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Java8 Collectors其它一些方法的使用

java8 来源:lgx06 1次浏览
package com.lgx.jdk8.part02;

import java.util.*;
import java.util.stream.Collectors;

/**
 * Collectors其它一些方法的使用
 */
public class Test11Collectors {
    public static void main(String[] args) {
        User user1 = new User("zhangsan", 60, 20);
        User user2 = new User("lisi", 80, 23);
        User user3 = new User("zhangsan", 80, 24);
        User user4 = new User("wangwu", 50, 24);
        User user5 = new User("wangwu2", 50, 24);

        List<User> userList = Arrays.asList(user1, user2, user3, user4, user5);

        //算出分数最小的那个并输出
        userList.stream().collect(Collectors.minBy(Comparator.comparingInt(User::getScore))).ifPresent(System.out::println);

        //算出分数最大的那个并输出(无法做到多个并列的时候求值)
        Optional optional = userList.stream().collect(Collectors.maxBy(Comparator.comparingInt(User::getScore)));
        //optional.isPresent(System.out::println);//isPresent是判断是否存在,不能接受参数
        optional.ifPresent(System.out::println);//直接使用时ifPresent

        //算出分数平均值并输出
        double averagint = userList.stream().collect(Collectors.averagingInt(User::getScore));
        System.out.println("averagint = " + averagint);

        //算出分数总和并输出
        int summingInt = userList.stream().collect(Collectors.summingInt(User::getScore));
        System.out.println("summingInt = " + summingInt);

        //算出汇总信息
        IntSummaryStatistics intSummaryStatistics = userList.stream().collect(Collectors.summarizingInt(User::getScore));
        System.out.println("intSummaryStatistics = " + intSummaryStatistics);

        //拼接名字
        String nameStrs = userList.stream().map(User::getName).collect(Collectors.joining(", "));
        System.out.println("nameStrs = " + nameStrs);

        //拼接名字,调用另外一个方法,可以加前缀和后缀
        String nameStrs2 = userList.stream().map(User::getName).collect(Collectors.joining(", ", "[", "]"));
        System.out.println("nameStrs2 = " + nameStrs2);

        //分组:按照分数(返回的map的key是根据分组的条件来决定的,score是int,那么key就是Integer)
        Map<Integer, List<User>> scoreUsers = userList.stream().collect(Collectors.groupingBy(User::getScore));
        System.out.println("scoreUsers = " + scoreUsers);

        //二级分组:线按照分数分组,返回一个Map<Integer, List<User>>, 在根据用户名分组
        Map<Integer, Map<String, List<User>>> scoreNameUsers = userList.stream().collect(Collectors.groupingBy(User::getScore, Collectors.groupingBy(User::getName)));
        System.out.println("scoreNameUsers = " + scoreNameUsers);

        //分区,是否及格
        Map<Boolean, List<User>> jigeUsers = userList.stream().collect(Collectors.partitioningBy(user -> user.getScore() >= 60));
        System.out.println("jigeUsers = " + jigeUsers);

        //二级分区,是否及格,及格里面是否大于80
        Map<Boolean, Map<Boolean, List<User>>> youxiuUsers = userList.stream().collect(Collectors.partitioningBy(user -> user.getScore() >= 60, Collectors.partitioningBy(user -> user.getScore() >= 80)));
        System.out.println("youxiuUsers = " + youxiuUsers);

        //分区,是否及格,算出及格的个数
        Map<Boolean, Long> jigeUserCount = userList.stream().collect(Collectors.partitioningBy(user -> user.getScore() >= 60, Collectors.counting()));
        System.out.println("jigeUserCount = " + jigeUserCount);

        //先按照名字分组,获取每个分组分数最小的
        Map<String, User> UserCount = userList.stream().collect(Collectors.groupingBy(User::getName, Collectors.collectingAndThen(Collectors.minBy(Comparator.comparingInt(User::getScore)), Optional::get)));
        System.out.println("UserCount = " + UserCount);


    }
}


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