java8-stream常用操作(1)

前言

java8的Stream 流式操作，用於對集合進行投影、轉換、過濾、排序、去重等，更進一步地說，這些操作能鏈式串聯在一起使用，類似於 SQL 語句，可以大大簡化程式碼。下面我就將平時常用的一些stream操作做一些彙總。

通過 IntStream 構造基本型別的流

        //在java8之前，遍歷0-99的基本操作

        for(int i = 0;i<100;i++){

            System.out.println("i:"+i);

        }

        //使用IntStream序列遍歷

        IntStream.rangeClosed(0,99).forEach(i->{

            System.out.println("i:"+i);

        });

        //使用IntStream並行遍歷

        IntStream.rangeClosed(0,99).parallel().forEach(i->{

            System.out.println("i:"+i);

        });

定義一個學生的結構體，為後續操作做準備

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 + '\'' +

                '}';

    }

}

//構造資料

		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,"鄭州"));

        students.add(new Student("stu003",45,90,"鄭州"));

        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

根據特定的條件進行過濾

//filter 實現過濾操作，這邊過濾出年齡大於30的學生

students.stream().filter(s->s.getAge()>30).forEach(System.out::println);

map

元素進行轉化

 //map 對流每一個元素進行轉化，將student的age轉化成流

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

sort 排序

根據條件進行排序操作

//sort 按年齡排序，倒序

students.stream().sorted(Comparator.comparing(Student::getAge).reversed()).forEach(System.out::println);

distinct 去重

對流元素進行去重操作

//distinct 去重,因為實現了hashcode和equals，名稱為stu001的學生重複了

students.stream().distinct().forEach(System.out::println);

將集合組成一個list

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

將集合組成一個set

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

最大值

求出年齡最大的學生

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

最大值

求出年齡最最小的學生

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

join操作

按照逗號進行join,用[]包裹起來

//join操作，姓名join

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

toMap操作

根據指定key,value生成一個map

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