Java Stream 常用聚合操作(sum、count、max、min、average)的使用

1、使用Stream的sum()求和

可以使用Stream的sum()方法对数组、列表和字典进行求和，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) {

    //数组
    int[] intArray = new int[]{1, 2, 3,4};
    var result = Arrays.stream(intArray).sum();
    System.out.println(result);
    
    //列表
    List<Long> list = new ArrayList<>(Collections.nCopies(5, 0L));
    list.set(0,1L);
    list.set(1,2L);
    list.set(2,3L);
    list.set(3,4L);
    list.set(4,5L);
    System.out.println(list.stream().mapToDouble(e->e).sum());
    List<Person> pList = new ArrayList<>(Collections.nCopies(5, new Person(1)));
    System.out.println(pList.stream().mapToDouble(e->e.age).sum()); 
    
    Map<String, Integer> m1= new HashMap<>();
    m1.put("C",1);
    m1.put("Java",2);
    m1.put("Python",3);
    System.out.println(m1.values().stream().mapToDouble(d->d).sum()); 
    System.exit(0); //success
  }
}

2、使用Stream的count()计数

可以使用Stream的count()对满足指定条件的元素进行计数，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) {

    //数组
    int[] intArray = new int[]{1, 2, 3,4};
    var result = Arrays.stream(intArray).filter(e -> e>1).sum();
    System.out.println(result);
    
    //列表
    List<Long> list = new ArrayList<>(Collections.nCopies(5, 0L));
    list.set(0,1L);
    list.set(1,2L);
    list.set(2,3L);
    list.set(3,4L);
    list.set(4,5L);
    System.out.println(list.stream().filter(e -> e % 2 == 0).count());
    List<Person> pList = new ArrayList<>(Collections.nCopies(5, new Person(1)));
    System.out.println(pList.stream().filter(e -> e.getAge() > 0).count()); 
    
    Map<String, Integer> m1= new HashMap    <>();
    m1.put("C",1);
    m1.put("Java",2);
    m1.put("Python",3);
    System.out.println(m1.values().stream().filter(e -> e % 2 == 0).count()); 
    System.exit(0); //success
  }
}

3、使用Stream计算最大值、最小值和平均值

可以使用Stream的max()计算最大值 、min()计算最小值和average()计算平均值，如下，

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

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) {

    //数组
    int[] intArray = new int[]{1, 2, 3,4};
    OptionalInt maxOptional = Arrays.stream(intArray).max();
    OptionalInt minOptional = Arrays.stream(intArray).min();
    OptionalDouble avgOptional = Arrays.stream(intArray).average();
    minOptional.ifPresent(e -> System.out.println("Min: " + e));
    maxOptional.ifPresent(e -> System.out.println("Max: " + e));
    avgOptional.ifPresent(e -> System.out.println("Avg: " + e));
    //list
    List<Person> list = Arrays.asList(new Person(11),new Person(22), new Person(33), new Person(44), new Person(55));
    Optional<Person> maxListOptional = list.stream().max(Comparator.comparingInt(Person::getAge));
    Optional<Person> minListOptional = list.stream().min(Comparator.comparingInt(Person::getAge));
    OptionalDouble avgListOptional = list.stream().mapToDouble(e->e.age).average();
    minListOptional.ifPresent(e -> System.out.println("Min: " + e.getAge()));
    maxListOptional.ifPresent(e -> System.out.println("Max: " + e.getAge()));
    avgListOptional.ifPresent(e -> System.out.println("Avg: " + e));
    System.exit(0); //success
  }
}