JAVA8-Collectors API: partitioningBy、reducing、summarizingLong、summarizingInt、summarizingDouble JAVA8-Collectors API: partitioningBy、reducing、summarizingLong、summarizingInt、summarizingDouble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、partitioningBy 分组 public static void testPartitioningByWithPredicate() { System.out.println("testPartitioningByWithPredicate"); Map<Boolean, List<Dish>> collect = menu.stream().collect(Collectors.partitioningBy(Dish::isVegetarian)); Optional.ofNullable(collect).ifPresent(System.out::println); }输出结果：testPartitioningByWithPredicate {false=[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='prawns', vegetarian=false, calories=300, type=FISH}, Dish{name='salmon', vegetarian=false, calories=450, type=FISH}], true=[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}]}2、partitioningBy 分组后，再处理求平均数 public static void testPartitioningByWithPredicateAndCollecotr() { System.out.println("testPartitioningByWithPredicateAndCollecotr"); Map<Boolean, Double> collect = menu.stream().collect(Collectors.partitioningBy(Dish::isVegetarian, Collectors.averagingInt(Dish::getCalories))); Optional.ofNullable(collect).ifPresent(System.out::println); }输出结果：testPartitioningByWithPredicateAndCollecotr {false=530.0, true=387.5}3、reducing public static void testReducingBinaryOperator() { System.out.println("testReducingBinaryOperator"); Optional<Dish> collect = menu.stream().collect(Collectors.reducing(BinaryOperator.maxBy(Comparator.comparingInt(Dish::getCalories)))); collect.ifPresent(System.out::println); }输出结果：testReducingBinaryOperator Dish{name='pork', vegetarian=false, calories=800, type=MEAT}4、reducing public static void testReducingBinaryOperatorAndIdentity() { System.out.println("testReducingBinaryOperatorAndIdentity"); Integer collect = menu.stream().map(Dish::getCalories).collect(Collectors.reducing(0, (d1, d2) -> d1 + d2)); System.out.println(collect); }输出结果：testReducingBinaryOperatorAndIdentity 42005、reducing public static void testReducingBinaryOperatorAndIdentityAndFunction() { System.out.println("testReducingBinaryOperatorAndIdentityAndFunction"); Integer collect = menu.stream().collect(Collectors.reducing(0, Dish::getCalories,(d1, d2) -> d1 + d2)); System.out.println(collect); }输出结果：testReducingBinaryOperatorAndIdentityAndFunction 42006、summarizingDouble public static void testSummarizingDouble() { System.out.println("testSummarizingDouble"); DoubleSummaryStatistics collect = menu.stream().collect(Collectors.summarizingDouble(Dish::getCalories)); Optional.ofNullable(collect).ifPresent(System.out::println); }输出结果：testSummarizingDouble DoubleSummaryStatistics{count=9, sum=4200.000000, min=120.000000, average=466.666667, max=800.000000}7、summarizingInt public static void testSummarizingInt() { System.out.println("testSummarizingDouble"); IntSummaryStatistics collect = menu.stream().collect(Collectors.summarizingInt(Dish::getCalories)); Optional.ofNullable(collect).ifPresent(System.out::println); }输出结果：testSummarizingDouble IntSummaryStatistics{count=9, sum=4200, min=120, average=466.666667, max=800}8、summarizingLong public static void testSummarizingLong() { System.out.println("testSummarizingLong"); LongSummaryStatistics collect = menu.stream().collect(Collectors.summarizingLong(Dish::getCalories)); Optional.ofNullable(collect).ifPresent(System.out::println); }输出结果：testSummarizingLong LongSummaryStatistics{count=9, sum=4200, min=120, average=466.666667, max=800}