        BigDecimal height;//身高

        public User(Integer id, BigDecimal height) {
            this.id = id;
            this.height = height;
        }

        public Integer getId() {
            return id;
        }

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

        public BigDecimal getHeight() {
            return height;
        }

        public void setHeight(BigDecimal height) {
            this.height = height;
        }

        @Override
        public String toString() {
            return "User{" +
                    "id=" + id +
                    ", height=" + height +
                    '}';
        }
    }

}