按子组收集数据

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,Long> typesCount =
                menu.stream().collect(Collectors.groupingBy(Dish::getType,Collectors.counting()));
        System.out.println(typesCount);
    }
}

package com.ant.jdk8.chap06;

import java.util.*;
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, Optional<Dish>> mostCaloriesByType =
                menu.stream().collect(Collectors.groupingBy(Dish::getType,
                        Collectors.maxBy(Comparator.comparingInt(Dish::getCalories))));
        System.out.println(mostCaloriesByType);
    }
}

1. 把收集器的结果转换为另一种类型：

package com.ant.jdk8.chap06;

import java.util.*;
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, Dish> mostCaloriesByType =
                menu.stream().collect(Collectors.groupingBy(Dish::getType,
                        Collectors.collectingAndThen(
                                Collectors.maxBy(Comparator.comparingInt(Dish::getCalories)),Optional::get)));
        System.out.println(mostCaloriesByType);
    }
}

2. 与groupingBy联合使用的其他收集器的例子：

package com.ant.jdk8.chap06;

import java.util.*;
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, Integer> totalCaloriesByType =
                menu.stream().collect(Collectors.groupingBy(Dish::getType,
                        Collectors.summingInt(Dish::getCalories)));
        System.out.println(totalCaloriesByType);
    }
}

package com.ant.jdk8.chap06;

import java.util.*;
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, Set<CaloricLevel>> caloricLevelByType =
                menu.stream().collect(Collectors.groupingBy(Dish::getType,
                        Collectors.mapping(d->{if(d.getCalories()<=400) return CaloricLevel.DIET;
                        else if(d.getCalories()<=700) return CaloricLevel.NORMAL;
                        else return CaloricLevel.FAT;},
                                Collectors.toSet())));
        System.out.println(caloricLevelByType);
    }
}