java8-stream常用操作(1)

前言

java8的Stream 流式操作，用于对集合进行投影、转换、过滤、排序、去重等，更进一步地说，这些操作能链式串联在一起使用，类似于 SQL 语句，可以大大简化代码。下面我就将平时常用的一些stream操作做一些汇总。

通过 IntStream 构造基本类型的流

		//在java8之前，遍历0-99的基本操作
        for(int i = 0;i<100;i++){
            System.out.println("i:"+i);
        }

        //使用IntStream串行遍历
        IntStream.rangeClosed(0,99).forEach(i->{
            System.out.println("i:"+i);
        });

        //使用IntStream并行遍历
        IntStream.rangeClosed(0,99).parallel().forEach(i->{
            System.out.println("i:"+i);
        });

定义一个学生的结构体，为后续操作做准备

class Student{
    private String name;
    private int age;
    private int score;
    private String city;

    public Student(String name, int age, int score, String city) {
        this.name = name;
        this.age = age;
        this.score = score;
        this.city = city;
    }

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public int getScore() {
        return score;
    }

    public void setScore(int score) {
        this.score = score;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age &&
                score == student.score &&
                Objects.equals(name, student.name) &&
                Objects.equals(city, student.city);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age, score, city);
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", score=" + score +
                ", city='" + city + '\'' +
                '}';
    }
}


//构造数据
		List<Student> students = new ArrayList<>();
        students.add(new Student("stu001",18,70,"合肥"));
        students.add(new Student("stu001",18,70,"合肥"));
        students.add(new Student("stu002",23,80,"郑州"));
        students.add(new Student("stu003",45,90,"郑州"));
        students.add(new Student("stu004",32,73,"合肥"));
        students.add(new Student("stu005",66,76,"南京"));
        students.add(new Student("stu006",34,79,"杭州"));
        students.add(new Student("stu007",28,90,"合肥"));
        students.add(new Student("stu008",19,100,"杭州"));

filter

根据特定的条件进行过滤

//filter 实现过滤操作，这边过滤出年龄大于30的学生
students.stream().filter(s->s.getAge()>30).forEach(System.out::println);

map

元素进行转化

 //map 对流每一个元素进行转化，将student的age转化成流
students.stream().map(Student::getAge).forEach(System.out::println);

sort 排序

根据条件进行排序操作

//sort 按年龄排序，倒序
students.stream().sorted(Comparator.comparing(Student::getAge).reversed()).forEach(System.out::println);

distinct 去重

对流元素进行去重操作

//distinct 去重,因为实现了hashcode和equals，名称为stu001的学生重复了
students.stream().distinct().forEach(System.out::println);

将集合组成一个list

students.stream().map(Student::getAge).collect(Collectors.toList()).forEach(System.out::println);

将集合组成一个set

students.stream().map(Student::getAge).collect(Collectors.toSet()).forEach(System.out::println);

最大值

求出年龄最大的学生

System.out.println(students.stream().max(Comparator.comparingInt(Student::getAge)).get());

最大值

求出年龄最最小的学生

System.out.println(students.stream().min(Comparator.comparingInt(Student::getAge)).get());

join操作

按照逗号进行join,用[]包裹起来

//join操作，姓名join
System.out.println(students.stream().map(Student::getName).collect(Collectors.joining(",","[","]")));

toMap操作

根据指定key,value生成一个map

students.stream().collect(Collectors.toMap(Student::getName,student -> student,(x1,x2)->x2)).entrySet().forEach(System.out::println);

groupingBy

根据城市对学生进行分组

   students.stream().collect(Collectors.groupingBy(Student::getCity))
           .forEach((k,v)->{
                System.out.println(k+" = "+v);
            });

根据城市对学生进行分组,统计每个分组的count

   students.stream().collect(Collectors.groupingBy(Student::getCity,Collectors.counting()))
                .forEach((k,v)->{
                    System.out.println(k+" = "+v);
                });

统计每个分组的分数平均值

    students.stream().collect(Collectors.groupingBy(Student::getCity,Collectors.averagingInt(Student::getScore)))
                .forEach((k,v)->{
                    System.out.println(k+" = "+v);
                });

分组之转换分组结果List -> List

students.stream().collect(
                Collectors.groupingBy(Student::getCity, Collectors.mapping(Student::getName, Collectors.toList())))
        .forEach((k,v)->{
            System.out.println(k+" = "+v);
        });

//打印结果
//        合肥 = [stu001, stu001, stu004, stu007]
//        郑州 = [stu002, stu003]
//        杭州 = [stu006, stu008]
//        南京 = [stu005]
       

自定义key

        students.stream().collect(Collectors.groupingBy(student -> "key_"+student.getCity()))
                .forEach((k,v)->{
                    System.out.println(k+" = "+v);
                });