Collectors工具類的使用

/**
 * 
 */
package com.gewb.stream;

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

/**
 * @author Bingo.Ge
 * @date 2020年7月3日
 */
public class CollectorsAction {
	
	private final static List<Dish> menu = Arrays.asList(
			new Dish("pork", false, 800, Dish.Type.MEAT),
			new Dish("beef", false, 700, Dish.Type.MEAT),
			new Dish("chicken", false, 400, Dish.Type.MEAT),
			new Dish("french fries", true, 530, Dish.Type.OTHER));
	
	
	public static void main(String[] args) {
		testAverageingDouble();
		testCollectingAndThen();
		testGroupByFunction();
		testSummarizingInt();
	}
	
	
	private static void testAverageingDouble() {
		System.out.println("===================testAverageingDouble===================");
		Optional.ofNullable(menu.stream().collect(Collectors.averagingDouble(Dish::getCalories))).ifPresent(System.out::println);
	}
	
	private static void testCollectingAndThen() {
		System.out.println("===================testCollectingAndThen===================");
		Optional.ofNullable(menu.stream().collect(Collectors
				.collectingAndThen(Collectors.averagingInt(Dish::getCalories), a -> "average calories is:" + a)))
				.ifPresent(System.out::println);
	}
	
	private static void testGroupByFunction() {
		System.out.println("===================testGroupByFunction===================");
		Optional.ofNullable(menu.stream().collect(Collectors.groupingBy(Dish::getType, Collectors.counting()))).ifPresent(System.out::println);
	}
	
	
	private static void testSummarizingInt() {
		System.out.println("===================testSummarizingInt===================");
		Optional.ofNullable(menu.stream().collect(Collectors.summarizingInt(Dish::getCalories))).ifPresent(System.out::println);
	}
	
}

輸出結果：

===================testAverageingDouble===================
607.5
===================testCollectingAndThen===================
average calories is:607.5
===================testGroupByFunction===================
{MEAT=3, OTHER=1}
===================testSummarizingInt===================
IntSummaryStatistics{count=4, sum=2430, min=400, average=607.500000, max=800}