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How to use Java Collectors tool class

王林
Release: 2023-05-15 21:07:04
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    Method

    ● maxBy: Get the largest element in the stream; minBy: Get the smallest element in the stream

    ● joining: merge, merge The elements in the stream are concatenated in the form of strings

    ● summingInt: map the elements in the stream into int type elements, and sum them

    ● averagingInt: map the elements in the stream into elements of type int, find the average value

    ● summarizingInt: Map the elements in the stream into elements of type int, and obtain the description information

    Practice instructions

    1. Prerequisite Condition

    Person class

    package com.example;
    import lombok.AllArgsConstructor;
    import lombok.Data;
    import org.springframework.context.annotation.Configuration;
    import java.util.Objects;
    /**
     * @BelongsProject: StreamOperate
     * @BelongsPackage: com.example
     * @CreateTime: 2023-05-01  11:18
     * @Description: Person实体类
     * @Version: 1.0
     */
    public class Person implements Comparable<Person>{
        public String getName() {
            return name;
        }
        public Person setName(String name) {
            this.name = name;
            return this;
        }
        public int getAge() {
            return age;
        }
        public Person setAge(int age) {
            this.age = age;
            return this;
        }
        public int getScore() {
            return score;
        }
        public Person setScore(int score) {
            this.score = score;
            return this;
        }
        private String name;
        private int age;
        private int score;
        public Person(String name, int age, int score) {
            this.name = name;
            this.age = age;
            this.score = score;
        }
        public Person() {
        }
        @Override
        public String toString() {
            return "Person{" +
                    "name=&#39;" + name + &#39;\&#39;&#39; +
                    ", age=" + age +
                    ", score=" + score +
                    &#39;}&#39;;
        }
        @Override
        public boolean equals(Object o) {
            //地址相同,为true
            if (this == o) return true;
            //为null,并且类型不一样,为false
            if (o == null || getClass() != o.getClass()) return false;
            //向下转型,再去比较属性值
            Person person = (Person) o;
            //如果属性值相同,最后的结果为true
            return age == person.age && score == person.score && Objects.equals(name, person.name);
            //return false;
        }
        @Override
        public int hashCode() {
            return Objects.hash(name, age, score);
        }
        @Override
        public int compareTo(Person o) {
            return this.getScore()-o.getScore();
        }
    }
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    Data class

    package com.example;
    import org.springframework.context.annotation.Configuration;
    import java.util.ArrayList;
    /**
     * @BelongsProject: StreamOperate
     * @BelongsPackage: com.example
     * @CreateTime: 2023-05-01  11:08
     * @Description: Data类
     * @Version: 1.0
     */
    public class Data {
        public static ArrayList<Person> getData() {
            ArrayList<Person> personList = new ArrayList<>();
            personList.add(new Person("张三", 18, 90));
            personList.add(new Person("李四", 19, 100));
            personList.add(new Person("王五", 17, 60));
            personList.add(new Person("赵六", 18, 89));
            personList.add(new Person("孙七", 20, 96));
            personList.add(new Person("郑十", 20, 46));
            personList.add(new Person("周八", 20, 96));
            personList.add(new Person("周八", 20, 96));
            personList.add(new Person("吴九", 20, 45));
            personList.add(new Person("邓十一", 20, 35));
            personList.add(new Person("刘十二", 20, 99));
            personList.add(new Person("小十三", 20, 56));
            personList.add(new Person("小十三", 20, 56));
            return personList;
        }
    }
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    2. Operation

    maxBy: Get the largest element in the stream; minBy: Get the smallest element in the stream

    public static void main(String[] args) {
            Stream<Person> stream = Data.getData().stream();
            //maxBy:获取流中最大元素;minBy:获取流中最小元素
            System.out.println(Data.getData().stream().collect(Collectors.maxBy((ele1, ele2) -> ele1.getScore() - ele2.getScore())));
            System.out.println(Data.getData().stream().collect(Collectors.minBy((ele1, ele2) -> ele1.getAge() - ele2.getAge())));
    }
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    joining: Merge, join the elements in the stream in the form of a string

    public static void main(String[] args) {
            Stream<Person> stream = Data.getData().stream();
            //joining:合并,将流中的元素,以字符串的形式拼接起来
            //将集合中person对象的姓名拼接成一个字符串
            System.out.println(Data.getData().stream().map(Person::getName).collect(Collectors.joining()));
            System.out.println(Data.getData().stream().map(Person::getName).collect(Collectors.joining("-")));
            System.out.println(Data.getData().stream().map(Person::getName).collect(Collectors.joining("-", "{", "}")));
    }
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    summingInt: Map the elements in the stream into elements of type int, and sum them

    public static void main(String[] args) {
            Stream<Person> stream = Data.getData().stream();
    				//summingInt:把流中的元素映射成int类型的元素,求和
            System.out.println(Data.getData().stream().collect(Collectors.summingInt(Person::getScore)));
    }
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    averagingInt: Map the elements in the stream to elements of type int, and find the average value

    public static void main(String[] args) {
            Stream<Person> stream = Data.getData().stream();
            //averagingInt:把流中的元素映射成int类型的元素,求平均值
            System.out.println(Data.getData().stream().collect(Collectors.averagingInt(Person::getScore)));
    }
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    summarizingInt: Map the elements in the stream to elements of type int, and get the description information

    Requirement: Replace Person objects with scores greater than or equal to 80 in the stream with their names

     public static void main(String[] args) {
            Stream<Person> stream = Data.getData().stream();
            //summarizingInt:把流中的元素映射成int类型的元素,获取描述信息
            IntSummaryStatistics collect = Data.getData().stream().collect(Collectors.summarizingInt(Person::getScore));
            System.out.println(collect);
            System.out.println(collect.getCount());
            System.out.println(collect.getSum());
            System.out.println(collect.getMax());
            System.out.println(collect.getMax());
            System.out.println(collect.getAverage());
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    Output result:

    Java Collectors工具类怎么使用

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