Records -- current status

There are a number of potentially open details on the design for records. My inclination is to start with the simplest thing that preserves the flexibility and expectations we want, and consider opening up later as necessary. One of the biggest issues, which Kevin raised as a must-address issue, is having sufficient support for precondition validation. Without foreclosing on the ability to do more later with declarative guards, I think the recent construction proposal meets the requirement for lightweight enforcement with minimal or no duplication. I'm hopeful that this bit is "there". Our goal all along has been to define records as being “just macros” for a finer-grained set of features. Some of these are motivated by boilerplate; some are motivated by semantics (coupling semantics of API elements to state.) In general, records will get there first, and then ordinary classes will get the more general feature, but the default answer for "can you relax records, so I can use it in this case that almost but doesn't quite fit" should be "no, but there will probably be a feature coming that makes that class simpler, wait for that." Some other open issues (please see my writeup at http://cr.openjdk.java.net/~briangoetz/amber/datum.html for reference), and my current thoughts on these, are outlined below. Comments welcome! - Extension. The proposal outlines a notion of abstract record, which provides a "width subtyped" hierarchy. Some have questioned whether this carries its weight, especially given how Scala doesn't support case-to-case extension (some see this as a bug, others as an existence proof.) Records can implement interfaces. - Concrete records are final. Relaxing this adds complexity to the equality story; I'm not seeing good reasons to do so. - Additional constructors. I don't see any reason why additional constructors are problematic, especially if they are constrained to delegate to the default constructor (which in turn is made far simpler if there can be statements ahead of the this() call.) Users may find the lack of additional constructors to be an arbitrary limitation (and they'd probably be right.) - Static fields. Static fields seem harmless. - Additional instance fields. These are a much bigger concern. While the primary arguments against them are of the "slippery slope" variety, I still have deep misgivings about supporting unrestricted non-principal instance fields, and I also haven't found a reasonable set of restrictions that makes this less risky. I'd like to keep looking for a better story here, before just caving on this, as I worry doing so will end up biting us in the back. - Mutability and accessibility. I'd like to propose an odd choice here, which is: fields are final and package (protected for abstract records) by default, but finality can be explicitly opted out of (non-final) and accessibility can be explicitly widened (public). - Accessors. Perhaps the most controversial aspect is that records are inherently transparent to read; if something wants to truly encapsulate state, it's not a record. Records will eventually have pattern deconstructors, which will expose their state, so we should go out of the gate with the equivalent. The obvious choice is to expose read accessors automatically. (These will not be named getXxx; we are not burning the ill-advised Javabean naming conventions into the language, no matter how much people think it already is.) The obvious naming choice for these accessors is fieldName(). No provision for write accessors; that's bring-your-own. - Core methods. Records will get equals, hashCode, and toString. There's a good argument for making equals/hashCode final (so they can't be explicitly redeclared); this gives us stronger preservation of the data invariants that allow us to safely and mechanically snapshot / serialize / marshal (we'd definitely want this if we ever allowed additional instance fields.) No reason to suppress override of toString, though. Records could be safely made cloneable() with automatic support too (like arrays), but not clear if this is worth it (its darn useful for arrays, though.) I think the auto-generated getters should be final too; this leaves arrays as second-class components, but I am not sure that bothers me.