Constructor Interfaces

Brian Goetz brian.goetz at oracle.com
Wed Jan 25 15:11:00 UTC 2023


This is a good example of a "you can, but you probably shouldn't" 
language feature.  The power-to-weight ratio isn't favorable; it is a 
lot of new machinery and concept to move the ball forward a small 
amount. And as soon as the ball is moved forward by that amount, we will 
immediately be confronted by the next thing we can't do, and the 
solutions are likely to be an increasingly complex sequence of worse 
power-to-weight ratio ideas.  (Careful observers of Java history might 
note that this phenomenon is especially evident in any proposal 
surrounding annotations.)

As Michael K pointed out, other languages have explored more general, 
but more suitable, answers here; what you're looking for is a witness to 
conformance to a type class.  (Our friends in C# have pursued something 
similar through abstract statics; for various reasons, that's less of a 
good match for Java than for C#.)  This is not a small ask; its 
significant new complexity, but the power gained is much greater.  If we 
were to choose to invest in solving problems like this one, that would 
likely be the path, but this is a big lift and we have other big things 
on our plate right now.

As a general note, while it is fun to imagine new language features, 
language design needs to be a holistic process.  If we did a hundred 
"point" features like this, what are the chances that the whole would 
hold together?  If we did this feature, what other potential feature 
directions are we implicitly foreclosing on?  These are the questions we 
address ourselves to when choosing what features to consider and not.


On 1/25/2023 2:03 AM, Red IO wrote:
> Summary
> -------
>
> Enable a parameterized class to constrain the parameterized type to be 
> constructible with a given list of parameters.
>
>
>
> Motivation
> ----------
>
> It is possible since JDK 8 to get a constructor (method) reference of 
> an object. This allowed for the creation of an unknown class with a 
> known constructor reference. But currently the only way to obtain such 
> reference is at call site like this:
> Box<String> stringBox = new Box<>(String::new);
>
> It is inconvenient for the user to supply the the reference themselves 
> and can confuse them as the type of the parameter is something like 
> Supplier<String> which doesn't require the pased reference to be a 
> constructor.
> It also clutters api's like "toArray" which requires an IntFunction to 
> be type safe.
>
> Description
> -----------
>
> ConstructorInterface
> A ConstructorInterface is a special kind of interface similar to a 
> FunctionalInterface. It also has similar constraints. It only allows 
> abstract constructors and no other abstract methods. It can declare 
> multiple constructors though. The definition of such interface would 
> look similar to this:
>
> @ConstructorInterface //optional validation like FunctionalInterfaces
> public interface DefaultConstructible {
> new();
> new(char[] chars);
> }
>
> A parameterized type could declare this interface as a type bound for 
> its parameter and therefore enabling it to be constructed safely. Like 
> this:
> public class Box<E extends DefaultConstructible> {
> public Box() {
> E newElement = new E();
> }
> }
> The containing type is not forced to implement the ContructorInterface 
> explicitly. It is implicitly implemented if the required 
> constructor(s) is(are) present.
> public static void main(String[] args) {
> Box<String> stringBox = new Box<>(); //compiles because String has the 
> required constructors.
> Box<java.sql.Date> dateBox new Box<>(); error: java.sql.Data does not 
> satisfy the type bound DefaultConstructible
> }
> The interface might not be implemented by any class, since it doesn't 
> follow the inheritance rule that extending classes of those who 
> implement it also implement it. This requirement comes from the fact 
> that extending classes do not necessarily need to have the same 
> constructor signature and therefore don't qualify the requirements for 
> the interface. Another option would be that extending classes of 
> classes that implement a constructor interface explicitly are also 
> required to supply the necessary constructors.
>
> class Foo implements DefaultConstructable {
> //both required by the interface
> public Foo() {}
> public Foo(char[] chars) {}
> }
>
> class Bar extends Foo {
> //the requirement for the required constructors is passed down.
> public Bar() {}
> public Bar(char[] chars) {}
> }
>
>
>
> public static <T extends Foo> T createT() {
> return new T();
> }
>
> public <T extends Foo> T wrapper() {
> return createT();
> }
> This would technically work but would require a lot of static analysis 
> to find the real type of T to call its constructor.
> Restricting the use of "new T()" to type parameters that specify a 
> constructor interface directly and only allow those to be resolved 
> with a concrete type rather than another type parameter.
>
> Alternatives
> ------------
> An alternative would be to introduce new syntax to restrict the 
> ability of certain constructors on a parameter type. Like c# does (but 
> only for the default constructor) :
> public static T foo<T>() where T: new() {
> return new T();
> }
> In java:
> public static <T extends new()> T foo() {
> return new T();
> }
> The downside of this approach is obviously the introduction of new 
> syntax rather than reusing the interface/inheritance syntax.
>
> Another alternative to this approach could be to implement static 
> abstract methods. This would allow an interface to mandate a static 
> Factory Method. The downside of this approach is that it requires the 
> parameter class to actually implement the interface and the concept of 
> type erasure would need to be addressed for static abstract methods to 
> work. In contrast the ConstructorInterface enables every class that 
> matches its contract to pass the type bound.
>
>
>
> Risks and Assumptions
> ---------------------
>
> As mentioned before the restriction the interface is giving on a type 
> bound is different to normal interfaces, it restricts by its 
> containing abstract constructors not by the type itself. It also makes 
> use of the new operator on type variables.
>
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