Java: Rationale of the Object class not being declared abstract

Question

Why wasn\'t the java.lang.Object class declared to be abstract ?

Surely for an Object to be useful it needs added state or behaviour, an Object class is

Answer 1

I'll just throw in another reason that I've found Object to useful to instantiate on its own. I have a pool of objects I've created that has a number of slots. Those slots can contain any of a number of objects, all that inherit from an abstract class. But what do I put in the pool to represent "empty". I could use null, but for my purpose, it made more sense to insure that there was always some object in each slot. I can't instantiate the abstract class to put in there, and I wouldn't have wanted to. So I could have created a concrete subclass of my abstract class to represent "not a useful foo", but that seemed unnecessary when using an instance of Object was just as good..in fact better, as it clearly says that what's in the slot has no functionality. So when I initialize my pool, I do so by creating an Object to assign to each slot as the initial condition of the pool.

I agree that it might have made sense for the original Java crew to have defined a Placeholder object as a concrete subclass of Object, and then made Object abstract, but it doesn't rub me wrong at all that they went the way they did. I would then have used Placeholder in place of Object.

Answer 2

Does Object specify methods that classes extending it must implement in order to be useful? No, and therefor it needn't be abstract.

The concept of a class being abstract has a well defined meaning that does not apply to Object.

Answer 3

You never know when you might want to use a simple Object as a placeholder. Think of it as like having a zero in a numerical system (and null doesn't work for this, since null represents the absence of data).

Answer 4

How is Object any more offensive than null?

It makes a good place marker (as good as null anyway).

Also, I don't think it would be good design to make an object abstract without an abstract method that needs to go on it.

I'm not saying null is the best thing since sliced bread--I read an article the other day by the "Inventor" discussing the cost/value of having the concept of null... (I didn't even think null was inventable! I guess someone somewhere could claim he invented zero..) just that being able to instantiate Object is no worse than being able to pass null.