Thoughts on Array Initialization in JavaONE 2025
Brian Goetz
brian.goetz at oracle.com
Mon Jul 28 21:30:51 UTC 2025
This is the feature we sometimes refer to as "Collection Literals" or
"Structured Literals". It is on the roadmap, but there are some other
things ahead of it. (There was a poor proposal back in the Project Coin
days for this feature, which was thankfully rejected.)
(And, for fans of pattern matching, the dual of "structured literal" is
"structured pattern".)
On 7/28/2025 5:24 PM, Rob Ross wrote:
> In Python, arrays exist as a lower level library or via third-party
> libraries like NumPy. Unless you're doing specialized processing, you
> don't typically work with actually arrays. As with Java, you're
> normally working with lists.
> But the syntax in Python for working with lists makes list
> initialization simple and easy. I would love to see a similar syntax
> in Java for both List and Map initialization.
> E.g.,
> ```Python
> foo = [{"key": {"subkey": "value"}}]
> ```
> The simplest way I can duplicate this in Java is
> ```Java
> List<Map<String, Object>> foo = List.of(
> Map.of("key", Map.of("subkey", "value"))
> );
> ```
> Now, this doesn't look too bad. In Java this has created an immutable
> List, which is probably adequte in many cases. In the context of the
> actual code as a unit test, immutability is fine; but if you needed
> mutability:
> List<Map<String, Object>> foo = new ArrayList<>();
> Map<String, Object> outerMap = new HashMap<>();
> Map<String, String> innerMap = new HashMap<>();
>
> innerMap.put("subkey", "value");
> outerMap.put("key", innerMap);
> foo.add(outerMap);
>
> And you can see how the boilerplate starts to obscure the nature of
> the data for more complicated initializations:
> Python:
> result [{"key": {"subkey": [{"subsubkey": [{"subsubsubkey":
> [{"subsubsubsubkey": [1]}]}]}]}}]
>
> Java:
> List<Map<String, Object>> result = List.of(
> Map.of(
> "key", Map.of(
> "subkey", List.of(
> Map.of(
> "subsubkey", List.of(
> Map.of(
> "subsubsubkey", List.of(
> Map.of(
> "subsubsubsubkey", List.of(1)
> )
> )
> )
> )
> )
> )
> )
> )
> );
>
> So it would be nice if Java could use [] and {} as syntax for
> initializing Lists and Maps. I don't think it would be too complicated
> to add this syntax, e.g.,
>
> List<Integer> foo = [1, 2, 3]; // clear in context this is
> initializing a List, not an array.
>
> or more explicitly:
> List<Integer> foo = new ArrayList<>[1, 2, 3];
>
> or perhaps
> List<Integer> foo = new ArrayList<>([1, 2, 3]); // and [1,2,3] could
> be auto-boxed to a Collection object for the existing ArrayList
> constructor.
>
> I'm just talking about initialization syntax. The compiler should be
> able to determine the new syntax from context, just as it can tell the
> difference in {} between a block of code and an array initializer or
> anonymous class definition.
>
> I also could be totally wrong on how "easy" it would be to add this.
> But I do know it would simplify my unit testing code!
>
> - Rob
>
>
>
>
> On Sat, Jul 26, 2025 at 2:49 PM Brian Goetz <brian.goetz at oracle.com>
> wrote:
>
> All of the points you raise that "arrays are not the abstraction
> you are looking for" are absolutely true -- and well understood.
> The goal of improving array creation is not to help arrays win
> market share away from collections.
>
> But there is a reason that the language has arrays, and that
> reason hasn't gone away. Arrays are the bottom brick on which the
> tower of better data structures are built. You can't have
> ArrayList or HashMap without arrays. Arrays are part of the
> language for a reason. As we improve the language, sometimes
> arrays have to improve with it. (For example, if you have a
> non-implicitly-constructible value class NVA (one who has no valid
> default value), and someone wants to create an array of NVA![n],
> allowing the existing "zero the memory" array creation expression
> to create it would undermine the integrity of the runtime by
> exposing objects that are not in a valid state as determined by
> their constructor. So we can't allow that, which means we have to
> provide something else.)
>
> If we hadn't found the need to improve other aspects of
> initialization, we probably wouldn't have bothered to improve
> array initialization. But because we are improving initialization
> more broadly, we have to do the whole job. Not being able to
> create interesting arrays linguistically would forever look like a
> weird omission.
>
> Your suggestion -- "why not just" not improve array creation
> linguistically, and shunt any sort of exotic array creation to a
> privileged API -- leaves the user with little explanation for why
> the tower stands up. The bottom brick is not there, instead
> replaced by some sort of magic that seems to hold up the brick
> above it. While we expect relatively few programmers to program
> with arrays (ideally, just the guy who maintains ArrayList and
> HashMap, and the like), we should provide linguistic mechanisms
> for properly using core linguistic building blocks.
>
> I can imagine three implicit lines of thought for why you think
> such a low-level mechanism should be performed by a privileged
> library rather than a language feature:
>
> - The easier we make it to use, the more people will use it, and
> you would like fewer people to use it (as would we.) A
> scary-looking library will scare away more people than a
> pretty-looking language feature.
> - Language features are expensive, more expensive than libraries,
> so by shunting this to a library, we preserve resources to focus
> on more important things.
> - Array initialization appears to be competing for resources with
> features like collection literals, and you'd rather have the
> latter, so suggesting that we skip the former seems like it would
> bring the latter more quickly.
