JAVA8-Collectors API:averaging、collectingAndThen、counting、groupingBy

前置数据

    public static final 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),
            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, 300, Dish.Type.FISH),
            new Dish("salmon", false, 450, Dish.Type.FISH));

范例：

1、averagingDouble 求平均值

    public static void testAveragingDouble(){
        System.out.println("testAveragingDouble");
        //用reduce聚合求和
        Optional.ofNullable(menu.stream().map(Dish::getCalories).reduce(Integer::sum)).get().ifPresent(System.out::println);
        //用collectors averagingDouble求平均值
        Optional.ofNullable(menu.stream().collect(averagingDouble(Dish::getCalories))).ifPresent(System.out::println);
    }

输出结果：

testAveragingDouble
4200
466.6666666666667

2、averagingInt 求平均值

    public static void testAveragingInt(){
        System.out.println("testAveragingInt");
        //用collectors averagingDouble求平均值
        Optional.ofNullable(menu.stream().collect(averagingInt(Dish::getCalories))).ifPresent(System.out::println);
    }

输出结果：

testAveragingInt
466.6666666666667

3、averagingLong 求平均值

    public static void testAveragingLong(){
        System.out.println("testAveragingLong");
        //用collectors averagingLong求平均值
        Optional.ofNullable(menu.stream().collect(averagingLong(Dish::getCalories))).ifPresent(System.out::println);
    }

输出结果：

testAveragingLong
466.6666666666667

4、collectingAndThen(收集数据，处理数据)，将搜集结果，再做处理。

4.1、CollectingAndThen 求平均数后，拼接一句话

        Optional.ofNullable(menu.stream().collect(Collectors.collectingAndThen(averagingInt(Dish::getCalories),a->"This is Calories eques = "+a))).ifPresent(System.out::println);

输出结果：

testCollectingAndThen
This is Calories eques = 466.6666666666667

4.2、获取MEAT类，之后再往里面加入其它类型

        List<Dish> collect = menu.stream().filter(m -> Dish.Type.MEAT.equals(m.getType())).collect(toList());
        collect.add(new Dish("回锅肉", true, 550, Dish.Type.OTHER));
        collect.stream().forEach(System.out::println);

输出结果：

Dish{name='pork', vegetarian=false, calories=800, type=MEAT}
Dish{name='beef', vegetarian=false, calories=700, type=MEAT}
Dish{name='chicken', vegetarian=false, calories=400, type=MEAT}
Dish{name='回锅肉', vegetarian=true, calories=550, type=OTHER}

4.3、CollectingAndThen 如果想将收集结果设置为不可修改

        List<Dish> meatCollect = menu.stream().filter(m -> Dish.Type.MEAT.equals(m.getType())).collect(collectingAndThen(toList(), Collections::unmodifiableList));
        //修改时就会报错：Exception in thread "main" java.lang.UnsupportedOperationException
        meatCollect.add(new Dish("回锅肉", true, 550, Dish.Type.OTHER));
        collect.stream().forEach(System.out::println);

输出结果：

Exception in thread "main" java.lang.UnsupportedOperationException

5、counting 统计 返回Long类型

    public static void testCounting(){
        System.out.println("testCounting");
        Optional.ofNullable(menu.stream().collect(Collectors.counting())).ifPresent(System.out::println);
    }

输出结果：

testCounting
9

6、groupingBy 分组

public static void testGroupingByFunction(){
        System.out.println("testGroupingByFunction");
        Optional.of(menu.stream().collect(Collectors.groupingBy(Dish::getType))).ifPresent(System.out::println);
    }

输出结果：

testGroupingByFunction
{OTHER=[Dish{name='french fries', vegetarian=true, calories=530, type=OTHER}, Dish{name='rice', vegetarian=true, calories=350, type=OTHER}, Dish{name='season fruit', vegetarian=true, calories=120, type=OTHER}, Dish{name='pizza', vegetarian=true, calories=550, type=OTHER}], MEAT=[Dish{name='pork', vegetarian=false, calories=800, type=MEAT}, Dish{name='beef', vegetarian=false, calories=700, type=MEAT}, Dish{name='chicken', vegetarian=false, calories=400, type=MEAT}], FISH=[Dish{name='prawns', vegetarian=false, calories=300, type=FISH}, Dish{name='salmon', vegetarian=false, calories=450, type=FISH}]}

7、groupingBy 分组后统计

    public static void testGroupingByFunctionAndCollector(){
        System.out.println("testGroupingByFunctionAndCollector");
        Optional.of(menu.stream().collect(Collectors.groupingBy(Dish::getType,counting()))).ifPresent(System.out::println);
    }

输出结果：

testGroupingByFunctionAndCollector
{OTHER=4, MEAT=3, FISH=2}

8、groupingBy 分组后求平均值，默认是HashMap,通过groupingBy修改返回类型

    public static void testGroupingByFunctionAndSuppilerAndCollector(){
        System.out.println("testGroupingByFunctionAndSuppilerAndCollector");
        Map<Dish.Type, Double> map = Optional.of(menu.stream().collect(groupingBy(Dish::getType, averagingInt(Dish::getCalories)))).get();
        //默认是返回HashMap类型
        Optional.ofNullable(map.getClass()).ifPresent(System.out::println);

        TreeMap<Dish.Type, Double> newMap = Optional.of(menu.stream().collect(groupingBy(Dish::getType, TreeMap::new, averagingInt(Dish::getCalories)))).get();
        //groupingBy:修改返回类型为TreeMap
        Optional.ofNullable(newMap.getClass()).ifPresent(System.out::println);
    }

输出结果：

testGroupingByFunctionAndSuppilerAndCollector
class java.util.HashMap
class java.util.TreeMap