On 26/08/2022 18:54, Ethan McCue wrote:
If the collections would decide whether or not to copy, I don't think
just requesting an immutable reference would be enough.
static <E> List<E> listCopy(Collection<? extends E> coll) {
if (coll instanceof List12 || (coll instanceof ListN && !
((ListN<?>)coll).allowNulls)) {
return (List<E>)coll;
} else {
return (List<E>)List.of(coll.toArray()); // implicit
nullcheck of coll
}
}
The two things that List.copyOf needs to know are that the list is
immutable, but also that it isn't a variant that might contain a null.
I really don't care about the null problem, that's a problem that the
designers of this basically brought upon themselves, not because of any
real inherit limitation that an immutable collection can't contain
`null`. What irks even more is that the `List` interface provides no way
to determine if an implementation is actively null hostile meaning that
this code is no longer safe (or strictly, never really was safe due to
rather weak guarantees made in the `List` interface):
List<?> aList = ... ; // a list from somewhere
if (aList.contains(null)) throw IllegalArgumentException(); //
this is unsafe, and will cause a NPE depending on the list type
This unfortunate choice was never that visible, but since `List.of` it
occurs more frequently in standard code, and highlights that a leniently
specified interface is mostly a useless interface.
So, I don't see the reason to jump through hoops to use the same type of
`List` that `List.of` or `List.copyOf` returns. All that is required is
that an immutable list is returned, which can be as simple as:
return Collections.unmodifiableList(clone());
Or:
return Collections.unmodifiableList(new ArrayList<>(this));
Or if already wrapped in the immutable wrapper simply `return this`.
So maybe instead of
List<T> y = x.immutableCopy();
It could be appropriate to use the spliterator approach and request a
copy which has certain characteristics.
static <E> List<E> listCopy(Collection<? extends E> coll) {
if (coll instanceof List<?> list) {
return list.copyWhere(EnumSet.of(IMMUTABLE, DISALLOW_NULLS));
} else {
return (List<E>)List.of(coll.toArray()); // implicit
nullcheck of coll
}
}
but that leaves open whether you would want to request the *presence*
of capabilities or the *absence* of them.
Maybe
List.of().copyWhere();
Could be defined to give a list where it is immutable and nulls aren't
allowed. And then
List.of(1, 2, 3).copyWhere(EnumSet.of(ADDABLE, NULLS_ALLOWED));
gives you a mutable copy where nulls are allowed.
This still does presume that making a copy if a capability isn't
present is the only use of knowing the capabilities - which from the
conversation so far isn't that unrealistic
I fear there are too many possibilities here to cover all use cases one
could think of: Appendable, Prependable, Insertable, Removable, Popable,
HeadRemovable(?), Permutable, Replacable, just to name a few. A copy to
create a modifiable version seems sufficient, and a custom solution is
probably in order if that would cause performance issues (like a wrapper
around an actual list that only allows specific functionality, like
implements Appendable<T>).
Perhaps with a method (or constructor) of the form:
<T extends List<T> & Appendable<T>> void giveMeAnAppendableList(T
appendable);
--John
On Fri, Aug 26, 2022 at 11:20 AM John Hendrikx
<john.hendr...@gmail.com> wrote:
On 24/08/2022 15:38, Ethan McCue wrote:
A use case that doesn't cover is adding to a collection.
Say as part of a method's contract you state that you take
ownership of a List. You aren't going to copy even if the list is
mutable.
Later on, you may want to add to the list. Add is supported on
ArrayList so you don't need to copy and replace your reference,
but you would if the list you were given was made with List.of or
Arrays.asList
I don't think this is a common enough use case that should be
catered for. It might be better handled with concurrent lists
instead.
The most common use case by far is wanting to make sure a
collection you've received is not going to be modified while you
are working with it. I don't think another proposal which does
cover the most common cases should be dismissed out of hand
because it doesn't support a rather rare use case.
--John
On Wed, Aug 24, 2022, 8:13 AM John Hendrikx
<john.hendr...@gmail.com> wrote:
Would it be an option to not make the receiver responsible
for the decision whether to make a copy or not? Instead put
this burden (using default methods) on the various collections?
If List/Set/Map had a method like this:
List<T> immutableCopy(); // returns a (shallow)
immutable copy if list is mutable (basically always copies,
unless proven otherwise)
Paired with methods on Collections to prevent collections
from being modified:
Collections.immutableList(List<T>)
This wrapper is similar to `unmodifiableList` except it
implements `immutableCopy` as `return this`.