>
> These are appealing-sounding arguments, but they don't point to
> either usability wins or project-management wins. Arrays are the
> right tool for some jobs. Collections are the right tool for
> others (most others.) But the way to encourage people to use the
> right tool is not to make the other tools harder to use. (We too
> would like to have linguistic support for creating sets, lists,
> maps, etc, but that feature is not competing with arrays, its
> waiting for something else.) Nor is the cost of a privileged
> array-construction API significantly cheaper than a language feature.
>
> Cheers,
> -Brian
>
>
>
>
>
>
>
>
>
> On 7/26/2025 5:03 PM, david Grajales wrote:
>>
>> Dear Amber developers,
>>
>> I recently watched the JavaONE 2025 session titled /“A New Model
>> for Java Object Initialization”/ and was particularly intrigued
>> by the proposed improvements to array initialization.
>>
>> https://www.youtube.com/watch?v=XtvR4kqK8lo
>> <https://urldefense.com/v3/__https://www.youtube.com/watch?v=XtvR4kqK8lo__;!!ACWV5N9M2RV99hQ!KIGGhMZzNwG-ujAKo8hQ1-VhBKB9SCO8frqQjcrv10FfH1nSh4FU3VcgQ9xf6CWkQCtBUfQOUMSQJP8bFg$>
>>
>>
>> I strongly agree that Java needs better mechanisms for
>> initializing data structures in a concise, expressive, and
>> stricter manner—similar in spirit to Python’s list comprehensions
>> and aligned with Strict initialization, required for Valhalla.
>> Such constructs can help avoid subtle bugs and the presence of
>> unintended |null| values. However, I remain skeptical about the
>> decision to focus this new model exclusively around arrays.
>>
>> As has been discussed over the past few months, arrays are not
>> ideal as a default abstraction, especially for students or in
>> enterprise applications. Arrays are a low-level construct with
>> several limitations:
>>
>> *
>>
>> They do not integrate well with generics.
>>
>> *
>>
>> They are of fixed size.
>>
>> *
>>
>> They lack methods and flexibility.
>>
>> *
>>
>> They are syntactically and semantically inconsistent with the
>> rest of the Java Collections Framework.
>>
>> In many ways, arrays are a legacy feature inherited from
>> C/C++—much like the original |switch| statement—that carry
>> forward certain limitations that Java has otherwise worked hard
>> to overcome.
>>
>> Given these issues, Why not just create an small API that
>> facilitates the creation of the most used data structures with
>> strict initialization?
>>
>> For example:
>>
>>
>> void main(){
>>
>>
>> // toArray
>>
>> var array = StrictCollections.toArray(String.class, 5, i ->
>> "Item-" + i);
>>
>> IO.println("Array: " + Arrays.toString(array));
>>
>>
>> // toList
>>
>> var list = StrictCollections.toList(5, i -> "List-" + i);
>>
>> IO.println("List: " + list);
>>
>>
>> // toSet
>>
>> var set = StrictCollections.toSet(5, i -> "Set-" + (i % 3));
>>
>> IO.println("Set: " + set);
>>
>>
>>
>> var map = StrictCollections.toMap(
>>
>> 5,
>>
>> i -> "Key-" + i,
>>
>> i -> i * 100
>>
>> );
>>
>> IO.println("Map: " + map);
>>
>>
>> }
>>
>>
>>
>> public static class StrictCollections {
>>
>>
>> public static <T> T[] toArray(Class<T> clazz, int size,
>> IntFunction<T> function) {
>>
>> @SuppressWarnings("unchecked")
>>
>> T[] array = (T[]) Array.newInstance(clazz, size); // This
>> could be a frozen array once these are ready
>>
>> for (int i = 0; i < size; i++) {
>>
>> array[i] = function.apply(i);
>>
>> }
>>
>> return array;
>>
>> }
>>
>>
>> public static <T> ArrayList<T> toList(int size,
>> IntFunction<T> function) {
>>
>> var list = new ArrayList<T>(size);
>>
>> for (int i = 0; i < size; i++) {
>>
>> list.add(function.apply(i));
>>
>> }
>>
>> return list;
>>
>> }
>>
>>
>> public static <T> HashSet<T> toSet(int size, IntFunction<T>
>> function) {
>>
>> List<T> list = new ArrayList<>(size);
>>
>> for (int i = 0; i < size; i++) {
>>
>> list.add(function.apply(i));
>>
>> }
>>
>> return new HashSet<>(list);
>>
>> }
>>
>>
>> public static <K, V> HashMap<K, V> toMap(int size,
>> IntFunction<K> kFunction, IntFunction<V> vFunction) {
>>
>> HashMap<K, V> map = new HashMap<>(size);
>>
>> for (int i = 0; i < size; i++) {
>>
>> map.put(kFunction.apply(i), vFunction.apply(i));
>>
>> }
>>
>> return map;
>>
>> }
>>
>> }
>>
>> While this is admittedly a rough sketch developed in just a few
>> minutes, I believe a similar—much more thoroughly
>> designed—approach could provide much greater flexibility with far
>> less complexity than introducing a dedicated array-specific
>> feature. It would also extend naturally to a broader range of use
>> cases --Such as being able to be combined with the Stream API in
>> a much more ergonomic way--. Furthermore, as value classes and
>> parametric JVM start to make it into the language and the JVM,
>> the advantages of arrays and primitive types will diminish
>> further. In that context, arrays will become even less compelling
>> in the future.
>>
>> If Java is to introduce a safe, expressive, and idiomatic strict
>> initialization literal for data structures, I would argue it
>> should primarily support |List|, |Set|, and |Map|—especially
>> |Map|, which remains one of the least ergonomic structures to
>> initialize in the language today, particularly when compared to
>> alternatives in Dart, Python, or even JavaScript objects. Data
>> structures that are much more used.
>>
>>
>> Thank you so much for all your work and always yours
>>
>>
>
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