主要API

• Collectors.maxBy
• Collectors.minBy
• 计数

long howManyDishes = menu.stream().collect(Collectors.counting());
long howManyDishes = menu.stream().count();

• Collectors.summingInt
• Collectors.summingLong
• Collectors.summingDouble
• 连接字符串

String shortMenu = menu.stream().map(Dish::getName).collect(joining());

• 分组

Map<Dish.Type, List<Dish>> dishesByType = menu.stream().collect(groupingBy(Dish::getType));
Map<Dish.Type, Long> typesCount = menu.stream().collect( groupingBy(Dish::getType, counting())); //分组汇总

• partitioningBy：分区