guava/src/com/google/common/primitives/Ints.java
public static int[] toArray(Collection<? extends Number> collection) {
    if (collection instanceof IntArrayAsList) {
      return ((IntArrayAsList) collection).toIntArray();
    }

    Object[] boxedArray = collection.toArray();
    int len = boxedArray.length;
    int[] array = new int[len];
    for (int i = 0; i < len; i++) {
      // checkNotNull for GWT (do not optimize)
      array[i] = ((Number) checkNotNull(boxedArray[i])).intValue();
    }
    return array;
  }
public static int[] toArray(Collection<? extends Number> collection) {
    if (collection instanceof IntArrayAsList) {
      return ((IntArrayAsList) collection).toIntArray();
    }

    Object[] boxedArray = collection.toArray();
    int len = boxedArray.length;
    int[] array = new int[len];
    for (int i = 0; i < len; i++) {
      // checkNotNull for GWT (do not optimize)
      array[i] = ((Number) checkNotNull(boxedArray[i])).intValue();
    }
    return array;
  }
Returns an array containing each value of {@code collection}, converted to a {@code int} value
in the manner of {@link Number#intValue}.

<p>Elements are copied from the argument collection as if by {@code collection.toArray()}.
Calling this method is as thread-safe as calling that method.

@param collection a collection of {@code Number} instances
@return an array containing the same values as {@code collection}, in the same order, converted
to primitives
@throws NullPointerException if {@code collection} or any of its elements is null
@since 1.0 (parameter was {@code Collection<Integer>} before 12.0)