继续之前写的

package demo;

import java.util.*;

import static java.util.Arrays.asList;

public class Dish {

    private final String name;
    private final boolean vegetarian;
    private final int calories;
    private final Type type;

    public Dish(String name, boolean vegetarian, int calories, Type type) {
        this.name = name;
        this.vegetarian = vegetarian;
        this.calories = calories;
        this.type = type;
    }

    public String getName() {
        return name;
    }

    public boolean isVegetarian() {
        return vegetarian;
    }

    public int getCalories() {
        return calories;
    }

    public Type getType() {
        return type;
    }

    public enum Type { MEAT, FISH, OTHER }

    @Override
    public String toString() {
        return name;
    }

    public static final List<Dish> menu =
            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),
                    new Dish("rice", true, 350, Dish.Type.OTHER),
                    new Dish("season fruit", true, 120, Dish.Type.OTHER),
                    new Dish("pizza", true, 550, Dish.Type.OTHER),
                    new Dish("prawns", false, 400, Dish.Type.FISH),
                    new Dish("salmon", false, 450, Dish.Type.FISH));

    public static final Map<String, List<String>> dishTags = new HashMap<>();

    static {
        dishTags.put("pork", asList("greasy", "salty"));
        dishTags.put("beef", asList("salty", "roasted"));
        dishTags.put("chicken", asList("fried", "crisp"));
        dishTags.put("french fries", asList("greasy", "fried"));
        dishTags.put("rice", asList("light", "natural"));
        dishTags.put("season fruit", asList("fresh", "natural"));
        dishTags.put("pizza", asList("tasty", "salty"));
        dishTags.put("prawns", asList("tasty", "roasted"));
        dishTags.put("salmon", asList("delicious", "fresh"));
    }
}

第一个类是各种食物
下面用流处理他

package demo;
import java.util.*;
import static java.util.stream.Collectors.*;
import static demo.Dish.dishTags;
import static demo.Dish.menu;

public class Grouping {

    enum CaloricLevel { DIET, NORMAL, FAT };

    public static void main(String ... args) {
        System.out.println("Dishes grouped by type: " + groupDishesByType());
     /*   System.out.println("Dish names grouped by type: " + groupDishNamesByType());
        System.out.println("Dish tags grouped by type: " + groupDishTagsByType());
        System.out.println("Caloric dishes grouped by type: " + groupCaloricDishesByType());
        System.out.println("Dishes grouped by caloric level: " + groupDishesByCaloricLevel());
        System.out.println("Dishes grouped by type and caloric level: " + groupDishedByTypeAndCaloricLevel());
        System.out.println("Count dishes in groups: " + countDishesInGroups());
        System.out.println("Most caloric dishes by type: " + mostCaloricDishesByType());
        System.out.println("Most caloric dishes by type: " + mostCaloricDishesByTypeWithoutOprionals());
        System.out.println("Sum calories by type: " + sumCaloriesByType());
        System.out.println("Caloric levels by type: " + caloricLevelsByType());*/
       
    }
 

	private static Map<Dish.Type, List<Dish>> groupDishesByType() {
        return menu.stream().collect(groupingBy(Dish::getType));
    }

    private static Map<Dish.Type, List<String>> groupDishNamesByType() {
        return menu.stream().collect(groupingBy(Dish::getType, mapping(Dish::getName, toList())));
    }

    private static Map<Dish.Type, Set<String>> groupDishTagsByType() {
        return menu.stream().collect(groupingBy(Dish::getType, flatMapping(dish -> dishTags.get( dish.getName() ).stream(), toSet())));
    }

    private static Map<Dish.Type, List<Dish>> groupCaloricDishesByType() {
//        return menu.stream().filter(dish -> dish.getCalories() > 500).collect(groupingBy(Dish::getType));
        return menu.stream().collect(groupingBy(Dish::getType, filtering(dish -> dish.getCalories() > 500, toList())));
    }

    private static Map<CaloricLevel, List<Dish>> groupDishesByCaloricLevel() {
        return menu.stream().collect(
                groupingBy(dish -> {
                    if (dish.getCalories() <= 400) return CaloricLevel.DIET;
                    else if (dish.getCalories() <= 700) return CaloricLevel.NORMAL;
                    else return CaloricLevel.FAT;
                } ));
    }

    private static Map<Dish.Type, Map<CaloricLevel, List<Dish>>> groupDishedByTypeAndCaloricLevel() {
        return menu.stream().collect(
                groupingBy(Dish::getType,
                        groupingBy((Dish dish) -> {
                            if (dish.getCalories() <= 400) return CaloricLevel.DIET;
                            else if (dish.getCalories() <= 700) return CaloricLevel.NORMAL;
                            else return CaloricLevel.FAT;
                        } )
                )
        );
    }

    private static Map<Dish.Type, Long> countDishesInGroups() {
        return menu.stream().collect(groupingBy(Dish::getType, counting()));
    }

    private static Map<Dish.Type, Optional<Dish>> mostCaloricDishesByType() {
        return menu.stream().collect(
                groupingBy(Dish::getType,
                        reducing((Dish d1, Dish d2) -> d1.getCalories() > d2.getCalories() ? d1 : d2)));
    }

    private static Map<Dish.Type, Dish> mostCaloricDishesByTypeWithoutOprionals() {
        return menu.stream().collect(
                groupingBy(Dish::getType,
                        collectingAndThen(
                                reducing((d1, d2) -> d1.getCalories() > d2.getCalories() ? d1 : d2),
                                Optional::get)));
    }

    private static Map<Dish.Type, Integer> sumCaloriesByType() {
        return menu.stream().collect(groupingBy(Dish::getType,
                summingInt(Dish::getCalories)));
    }

    private static Map<Dish.Type, Set<CaloricLevel>> caloricLevelsByType() {
        return menu.stream().collect(
                groupingBy(Dish::getType, mapping(
                        dish -> { if (dish.getCalories() <= 400) return CaloricLevel.DIET;
                        else if (dish.getCalories() <= 700) return CaloricLevel.NORMAL;
                        else return CaloricLevel.FAT; },
                        toSet() )));
    }
}

上面是流的分组测试

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