Then for the various scenario's, where `x` is an untrusted
source of List with unknown status:
// Create a defensive copy; result is a private list
that cannot be modified:
List<T> y = x.immutableCopy();
// Create a defensive copy for sharing, promising it
won't ever change:
List<T> y = Collections.immutableList(x.immutableCopy());
// Create a defensive copy for mutating:
List<T> y = new ArrayList<>(x); // same as always
// Create a mutable copy, modify it, then expose as
immutable:
List<T> y = new ArrayList<>(x); // same as always
y.add( <some element> );
List<T> z = Collections.immutableList(y);
y = null; // we promise `z` won't change again by
clearing the only path to mutating it!
The advantage would be that this information isn't part of
the type system where it can easily get lost. The actual
implementation knows best whether a copy must be made or not.
Of course, the immutableList wrapper can be used incorrectly
and the promise here can be broken by keeping a reference to
the original (mutable) list, but I think that's an acceptable
trade-off.
--John
PS. Chosen names are just for illustration; there is some
discussion as what "unmodifiable" vs "immutable" means in the
context of collections that may contain elements that are
mutable. In this post, immutable refers to shallow immutability .
On 24/08/2022 03:24, Ethan McCue wrote:
Ah, I'm an idiot.
There is still a proposal here somewhere...maybe. right now
non jdk lists can't participate in the special casing?
On Tue, Aug 23, 2022, 9:00 PM Paul Sandoz
<paul.san...@oracle.com> wrote:
List.copyOf already does what you want.
https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/List.java#L1068
https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/ImmutableCollections.java#L168
Paul.
> On Aug 23, 2022, at 4:49 PM, Ethan McCue
<et...@mccue.dev> wrote:
>
> Hi all,
>
> I am running into an issue with the collections
framework where I have to choose between good semantics
for users and performance.
>
> Specifically I am taking a java.util.List from my
users and I need to choose to either
> * Not defensively copy and expose a potential footgun
when I pass that List to another thread
> * Defensively copy and make my users pay an
unnecessary runtime cost.
>
> What I would really want, in a nutshell, is for
List.copyOf to be a no-op when used on lists made with
List.of().
>
> Below the line is a pitch I wrote up on reddit 7
months ago for a mechanism I think could accomplish
that. My goal is to share the idea a bit more widely and
to this specific audience to get feedback.
>
>
https://www.reddit.com/r/java/comments/sf8qrv/comment/hv8or92/?utm_source=share&utm_medium=web2x&context=3
<https://www.reddit.com/r/java/comments/sf8qrv/comment/hv8or92/?utm_source=share&utm_medium=web2x&context=3>
>
> Important also for context is Ron Pressler's comment
above.
> --------------
>
> What if the collections api added more marker
interfaces like RandomAccess?
>
> It's already a common thing for codebases to make
explicit null checks at error boundaries because the
type system can't encode null | List<String>.
>
> This feels like a similar problem.
> If you have a List<T> in the type system then you
don't know for sure you can call any methods on it until
you check that its not null. In the same way, there is a
set of methods that you don't know at the type/interface
level if you are allowed to call.
>
> If the List is actually a __
> Then you can definitely call
> And you know other reference holders might call
> And you can confirm its this case by
> null
> no methods
> no methods
> list == null
> List.of(...)
> get, size
> get, size
> ???
> Collections.unmodifiableList(...)
> get, size
> get, size, add, set
> ???
> Arrays.asList(...)
> get, size, set
> get, size, set
> ???
> new ArrayList<>()
> get, size, add, set
> get, size, add, set
> ???
> While yes, there is no feasible way to encode these
things in the type system. Its not impossible to encode
it at runtime though.
> interface FullyImmutable {
> // So you know the existence of this implies the absence
> // of the others
> default Void cantIntersect() { return null; }
> }
>
> interace MutationCapability {
> default String cantIntersect() { return ""; }
> }
>
> interface Addable extends MutationCapability {}
> interface Settable extends MutationCapability {}
>
> If the List is actually a __
> Then you can definitely call
> And you know other reference holders might call
> And you can confirm its this case by
> null
> no methods
> no methods
> list == null
> List.of(...)
> get, size
> get, size
> instanceof FullyImmutable
> Collections.unmodifiableList(...)
> get, size
> get, size, add, set
> !(instanceof Addable) && !(instanceof Settable)
> Arrays.asList(...)
> get, size, set
> get, size, set
> instanceof Settable
> new ArrayList<>()
> get, size, add, set
> get, size, add, set
> instanceof Settable && instanceof Addable
> In the same way a RandomAccess check let's
implementations decide whether they want to try an
alternative algorithm or crash, some marker "capability"
interfaces would let users of a collection decide if
they want to clone what they are given before working on it.
>
>
> --------------
>
> So the applicability of this would be that the list
returned by List.of could implement FullyImmutable,
signifying that there is no caller which might have a
mutable handle on the collection. Then List.of could
check for this interface and skip a copy.
>
>
