Java 集合 pack() 方法

Question

使用 Java 流实现集合的 pack() 操作的好方法是什么？这是我想要完成的:

List<String> items = Arrays.asList("A", "A", "B", "B", "A", "C", "C", "A", "A", "A");
List<List<String>> packs = items.stream().pack();
// packs:  [[A,A],[B,B],[A],[C,C],[A,A,A]]

或者，打包操作可以返回 (index, element, count) 形式的元组列表:

[(0, A, 2), (2, B, 2), (4, A, 1), (5, C, 2), (7, A, 3)]

目前，我通过以下方式使用可变累加器实现了这一点:

Packer<String> packer = new Packer<>();
items.stream().forEach(packer);
List<Triple<Integer, T, Integer>> packs = packer.get();

public class Packer<T> implements Consumer<T>, Supplier<List<Triple<Integer, T, Integer>>>
{
    private List<Triple<Integer, T, AtomicInteger>> result = new ArrayList<>();
    private Optional<Triple<Integer, T, AtomicInteger>> currentElement = Optional.empty();
    private int count = 0;

    @Override
    public void accept(T t)
    {
        if (currentElement.isPresent() && currentElement.get().getMiddle().equals(t))
        {
            currentElement.get().getRight().incrementAndGet();
        }
        else
        {
            currentElement = Optional.of(Triple.of(count, t, new AtomicInteger(1)));
            result.add(currentElement.get());
        }
        count++;
    }

    @Override
    public List<Triple<Integer, T, Integer>> get()
    {
        return result.stream().map(x -> Triple.of(x.getLeft(), x.getMiddle(), x.getRight().get())).collect(Collectors.toList());
    }

}

Answer 1

您可以使用 StreamEx 折叠相邻元素:

List<List<String>> packs = StreamEx.of(items)
        .collapse(Object::equals, Collectors.toList())
        .collect(Collectors.toList());

输出:

[[A, A], [B, B], [A], [C, C], [A, A, A]]

JDoodle Demo

我不确定 StreamEx 是否支持索引折叠。