Java8之list.stream的常见使用

本文转自 https://blog.csdn.net/jhgnqq/article/details/123679622 感谢楼主分享

import org.junit.Before;
import org.junit.Test;

import java.util.Arrays;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

public class StreamDemo {
    List<Student> list = null;
    //初始化数据
    @Before
    public void beforetest() {
        list = Arrays.asList(
                new Student("Tom", 18, 88, 90),
                new Student("Jerry", 20, 77, 89),
                new Student("Lily", 17, 98, 79),
                new Student("Lucy", 19, 70, 80),
                new Student("LiLei", 18, 88, 90),
                new Student("HanMeiMei", 21, 87, 79));
    }


    @Test
    public void streamtest() {
        // filter 过滤器返回还是一个stream流对象
        //查询math成绩大于80的学生并遍历输出
        list.stream().filter(e->e.getMath()>80).forEach(System.out::println);//.forEach(e->System.out.println(e))
        //统计数量count
        System.out.println(list.stream().count());
        //如统计总分大于160的人数
        System.out.println(list.stream().filter(e->e.getEnglish()+e.getMath()>160).count());
        //limit  取前n个值
        list.stream().limit(3).forEach(System.out::println);
        //skip 跳过前n个
        list.stream().skip(2).forEach(System.out::println);
        //distinct 去除重复数据
        list.stream().distinct().forEach(System.out::println);
        //map 映射元素可以对元素进行操作   例如对每个学生年龄加1
        list.stream().map(e->{
            e.setAge(e.getAge()+1);
            return e;
        }).forEach(System.out::println);
        //sorted 排序
        //升序
        list.stream().sorted((a,b)->{
            return a.getEnglish().compareTo(b.getEnglish());
        });
        //降序
        list.stream().sorted((a,b)->{
            return b.getEnglish().compareTo(a.getEnglish());
        });
        //返回第一个元素
        Optional<Student> first = list.stream().findFirst();
        System.out.println(first.get());
        //返回任意一个元素
        System.out.println(list.stream().findAny().get());
        //anyMatch 是否匹配任意一元素  检查是否包含名字为Tom的
        System.out.println(list.stream().anyMatch(e->e.getName().equals("Tom")));
        //allMatch 是否匹配所有元素
        System.out.println(list.stream().allMatch(e->e.getName().equals("Tom")));
        //noneMatch  是否未匹配所有元素
        System.out.println(list.stream().noneMatch(e->e.getName().equals("Tom")));
        //findFirst 返回元素中第一个值
        Student student = list.stream().findFirst().get();
        //findAny 返回元素中任意一个值
        Student student1 = list.stream().findAny().get();
        //max 返回最大值 查询英语成绩最高的学生
        Student student2 = list.stream().max((l1,l2)->l2.getEnglish().compareTo(l1.getEnglish())).get();
        //min 最小值  将上面l1，l2位置对调
        Student student3 = list.stream().max((l1,l2)->l2.getEnglish().compareTo(l1.getEnglish())).get();
        //将流对象转为list
        list.stream().filter(e->e.getMath()>80).collect(Collectors.toList());
        //将流转未set
        list.stream().filter(e->e.getMath()>80).collect(Collectors.toSet());
        //对对象中的某项进行统计
        IntSummaryStatistics c = list.stream().collect(Collectors.summarizingInt(Student::getEnglish));
        System.out.println(c);//IntSummaryStatistics{count=6, sum=507, min=79, average=84.500000, max=90}
    }

}

//entity
import lombok.Data;
@data
public class Student {
    private String name;
    private Integer age;
    private Integer math;
    private Integer english;
}

 另外一篇应用举例  (16条消息) Java8之list.stream的常见使用_大白给小白讲故事的博客-CSDN博客_list stream

public static void main(String[] args) {
        List<Student> list = Lists.newArrayList();
        list.add(new Student("测试", "男", 18));
        list.add(new Student("开发", "男", 20));
        list.add(new Student("运维", "女", 19));
        list.add(new Student("DBA", "女", 22));
        list.add(new Student("运营", "男", 24));
        list.add(new Student("产品", "女", 21));
        list.add(new Student("经理", "女", 25));
        list.add(new Student("产品", "女", 21));
 
        //求性别为男的学生集合
        List<Student> l1 = list.stream().filter(student -> student.sex.equals("男")).collect(toList());
        
        //map的key值true为男，false为女的集合
        Map<Boolean, List<Student>> map = list.stream().collect(partitioningBy(student -> student.getSex().equals("男")));
 
        //求性别为男的学生总岁数
        Integer sum = list.stream().filter(student -> student.sex.equals("男")).mapToInt(Student::getAge).sum();
 
        //按性别进行分组统计人数
        Map<String, Integer> map = list.stream().collect(Collectors.groupingBy(Student::getSex, Collectors.summingInt(p -> 1)));
 
        //判断是否有年龄大于25岁的学生
        boolean check = list.stream().anyMatch(student -> student.getAge() > 25);
 
        //获取所有学生的姓名集合
        List<String> l2 = list.stream().map(Student::getName).collect(toList());
 
        //求所有人的平均年龄
        double avg = list.stream().collect(averagingInt(Student::getAge));
 
        //求年龄最大的学生
        Student s = list.stream().reduce((student, student2) -> student.getAge() > student2.getAge() ? student:student2).get();
        
        Student stu = list.stream().collect(maxBy(Comparator.comparing(Student::getAge))).get();
 
        //按照年龄从小到大排序
        List<Student> l3 = list.stream().sorted((s1, s2) -> s1.getAge().compareTo(s2.getAge())).collect(toList());
 
        //求年龄最小的两个学生
        List<Student> l4 = l3.stream().limit(2).collect(toList());
 
        //获取所有的名字，组成一条语句
        String str = list.stream().map(Student::getName).collect(Collectors.joining(",", "[", "]"));
        
        //获取年龄的最大值、最小值、平均值、求和等等
        IntSummaryStatistics intSummaryStatistics = list.stream().mapToInt(Student::getAge).summaryStatistics();
        System.out.println(intSummaryStatistics.getMax());
        System.out.println(intSummaryStatistics.getCount());
    }
 
    @Data
    @AllArgsConstructor
    static class Student{
        String name;
        String sex;
        Integer age;
 
    }