JDK8的Stream实现统计（求最大值最小值求和等操作）

数据准备

@Data
public class User {

    private Long id;
    private Integer schoolId;
    private String userName;
    private String edu;
    private double price;



    public static List<User> users = new ArrayList<>();

    static {
        User u1 = new User();
        u1.setId(1001L);
        u1.setSchoolId(100);
        u1.setUserName("小1");
        u1.setEdu("001");
        u1.setPrice(0.01);

        User u2 = new User();
        u2.setId(1002L);
        u2.setSchoolId(100);
        u2.setUserName("小2");
        u2.setEdu("002");
        u2.setPrice(0.20);


        User u3 = new User();
        u3.setId(2010L);
        u3.setSchoolId(200);
        u3.setUserName("小3");
        u3.setEdu("001");
        u3.setPrice(3.00);


        User u4 = new User();
        u4.setId(3001L);
        u4.setSchoolId(300);
        u4.setEdu("001");
        u4.setPrice(40.0);

        users.add(u1);
        users.add(u2);
        users.add(u3);
        users.add(u4);
    }
}

1. 使用mapToXxx进行统计

public class TestMapToXx {
    public static void main(String[] args) {
        //将原始对象直接映射为Int类型
        IntStream intStream = User.users.stream().mapToInt(User::getSchoolId);
        //最大值
        OptionalInt max = intStream.max();
        //最小值
        OptionalInt min = intStream.min();
        //求和
        int sum = intStream.sum();
        //统计值
        long count = intStream.count();
        //平均值
        OptionalDouble average = intStream.average();

        //若是空指针，直接返回0.00
        double avg = average.orElse(0.00);

        DoubleStream doubleStream = User.users.stream().mapToDouble(User::getPrice);

        LongStream longStream = User.users.stream().mapToLong(User::getId);
    }
}

2. 使用reduce进行统计

public class TestMapToXx {
    public static void main(String[] args) {
        //获取最大值
        OptionalDouble maxReduce = User.users.stream().mapToDouble(User::getPrice).reduce(Double::max);
        double max = maxReduce.orElse(0);
        System.out.println(max);

        //求和
        OptionalDouble sumReduce = User.users.stream().mapToDouble(User::getPrice).reduce(Double::sum);
        double sum = sumReduce.orElse(0);
        System.out.println(sum);
    }
}

3. 使用summarizingXxx进行统计

    private static void test1() {
        DoubleSummaryStatistics collect = User.users.stream().collect(Collectors.summarizingDouble(User::getPrice));
        double average = collect.getAverage();
        long count = collect.getCount();
        double max = collect.getMax();
        double min = collect.getMin();
        double sum = collect.getSum();
    }

4. 使用min或者max

    private static void test1() {
        Optional<String> max = User.users.stream().map(User::getUserName).max((o1, o2) -> o2.compareTo(o1));
        //等效于正常顺序降序排序取最大值
        Optional<String> max1 = User.users.stream().map(User::getUserName).max(Comparator.reverseOrder());
        //正常的顺序求最大值
        Optional<Long> min = User.users.stream().map(User::getId).min(Comparator.naturalOrder());
    }

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