Why is BitSet not Iterable?

None of the methods in Iterable (foreach, iterator, and spliterator) is provided in BitSet. There is no stream() method in Iterable.

Furthermore the stream() method of BitSet does not return a stream over the bits of the bit set, but returns a stream over the indices of the bits whose values are set (which is kind of confusing TBH). Therefore, technically speaking there seems to be almost nothing in common with Iterable.

One reason (not the entire reason, maybe) is that Iterable would inefficient, because the bit indexes have to be boxed (*); the stream is able to use primitive ints.

There's an efficient way to iterate the bitset without using Iterable, as described in the Javadoc, so it's not really necessary.

---

(*) However, for bitsets with size 128 or smaller, boxing would be cheap, as cached boxed instances would be used.

BitSet is not a "true" member of the java collection framework, so technically, no need to implement Collection.iterator() and provide one.

public class BitSet implements Cloneable, java.io.Serializable 

More to the point, both would be ill-fitted togethoer.

BitSet are not generic, unlike java.util.Iterator; BitSet provides ad-hoc methods with special features for side-effects and random addressing, unlike Iterator.