> From: "Brian Goetz" <brian.go...@oracle.com>
> To: "amber-spec-experts" <amber-spec-experts@openjdk.java.net>
> Sent: Wednesday, May 18, 2022 9:18:01 PM
> Subject: Pattern matching: next steps after JEP 405
> JEP 405 has been proposed to target for 19. But, it has some loose ends that
> I'd
> like to refine before it eventually becomes a final feature. These include:
[...]
> - Varargs patterns. Records can be varargs, but we have an asymmetry where we
> can use varargs in constructors but not in deconstruction. This should be
> rectified. The semantics of this is straightforward; given
> record Foo(int x, int y, int... zs) { }
> just as
> new Foo(x, y, z1, z2)
> is shorthand for
> new Foo(x, y, new int[] { z1, z2 })
> we also can express
> case Foo(var x, var y, var z1, var z2)
> as being shorthand for
> case Foo(var x, var y, int[] { var z1, var z2 })
> This means that varargs drags in array patterns.
Thinking a bit about the varargs pattern, introducing them is not a good idea
because a varargs record is not a safe construct by default,
- varargs are arrays, and arrays are mutable in Java, so varargs records are
not immutable by default
- equals() and hashCode() does not work as is too.
The record Foo should be written
record Foo(int x, int y, ... zs) {
Foo {
zs = zs.clone();
}
public int[] zs() {
return zs.clone();
}
public boolean equals(Object o) {
return o instanceof Foo foo && x == foo.x && y == foo.y && Arrays.equals(zs,
foo.zs);
}
public int hashCode() {
return hash(x, y, Arrays.hashCode(zs));
}
}
Given that most people will forget that the default behavior of a varargs
record is not the right one, introducing a specific pattern for varargs record
to mirror them is like giving a gentle nudge to somebody on a cliff.
Note that, it does not mean that we will not support varargs record, because
one can still write either
case Foo(int x, int y, int[] zs)
or
case Foo(int x, int y, int[] { int... zs }) // or a similar syntax that mix a
record pattern and an array pattern
but just that there will be no streamlined syntax for a varargs record.
Rémi
