Collector与Collectors之间的关系

作为collect方法的参数，Collector是一个接口，它是一个可变的汇聚操作，将输入元素累计到一个可变的结果容器中；它会在所有元素都处理完毕后，将累积的结果转换为一个最终的表示（这是一个可选操作）；

Collectors本身提供了关于Collector的常见汇聚实现，Collectors的内部类CollectorImpl实现了Collector接口，Collectors本身实际上是一个工厂。

Collectors提供哪些方法

public class CollectorsDemo {

public static void main(String[] args) {

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));

//maxby  test  筛选出最大的元素
Comparator<Dish> dishCaloriesComparator =
comparingInt(Dish::getCalories);
menu.stream().collect(maxBy(dishCaloriesComparator))
.ifPresent(a -> System.out.println("max calories --->" + a.getCalories()));

//groupingby test  将元素的某个属性提取出来进行分组
Map<Dish.Type, List<Dish>> map = menu.stream().collect(groupingBy(Dish::getType));
for (Map.Entry<Dish.Type, List<Dish>> entry : map.entrySet()) {
System.out.println(entry.getKey() + "---" + entry.getValue().size());
}

//summingInt test  计算所有元素的和
int totalCalories = menu.stream().collect(summingInt(Dish::getCalories));

//averagingInt test  计算所有元素的平均值
double averageCalories = menu.stream().collect(averagingInt(Dish::getCalories));

//summarizingInt test  一次获取多个值, count, sum, min, max, average
IntSummaryStatistics menuStatistics =
menu.stream().collect(summarizingInt(Dish::getCalories));
System.out.println(menuStatistics.toString());

//join  test  joining(",") 可以指定连接符，避免拼接字符串，还要对最后一个元素进行特殊处理
// 如：每个元素拼接 element+"," 最后去掉最后一个"，"号
String shortMenu = menu.stream().map(Dish::getName).collect(joining(","));
System.out.println("join test --->" + shortMenu);

//reducing 实现
menu.stream().collect(Collectors.reducing(
0,               //初始值
Dish::getCalories,      //转换函数
(a, b) -> a + b));           //累计函数
menu.stream().collect(Collectors.reducing(0, Dish::getCalories, Integer::sum));

//获取热量最高的值，默认值有可能为空所以返回Optional，
menu.stream().collect(Collectors.reducing(
(d1, d2) -> d1.getCalories() > d2.getCalories() ? d1 : d2))
.ifPresent(a -> System.out.println("max calories---" + a.getCalories()));

// 根据需要修改key，修改lambda表达式里面的逻辑
Map<CaloricLevel, List<Dish>> dishesByCaloricLevel = menu.stream().collect(
groupingBy(dish -> {
return getLevel(dish);
}));

//二级分组
Map<Dish.Type, Map<CaloricLevel, List<Dish>>> dishesByTypeCaloricLevel =
menu.stream().collect(
groupingBy(Dish::getType,
groupingBy(dish -> {
return getLevel(dish);
})
)
);

//获取每种类型中热量最高的dish
Map<Dish.Type, Optional<Dish>> mostCaloricByType =
menu.stream()
.collect(groupingBy(Dish::getType,
maxBy(comparingInt(Dish::getCalories))));

//将optional直接转换要用collectingAndThen
Map<Dish.Type, Dish> mostCaloricDish =
menu.stream()
.collect(groupingBy(Dish::getType,
Collectors.collectingAndThen(
maxBy(comparingInt(Dish::getCalories)),
Optional::get)));

//格式化value的方法
Map<Dish.Type, Set<CaloricLevel>> caloricLevelsByType =
menu.stream().collect(
groupingBy(Dish::getType, Collectors.mapping(
dish -> {
return getLevel(dish);
},
Collectors.toSet())));
//分区partitioningBy test,传入谓词
Map<Boolean, List<Dish>> partitionedMenu =
menu.stream().collect(Collectors.partitioningBy(Dish::isVegetarian));

//只返回符合条件的
List<Dish> vegetarianDishes =
menu.stream().filter(Dish::isVegetarian).collect(Collectors.toList());
System.out.println(vegetarianDishes.size());

}

public enum CaloricLevel {DIET, NORMAL, FAT}

public static CaloricLevel getLevel(Dish dish){
if (dish.getCalories() <= 400) return CaloricLevel.DIET;
else if (dish.getCalories() <= 700) return CaloricLevel.NORMAL;
else return CaloricLevel.FAT;
}
}