对于我的示例,拥有汽车对象并根据模型(分组依据)找到最小和最大价格值.
For my example, having car object and found that min and max price value based on model (group by).
List<Car> carsDetails = UserDB.getCarsDetails(); Map<String, DoubleSummaryStatistics> collect4 = carsDetails.stream() .collect(Collectors.groupingBy(Car::getMake, Collectors.summarizingDouble(Car::getPrice))); collect4.entrySet().forEach(e->System.out.println(e.getKey()+" "+e.getValue().getMax()+" "+e.getValue().getMin())); output : Lexus 94837.79 17569.59 Subaru 96583.25 8498.41 Chevrolet 99892.59 6861.85但我找不到哪些汽车对象具有最高和最低价格.我该怎么做?
But I couldn't find which car objects have max and min price. How can I do that?
推荐答案如果您只对每组一辆 Car 感兴趣,您可以使用,例如
If you were interested in only one Car per group, you could use, e.g.
Map<String, Car> mostExpensives = carsDetails.stream() .collect(Collectors.toMap(Car::getMake, Function.identity(), BinaryOperator.maxBy(Comparatorparing(Car::getPrice)))); mostExpensives.forEach((make,car) -> System.out.println(make+" "+car));但既然你想要最贵和最便宜的,你需要这样的东西:
But since you want the most expensive and the cheapest, you need something like this:
Map<String, List<Car>> mostExpensivesAndCheapest = carsDetails.stream() .collect(Collectors.toMap(Car::getMake, car -> Arrays.asList(car, car), (l1,l2) -> Arrays.asList( (l1.get(0).getPrice()>l2.get(0).getPrice()? l2: l1).get(0), (l1.get(1).getPrice()<l2.get(1).getPrice()? l2: l1).get(1)))); mostExpensivesAndCheapest.forEach((make,cars) -> System.out.println(make +" cheapest: "+cars.get(0)+" most expensive: "+cars.get(1)));由于没有等效于 DoubleSummaryStatistics 的通用统计对象,因此该解决方案存在一些不便.如果这种情况不止一次发生,那么值得用这样的类来填补空白:
This solution bears a bit of inconvenience due to the fact that there is no generic statistics object equivalent to DoubleSummaryStatistics. If this happens more than once, it’s worth filling the gap with a class like this:
/** * Like {@code DoubleSummaryStatistics}, {@code IntSummaryStatistics}, and * {@code LongSummaryStatistics}, but for an arbitrary type {@code T}. */ public class SummaryStatistics<T> implements Consumer<T> { /** * Collect to a {@code SummaryStatistics} for natural order. */ public static <T extends Comparable<? super T>> Collector<T,?,SummaryStatistics<T>> statistics() { return statistics(Comparator.<T>naturalOrder()); } /** * Collect to a {@code SummaryStatistics} using the specified comparator. */ public static <T> Collector<T,?,SummaryStatistics<T>> statistics(Comparator<T> comparator) { Objects.requireNonNull(comparator); return Collector.of(() -> new SummaryStatistics<>(comparator), SummaryStatistics::accept, SummaryStatistics::merge); } private final Comparator<T> c; private T min, max; private long count; public SummaryStatistics(Comparator<T> comparator) { c = Objects.requireNonNull(comparator); } public void accept(T t) { if(count == 0) { count = 1; min = t; max = t; } else { if(cpare(min, t) > 0) min = t; if(cpare(max, t) < 0) max = t; count++; } } public SummaryStatistics<T> merge(SummaryStatistics<T> s) { if(s.count > 0) { if(count == 0) { count = s.count; min = s.min; max = s.max; } else { if(cpare(min, s.min) > 0) min = s.min; if(cpare(max, s.max) < 0) max = s.max; count += s.count; } } return this; } public long getCount() { return count; } public T getMin() { return min; } public T getMax() { return max; } @Override public String toString() { return count == 0? "empty": (count+" elements between "+min+" and "+max); } }将其添加到您的代码库后,您可以像这样使用它
After adding this to your code base, you may use it like
Map<String, SummaryStatistics<Car>> mostExpensives = carsDetails.stream() .collect(Collectors.groupingBy(Car::getMake, SummaryStatistics.statistics(Comparatorparing(Car::getPrice)))); mostExpensives.forEach((make,cars) -> System.out.println(make+": "+cars));如果 getPrice 返回 double,则使用 ComparatorparingDouble(Car::getPrice) 而不是 可能更有效Comparatorparing(Car::getPrice).
If getPrice returns double, it may be more efficient to use ComparatorparingDouble(Car::getPrice) instead of Comparatorparing(Car::getPrice).
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