归约和汇总

Posted 2021-01-04 i-hard-working

1. 查找流中的最大值和最小值：

package com.ant.jdk8.chap06;

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class MaxByMinByDemo {
    public static void main(String[] args) {
        List<Dish> menu = Arrays.asList(
                new Dish("pork", false, 800, Type.MEAT),
                new Dish("beef", false, 700, Type.MEAT),
                new Dish("chicken", false, 400, Type.MEAT),
                new Dish("french fries", true, 530, Type.OTHER),
                new Dish("rice", true, 350, Type.OTHER),
                new Dish("season fruit", true, 120, Type.OTHER),
                new Dish("pizza", true, 550, Type.OTHER),
                new Dish("prawns", false, 300, Type.FISH),
                new Dish("salmon", false, 450, Type.FISH) );
        menu.stream()
                .collect(Collectors.maxBy(Comparator.comparingInt(Dish::getCalories)))
                .ifPresent(d-> System.out.println(d.getCalories()+"->"+d.getName()));
    }
}

2. 汇总：

int totalCalories = menu.stream().collect(Collectors.summingInt(Dish::getCalories));
System.out.println("totalCalories:"+totalCalories);

IntSummaryStatistics menuStatistics = menu.stream().collect(Collectors.summarizingInt(Dish::getCalories));
System.out.println(menuStatistics);

3. 连接字符串：

String menuStr = menu.stream().map(Dish::getName).collect(Collectors.joining(", "));
System.out.println(menuStr);

4. 广义的归约汇总：

 

int totalCalories1 = menu.stream().collect(Collectors.reducing(0,Dish::getCalories,Integer::sum));
int totalCalories2 = menu.stream().map(Dish::getCalories).reduce(Integer::sum).get();
int totalCalories3 = menu.stream().mapToInt(Dish::getCalories).sum();
System.out.println(totalCalories1+","+totalCalories2+","+totalCalories3);