Collection::getAny discussion

Stuart Marks stuart.marks at
Fri Apr 30 18:49:48 UTC 2021

Hi Henri,

I've changed the subject of this thread because I think it's out of scope of the 
ReversibleCollection proposal. I don't mean to say that we can't talk about it, but 
I would like it to be decoupled from ReversibleCollection. I'm somewhat arbitrarily 
calling it "Collection::getAny" because something similar to that was mentioned by 
both Remi and Peter elsewhere in this thread. There is also a bunch of history in 
the bug database that contains related ideas.

Before we dive in, I want to explain why this is separate from ReversibleCollection. 
Most of the ideas, including yours, involve an implementation that does something 
like `iterator().next()`, in other words, getting the "first" element from an 
Iterator. Hey, there's getFirst() in ReversibleCollection, let's use that! No. The 
"first" element of an iterator is in general an arbitrary element, which is 
different from the "first" element in the structural ordering of elements provided 
by a ReversibleCollection. The "arbitrary" notion is captured by "getAny" so that's 
what I'll use as a working term. (Of course any actual API we might add would have a 
better name if we can find one.)

For a historical perspective, let's dig into the bug database and take a look at 
this bug:

This requests a method Collection.get(Object). This searches the collection for an 
element that equals() the argument and returns the element, or it returns null if 
the element isn't found. (Recall in those days it was impossible to add a method to 
an interface without breaking compatibility, so it also proposes various workarounds 
that are no longer necessary.)

There's a comment from Josh Bloch saying that Collection should have had a get() 
method as well as a no-arg remove() method. Well ok then! And he points to the 
then-new Queue interface that was delivered in Java SE 5. Queue adds the following 
methods that seem relevant to this discussion:

  * E element() -- gets the head element, throws NSEE if empty
  * E remove() -- removes and returns the head element, throws NSEE if empty

(It also adds peek() and poll(), which are similar to the above except they return 
null if empty.)

This is kind of odd, in that none of these methods satisfy what the bug's submitter 
was requesting, which is a one-arg get() method. Also, these methods are on Queue, 
which doesn't really help with collections in general.

You're asking for something that's somewhat different, which you called the "find 
the first element when there is only one" problem. Here, there's a precondition that 
the collection have a single element. (It's not clear to me what should happen if 
the collection has zero or more than one element.)

To throw a couple more variations into the mix, Guava has a couple Collectors (for 
streams) that do interesting things. The class is MoreCollectors:

and the collectors are:

  * onlyElement -- if source has 1 element, returns it; throws NSEE if empty, IAE if > 1
  * toOptional -- if source has 0 or 1 elements, returns an Optional; otherwise throws

These apply to streams, but I think you can see the applicability to Collection as 
well. In particular, your proposal is similar to what onlyElement would look like if 
it were on Collection.

Let's summarize the variations so far:

  * preconditions: exactly one element, at-most-one, at-least-one
  * behavior if preconditions not met: return null, return empty Optional, throw
  * remove element or just peek
  * match a particular element, or return an arbitrary element

That's a lot of variations!

Before we talk about specific APIs, though, I wanted to talk about use cases. Which 
of these variations are more useful or less useful? Which are likely to appear in 
code? Henri gave a fairly specific example with a reasonable "success" case 
(preconditions met) but it's not clear what should happen if the preconditions 
aren't met. Clearly an API would have to choose. What would the use site look like? 
In particular, does the use site always have to check for null, or catch an 
exception, or something?

Answers to these questions will help determine what APIs, if any, we might want to add.


There's another thing looming in the distance, which is pattern matching. You might 
have seen one of Brian Goetz's talks on pattern matching futures. You can get a 
glimpse of some upcoming pattern matching features in JEP 405.

In particular, this JEP extends pattern matching with an /extraction/ step, where, 
if there is a match, record or array components can be extracted and bound to local 
variables. This is a step closer to /deconstruction patterns/, where arbitrary 
classes and interfaces (not just records or arrays) can participate in pattern 
matching. (See discussion of this at the end of the JEP.)

Deconstruction patterns apply directly to this discussion. For example, Henri's 
example could be rewritten like this, using pattern matching:

     Set<String> s = ... ;
     if (s instanceof Set.containing(String string)) {
         // use string, which is the only element
     } else {
         // handle failure

This "containing" thing would be a deconstruction pattern declared on Set or 
Collection (or possibly elsewhere), and as I've written here, it implicitly matches 
sets that contain exactly one element. Taking a nod from JEP 405's varargs-style 
pattern, extracting an arbitrary element from a set that contains one or more 
elements might look like this:

     Set<String> s = ... ;
     if (s instanceof Set.containing(String string, ...)) { // varargs style
         // use string, which is an arbitrary element
     } else {
         // handle failure

This deconstruction pattern addresses a bunch of the design decisions all at once. 
It handles one or at-least-one element. The preconditions-not-met case is handled by 
matching failure. Matching failure also sidesteps the issue of whether we return 
null, return an Optional, or throw an exception. Since pattern matching is 
inherently conditional, you can't forget to check for null or catch an exception 
(and you won't have to deal with that darned Optional API). The only thing this 
doesn't handle is removing an element. I don't think we want pattern matching to 
have side effects, and element removal is probably pretty rare. But we can still 
discuss whether it's a valuable case to support.

In summary, I think it's important and useful to have a conversation about use 
cases, what problems we are trying to achieve, and what we think the code at the 
call site should look like. That can be used to drive API discussions, such as 
adding new methods. It can /also/ be used to drive discussion about deconstruction 
patterns that would be useful to add to collections. Furthermore, since the pattern 
matching language feature is being designed right now, this discussion can help by 
providing information of the form "libraries need to be able to do this kind of 
pattern matching".



On 4/28/21 6:23 AM, Henri Tremblay wrote:
> Hi,
> (I think the first version of this message never went through, so here it goes 
> just in case)
> I just read quickly the proposal and it made me think about another common issue. 
> I wonder if we could tackle it as well.
> I will call it the "find the first element when there is only one" problem.
> It happens on unordered collections like a HashSet. It forces to do
> Set<String> s = new HashSet<>();
> if (s.size() == 1) {
>    return s.iterator().next();
>    // or
>    return;
> }
> Which is a lot of ugliness and object instantiations just to get the first element.
> I would be nice to have a public T findFirst() directly on Iterable<T>. With a 
> default implementation returning iterator().next(). Things like ArrayList will 
> want to override will return elementData[0]. It would return null when the list is 
> empty. Or NoSuchElementException.
> It needs to be polished of course but will that be acceptable?
> Thanks,
> Henri

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