How to use Java Collectors tool class

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();
    }
}

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;
    }
}

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())));
}

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("-", "{", "}")));
}

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)));
}

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)));
}

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());

Output result:

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