多级分组

package com.ant.jdk8.chap06;

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

public class DishGroupDemo {
    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) );
        Map<Type,List<Dish>> dishesByType =
                menu.stream().collect(Collectors.groupingBy(Dish::getType));
        System.out.println(dishesByType);
    }
}

package com.ant.jdk8.chap06;

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

public class DishGroupDemo {
    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) );
        Map<CaloricLevel,List<Dish>> dishesByCaloricLeval =
                menu.stream().collect(
                        Collectors.groupingBy(
                                d->{ if(d.getCalories()<=400) return CaloricLevel.DIET;
                                else if(d.getCalories()<=700) return CaloricLevel.NORMAL;
                                else return CaloricLevel.FAT;})
                );
        System.out.println(dishesByCaloricLeval);
    }
}

多级分组：

package com.ant.jdk8.chap06;

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

public class DishGroupDemo {
    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) );
        Map<Type,Map<CaloricLevel,List<Dish>>> dishesByTypeCaloricLevel =
                menu.stream().collect(
                        Collectors.groupingBy(Dish::getType,
                        Collectors.groupingBy(d->{
                            if(d.getCalories()<=400) return CaloricLevel.DIET;
                        else if(d.getCalories()<=700) return CaloricLevel.NORMAL;
                        else return CaloricLevel.FAT;})));
        System.out.println(dishesByTypeCaloricLevel);
    }
}