java - 使泛型函数适用于多个非重叠类型

Question

我正在尝试编写一个函数，该函数从集合中随机抽取元素并将它们添加到新集合中。所以如果你想从 {1,2,3,4,5} 中提取 3 个元素，你可以得到 {5,3,4}。我想出了这个通用函数:

/**
 * Take a random sample without repeats of the specified size from a
 * collection. Note that if the collection you're sampling from contains
 * repeated elements, then the sample could also contain repeated elements.
 * Use a Set as an argument to avoid this.
 *
 * @param <T> The type of objects in the Collection
 * @param <E> The type of the collection
 * @param collection The collection
 * @param size The sample size of elements which you wish to extract from
 * the collection
 * @param factory A factory method for the collection E. Call with
 * "new::ArrayList" or something similar.
 * @return A random sample of the collection consisting of 'size' elements
 * without repeats (unless the original collection contained repeats).
 * @throws IllegalArgumentException if size is larger than the collection.size().
 */
public static <T, E extends Collection<T>> E drawRandomlyWithoutReplacement(List<T> collection, int size, Supplier<E> factory) {
    if (size > collection.size()) {
        throw new IllegalArgumentException("The sample size cannot be greater than the size of the collection.");
    }

    E list = factory.get();
    for (int i = 0; i < size; i++) {
        int r = MathUtils.randomInt(0, collection.size() - 1);
        list.add(collection.remove(r));
    }
    return list;
}

不幸的是，Collection 接口(interface)没有一个函数可以在您删除它时从集合中返回一个元素，但是 List 和 Vector(以及其他)有它。有没有一种方法可以让这个函数适用于列表和 vector ，而不必重载它 3 次？我尝试将第一个参数设为 C 类型其中 C extends List<T> | Vector<T>但不幸的是，这没有用。

Answer 1

Collection 接口(interface)不能删除，但它的Iterator。

public static <T, E extends Collection<T>> E drawRandomlyWithoutReplacement(Collection<T> collection, int size,
    Supplier<E> factory) {
  final int colSize = collection.size();
  if (size > colSize) {
    throw new IllegalArgumentException("The sample size cannot be greater than the size of the collection.");
  } else if(size == 0) {
    return factory.get();
  }

  Random rand = new Random();
  Set<Integer> sampleIndices = new TreeSet<>();
  while (sampleIndices.size() < size) {
    sampleIndices.add(rand.nextInt(colSize));
  }

  E result = factory.get();

  Iterator<T> collectionIterator = collection.iterator();
  Iterator<Integer> indexIterator = sampleIndices.iterator();
  int sampleIndex = indexIterator.next();
  for (int i = 0; i < colSize; i++) {
    T sample = collectionIterator.next();
    if (i == sampleIndex) {
      result.add(sample);
      collectionIterator.remove();
      if (indexIterator.hasNext()) {
        sampleIndex = indexIterator.next();
      } else {
        break;
      }
    }
  }
  return result;
}