JAVA8-Collectors API:summingDouble、summingInt、testSummingLong、toCollection、toConcurrentMap、toList、toSet、toMap JAVA8-Collectors API:summingDouble、summingInt、testSummingLong、toCollection、toConcurrentMap、toList、toSet、toMap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、summingDouble 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：testSummingDouble 4200.0 42002、summingDouble public static void testSummingInt() { System.out.println("testSummingInt"); Optional.ofNullable(menu.stream().collect(Collectors.summingInt(Dish::getCalories))).ifPresent(System.out::println); }输出结果：testSummingInt 42003、summingLong public static void testSummingLong() { System.out.println("testSummingLong"); Optional.ofNullable(menu.stream().collect(Collectors.summingLong(Dish::getCalories))).ifPresent(System.out::println); }输出结果：testSummingLong 42004、toCollection 返回Collection子类 public static void testToCollection() { System.out.println("testToCollection"); Optional.ofNullable(menu.stream().collect(Collectors.toCollection(LinkedList::new))).ifPresent(System.out::println); }输出结果：testToCollection [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='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}, Dish{name='prawns', vegetarian=false, calories=300, type=FISH}, Dish{name='salmon', vegetarian=false, calories=450, type=FISH}]5、toConcurrentMap public static void testToConcurrentMap() { System.out.println("testToConcurrentMap"); Optional.ofNullable(menu.stream().collect(Collectors.toConcurrentMap(Dish::getName,Dish::getCalories))).ifPresent(System.out::println); }输出结果：testToConcurrentMap {season fruit=120, chicken=400, pizza=550, salmon=450, beef=700, pork=800, rice=350, french fries=530, prawns=300}6、toConcurrentMap 按类型统计个数 public static void testToConcurrentMapWithBinaryOperator() { System.out.println("testToConcurrentMapWithBinaryOperator"); Optional.ofNullable(menu.stream().collect(Collectors.toConcurrentMap(Dish::getType,v->1L,(a,b)->a+b))).ifPresent(v->{ System.out.println(v); System.out.println(v.getClass()); }); }输出结果：testToConcurrentMapWithBinaryOperator {OTHER=4, MEAT=3, FISH=2} class java.util.concurrent.ConcurrentHashMap7、toConcurrentMap 按类型统计个数，返回指定类型Map public static void testToConcurrentMapWithBinaryOperatorAndSupplier() { System.out.println("testToConcurrentMapWithBinaryOperatorAndSupplier"); Optional.ofNullable(menu.stream().collect(Collectors.toConcurrentMap(Dish::getType,v->1L,(a,b)->a+b, ConcurrentSkipListMap::new))).ifPresent(v->{ System.out.println(v); System.out.println(v.getClass()); }); }输出结果：testToConcurrentMapWithBinaryOperatorAndSupplier {MEAT=3, FISH=2, OTHER=4} class java.util.concurrent.ConcurrentSkipListMap8、toList 转成list public static void testTolist() { System.out.println("testTolist"); Optional.ofNullable(menu.stream().filter(t->t.isVegetarian()).collect(Collectors.toList())).ifPresent(v->{ System.out.println(v); System.out.println(v.getClass()); }); }输出结果：testTolist [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}] class java.util.ArrayList9、toSett 转成Set public static void testToSet() { System.out.println("testToSet"); Optional.ofNullable(menu.stream().filter(t->t.isVegetarian()).collect(Collectors.toSet())).ifPresent(v->{ System.out.println(v); System.out.println(v.getClass()); }); }输出结果：testToSet [Dish{name='french fries', vegetarian=true, calories=530, type=OTHER}, Dish{name='season fruit', vegetarian=true, calories=120, type=OTHER}, Dish{name='pizza', vegetarian=true, calories=550, type=OTHER}, Dish{name='rice', vegetarian=true, calories=350, type=OTHER}] class java.util.HashSet10、toMappublic static void testToMap() { System.out.println("testToMap"); Optional.ofNullable(menu.stream().collect(Collectors.toMap(Dish::getName,Dish::getCalories))).ifPresent(v->{ System.out.println(v); System.out.println(v.getClass()); }); //转成线程安全的map Optional.ofNullable(menu.stream().collect(Collectors.collectingAndThen(Collectors.toMap(Dish::getName, Dish::getCalories), Collections::synchronizedMap))).ifPresent(v->{ System.out.println(v); System.out.println(v.getClass()); }); }输出结果：testToMap {season fruit=120, chicken=400, pizza=550, salmon=450, beef=700, pork=800, rice=350, french fries=530, prawns=300} class java.util.HashMap {season fruit=120, chicken=400, pizza=550, salmon=450, beef=700, pork=800, rice=350, french fries=530, prawns=300} class java.util.Collections$SynchronizedMap11、toMap 按类型统计个数public static void testToMapWithBinaryOperator() { System.out.println("testToMapWithBinaryOperator"); Optional.ofNullable(menu.stream().collect(Collectors.toMap(Dish::getType,v->1L,(a,b)->a+b))).ifPresent(v->{ System.out.println(v); System.out.println(v.getClass()); }); }输出结果：testToMapWithBinaryOperator {MEAT=3, FISH=2, OTHER=4} class java.util.HashMap12、toMap 按类型统计个数，返回指定类型Ma ##p public static void testToMapWithBinaryOperatorAndSupplier() { System.out.println("testToMapWithBinaryOperatorAndSupplier"); Optional.ofNullable(menu.stream().collect(Collectors.toMap(Dish::getType,v->1L,(a,b)->a+b, ConcurrentSkipListMap::new))).ifPresent(v->{ System.out.println(v); System.out.println(v.getClass()); }); }输出结果：testToMapWithBinaryOperatorAndSupplier {MEAT=3, FISH=2, OTHER=4} class java.util.concurrent.ConcurrentSkipListMap