使用 Java8 收集器收集索引

Question

我有一个形状流

Stream<Shape> shapes = Arrays.asList(TRIANGLE, CIRCLE, SQUARE, SQUARE, CIRCLE, TRIANGLE, TRIANGLE).stream();

Shape 在哪里

public enum Shape {TRIANGLE, CIRCLE, SQUARE}

如何编写将形状流分组为 Map<Shape, List<Integer>> 这样按形状收集索引？

public Map<Shape, List<Integer>> indexedPartition(Stream<Shape> shapes) {
//code here
}

在函数 indexedPartition 的当前示例输出中看起来像

TRIANGE -> {0, 5, 6} 
CIRCLE -> {1, 4} 
SQUARE -> {2, 3}

在 Scala 中我会做类似的事情

val indices = Stream.from(0)

object Shape extends Enumeration {
  type Shape = Value
  val CIRCLE, TRIANGLE, SQUARE = Value
}

val shapes = Stream(Shape.TRIANGLE, Shape.CIRCLE, Shape.SQUARE, Shape.SQUARE, Shape.CIRCLE, Shape.TRIANGLE, Shape.TRIANGLE)

(shapes zip indices).groupBy{ case (s, i) => s }.mapValues(l => l.map(_._2))
//res0: scala.collection.immutable.Map[Shape.Value,List[Int]] = Map(SQUARE -> List(2, 3), TRIANGLE -> List(0, 5, 6), CIRCLE -> List(1, 4))

我尝试在 java 中使用 Collectors.groupingBy，但我无法理解它。

Answer 1

通过索引流式传输：

import static java.util.stream.Collectors.groupingBy;


List<Shape> shapes = Arrays.asList(TRIANGLE, CIRCLE, SQUARE, SQUARE, CIRCLE, TRIANGLE, TRIANGLE);

result = IntStream.range(0, shapes.size())
        .boxed()
        .collect(groupingBy(shapes::get));

Answer 2

可以实现Collector，它记录在流处理结束时遇到的元素的索引。当然，这个数字对于无序流没有意义，如果你有改变大小的中间操作，比如filter 或flatMap，这个数字将不再反映原始数组中的位置。

class IndexCollector<K> {
    int total;
    Map<K,List<Integer>> map = new HashMap<>();

    public static <T> Collector<T,?,Map<T,List<Integer>>> get() {
        return Collector.of(IndexCollector<T>::new,
            (c,t) -> c.map.computeIfAbsent(t, x -> new ArrayList<>()).add(c.total++),
            (c1,c2) -> merge(c1, c2),
            c -> c.map);
    }
    static <T> IndexCollector<T> merge(IndexCollector<T> a, IndexCollector<T> b) {
        if(a.total == 0) return b;
        if(b.total != 0) {
            int offset = a.total;
            b.map.forEach((t,l) -> {
                List<Integer> target = a.map.computeIfAbsent(t, x -> new ArrayList<>());
                for(Integer i: l) target.add(i+offset);
            });
            a.total += b.total;
        }
        return a;
    }
}

可以像这样使用

Map<Shape, List<Integer>> map =
    Stream.of(TRIANGLE, CIRCLE, SQUARE, SQUARE, CIRCLE, TRIANGLE, TRIANGLE)
          .collect(IndexCollector.get());

map.forEach((shape,list) -> System.out.printf("%-9s%s%n", shape, list));