Making Object abstract

Brian Goetz brian.goetz at
Fri May 7 22:38:00 UTC 2021

I think the question is: "cleaner for whom."  The parenthetical remarks 
"except for Object" are mostly our mess to corral; "new Object()" 
working, even when Object is seen to be abstract, will be extremely 
confusing for new developers learning what abstract means.  It's not 
just that reflection would expose this, but the source code and Javadoc 
would to.  User mental models are a stakeholder in Object too.

On 5/7/2021 6:06 PM, Dan Smith wrote:
> I notice a number of times in current design documents that we state a rule and then have a parenthetical "except for Object, which is special". A lot of this specialness is due to Object behaving like an abstract class, even though it is not abstract.
> I think we'd have a cleaner design if we just updated Object to be abstract.
> Implications:
> - The expression 'new Object()' wouldn't normally be allowed, but we plan to special-case this anyway to produce an instance of IdentityObject.
> - Similarly, a 'new java/lang/Object' instruction would normally cause a InstantiationError, but we plan to special-case it.
> - Reflection would expose the ACC_ABSTRACT flag. <shrug>
> - The JVM story for opting in to primitive subclasses would need to work for class Object just as well as other abstract classes. We can design for that.
> - As long as this is a preview feature, we might need two different versions of Object.class, depending on whether --enable-preview is set. But we need to do that for Integer, etc., anyway, so should be doable.
> Am I missing anything that would be particularly disruptive?

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