Java Stream Collectors的maxBy() minBy()、groupingBy()、partitioningBy()的使用

1、Collectors.maxBy()/minBy()的使用

Collectors.maxBy()/minBy()可以根据指定的Comparator，计算stream中的最大值和最小值，例如，

import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {
   public static class Person {
     int age;
     Person (int age) {
         this.age = age;
     }
     public int getAge() {
         return age;
     }
     public void setAge(int age) {
         this.age = age;
     }

}
public static void main(String[] args) {

    List<Person> pList = Arrays.asList(new Person(11),new Person(22), new Person(33), new Person(44), new Person(55));

    Optional<Integer> maxByResult = pList.stream().map(p -> p.getAge()).collect(Collectors.maxBy(Comparator.naturalOrder()));
    System.out.println("stream() maxByResult ：" + maxByResult.get());
    Optional<Integer> minByResult = pList.stream().map(p -> p.getAge()).collect(Collectors.minBy(Comparator.naturalOrder()));
    System.out.println("stream() minByResult ：" + minByResult.get());
    System.exit(0); //success
  }
}

2、Collectors.groupingBy()的使用

Collectors.groupingBy()根据指定的属性进行分组，并返回一个Map，例如，

import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {
   public static class Person {
     int age;
     Person (int age) {
         this.age = age;
     }
     public int getAge() {
         return age;
     }
     public void setAge(int age) {
         this.age = age;
     }

}
public static void main(String[] args) {

    List<Person> pList = Arrays.asList(new Person(11),new Person(11),new Person(22), new Person(33), new Person(44), new Person(55));

    Map<Integer, Set<Person>> groupByResult = pList.stream()
                .collect(Collectors.groupingBy(Person::getAge,Collectors.toSet()));

    System.out.println("stream() groupByResult ：" + groupByResult);
    System.exit(0); //success
  }
}

3、Collectors.partitioningBy()的使用

Collectors.partitioningBy()返回一个Map，这个Map是以boolean值为key，将stream分成两部分，一部分是匹配PartitioningBy条件的，另一部分是不满足条件的：，如下，

import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {
   public static class Person {
     int age;
     Person (int age) {
         this.age = age;
     }
     public int getAge() {
         return age;
     }
     public void setAge(int age) {
         this.age = age;
     }

}
public static void main(String[] args) {

    List<Person> pList = Arrays.asList(new Person(11),new Person(11),new Person(22), new Person(33), new Person(44), new Person(55));

    Map<Boolean, List<Person>> partitionResult = pList.stream()
                .collect(Collectors.partitioningBy(p -> p.getAge() > 22));

    System.out.println("stream() groupByResult ：" + partitionResult);
    System.exit(0); //success
  }
}