java - 如何对对象集合进行排序

Question

比如我有课

class Person{
    Integer rank;
    Double profit;
    Person(Integer rank, Double profit){
        this.rank = rank;
        this.profit = profit;
    }
    Person(Double profit){
        this(0, profit);
    }
}

我想对按排名评估的盈利人员排序的列表进行排名。这样

rank(Arrays.asList(
    new Person(30), 
    new Person(20), 
    new Person(20), 
    new Person(10))
)

会生成列表

new Person(1, 30), 
new Person(2, 20), 
new Person(2, 20), 
new Person(3, 10)

我还想使用 Java 8 中的自定义 Collector(或类似的东西)来完成它，而不是使用简单的循环。

Answer 1

让我们使用一个自定义收集器，我们将所有来自外部的东西传递给它。收集器将从调用者那里获得排名和构造函数。我们想这样调用它:

public List<Person> rank(List<Person> people) {
        return people
                .stream()
                .sorted(Comparator.<Person>comparingDouble(x -> x.profit).reversed())
                .collect(new IntegerRankingCollector<>(
                        Comparator.comparingDouble(p -> p.profit),  // how to differentiate rankings
                        p -> p.rank,    // where to get rank for an element which was already ranked
                        (p, rank) -> new Person(rank, p.profit)     // how to create an element from another element values and a rank
                ));
    }

这个收集器可以像Collectors.toList()一样实现但是使用累加器方法:

1. 获取前一个元素的排名
2. 如果当前元素等级应该不同于上一个元素等级
3. 创建一个具有新等级的元素

这是它的外观，它应该适用于有序流:

public class IntegerRankingCollector<T> implements Collector<T, List<T>, List<T>> {
        ...

    public IntegerRankingCollector(Comparator<? super T> comparator, Function<T, Integer> ranker, BiFunction<T, Integer, T> creator) {
        this.comparator = comparator;
        this.ranker = ranker;
        this.creator = creator;
    }

    @Override
    public BiConsumer<List<T>, T> accumulator() {
        return (list, current) -> {
            ArrayList<T> right = new ArrayList<>();
            right.add(creator.apply(current, 1));
            combiner().apply(list, right);
        };
    }

    @Override
    public BinaryOperator<List<T>> combiner() {
        return (left, right) -> {
            int rankAdjustment = getRankAdjustment(left, right);
            for (T t : right)
                left.add(creator.apply(t, rankAdjustment + ranker.apply(t)));
            return left;
        };
    }

    private int getRankAdjustment(List<T> left, List<T> right) {
        Optional<T> lastElementOnTheLeft = optGet(left, left.size() - 1);
        Optional<T> firstElementOnTheRight = optGet(right, 0);

        if (!lastElementOnTheLeft.isPresent() || !firstElementOnTheRight.isPresent())
            return 0;
        else if (comparator.compare(firstElementOnTheRight.get(), lastElementOnTheLeft.get()) == 0)
            return ranker.apply(lastElementOnTheLeft.get()) - 1;
        else
            return ranker.apply(lastElementOnTheLeft.get());
    }

    private Optional<T> optGet(List<T> list, int index) {
        if (list == null || list.isEmpty())
            return Optional.empty();
        else
            return Optional.of(list.get(index));
    }

        ...
    }

为完整起见，这是该类的完整代码。我从 Collectors.toList 复制了其余部分:

public class IntegerRankingCollector<T> implements Collector<T, List<T>, List<T>> {

    private static final Set<Characteristics> CHARACTERISTICSS = Collections.unmodifiableSet(EnumSet.of(Characteristics.IDENTITY_FINISH));
    private Comparator<? super T> comparator;
    private BiFunction<T, Integer, T> creator;
    private Function<T, Integer> ranker;

    public IntegerRankingCollector(Comparator<? super T> comparator, Function<T, Integer> ranker, BiFunction<T, Integer, T> creator) {
        this.comparator = comparator;
        this.ranker = ranker;
        this.creator = creator;
    }

    @Override
    public BiConsumer<List<T>, T> accumulator() {
        return (list, current) -> {
            ArrayList<T> right = new ArrayList<>();
            right.add(creator.apply(current, 1));
            combiner().apply(list, right);
        };
    }

    @Override
    public BinaryOperator<List<T>> combiner() {
        return (left, right) -> {
            int rankAdjustment = getRankAdjustment(left, right);
            for (T t : right)
                left.add(creator.apply(t, rankAdjustment + ranker.apply(t)));
            return left;
        };
    }

    private int getRankAdjustment(List<T> left, List<T> right) {
        Optional<T> lastElementOnTheLeft = optGet(left, left.size() - 1);
        Optional<T> firstElementOnTheRight = optGet(right, 0);

        if (!lastElementOnTheLeft.isPresent() || !firstElementOnTheRight.isPresent())
            return 0;
        else if (comparator.compare(firstElementOnTheRight.get(), lastElementOnTheLeft.get()) == 0)
            return ranker.apply(lastElementOnTheLeft.get()) - 1;
        else
            return ranker.apply(lastElementOnTheLeft.get());
    }

    private Optional<T> optGet(List<T> list, int index) {
        if (list == null || list.isEmpty())
            return Optional.empty();
        else
            return Optional.of(list.get(index));
    }


    @Override
    public Supplier<List<T>> supplier() {
        return ArrayList::new;
    }

    @Override
    public Function<List<T>, List<T>> finisher() {
        return l -> l;
    }

    @Override
    public Set<Characteristics> characteristics() {
        return CHARACTERISTICSS;
    }
}