Anonymous Cast (I think)

[John Duffy]

Hi

I'm studying for the OCA/OCP exams and casts cause me real headaches...

I've wrapped my head around the idea that a Reference variable can "point to" an object of a subclass. And, I think I've wrapped my head around the notion of up casts and down casts. But when I see exam questions using what look like anonymous casts I'm really lost.

For example, an exam question may have a prelude such as...

class Dog {
    void bark() {}
}

class Hound extends Dog {
    void bark() {}
}

But will then have something like this without any assignment to a variable...

((Dog) new Hound()).bark();

I know it's cast, but what is it? Is it a Hound object being cast into a Dog object? Or, is it a Hound object being assigned to an anonymous Dog reference variable?

And how do I decide which bark method will be called?

The lack of assignment is really confusing me.

Any assistance would be much appreciated.

Kind regards

[Carey Brown]

Same as
When in doubt try it out

public class Dog
{

	public static void main( String[] args )
	{
		((Dog)new Hound()).bark();
	}

	public void bark()
	{
		System.out.println("Dog barks");
	}
}

class Hound extends Dog
{
	public void bark()
	{
		System.out.println( "Hound barks" );
	}
}

[Junilu Lacar]

The expression ((Dog) new Hound()) is an object reference expression, i.e., an expression that evaluates to an object reference. ((Dog) new Hound()).bark() is a method invocation expression.  A method invocation expression uses an explicit reference or an implied reference (this). What you have used so far prior to seeing this example is an object reference expression that uses a variable reference. That's a very common way to have an object reference but it's not only way, as you have seen.

It's the same thing as with math and algebraic expressions:

Let x = 7
(x + 5) = 12  (this is an algebraic equation that uses the variable "x")

((14 / 2) + 5) = 12 (this is a mathematical equation that is equivalent to the above)

In the second example, you just calculate the value in line, without using the variable x. Kind of the same thing that's happening in your code example.

[Junilu Lacar]

Also, "Anonymous cast" is not A Thing™ in Java, as far as I know. In fact, now that I think about it, it's a contradiction in terms. If you're going to cast something, you have to know exactly what you're going to cast it to. So yeah, I'm pretty sure now that "anonymous cast" is not a thing in Java.

[Campbell Ritchie]

John Duffy wrote:. . . Is it a Hound object being cast into a Dog object? Or, is it a Hound object being assigned to an anonymous Dog reference variable? . . .

Probably, neither nor. But much nearer to the latter.

You can cast the type of a reference however you like. In some instances, however, the javac tool will recognise that the cast could never succeed and it will decline to compile it:-

String number = "123";
Integer i = (Integer)number; // try getting that line to compile :P

You can cast as much as you like, but you cannot change the actual type of an object If it is a Hound once, it is a Hound for ever, and that cannot be changed. That may be necessary when you have different options for instantiation at runtime. Nobody would write this sort of code in real life, would they?

Dog d = option == 1 ? new Hound()     :
        option == 2 ? new Terrier()   :
        option == 3 ? new Mastiff()   :
        option == 4 ? new Sheepdog()  :
                      new Rottweiler();

But it will compile. Any of those references can be cast to (Dog)..., but does that make any difference? Maybe only a Hound has a followScent() method and only a Sheepdog has guideSheep(). But you cannot call either of those methods on a Dog reference.

Dog d = new Sheepdog();
d.guideSheep(); // won't compile
((Sheepdog)d).guideSheep(); // will compile, but may fail at runtime

That line 3 depends on d actually being a Sheepdog. What you are doing is reassuring the compiler that it is in fact a Sheepdog, and it will believe you. But any casts to a more specific type, officially called narrowing reference conversion are inherently error‑prone. If you cast your Hound object to (Dog)h, that means you are casting to a wider type, which always succeeds, and is never actually necessary. You are not creating an anonymous reference, but asserting to the javac tool that the object behind that reference will be that type.

You need to know about casts for the exam, but in real life, start suspecting that you have something wrong with your design if you start casting references types.

[John Duffy]

Hi all

Thank you for taking the time to reply to my question. I think I'm closer to understanding this now.

I've worked my way through the whole K&B OCP Exam Guide and I have found "casts", and how to determine whether they will generate a compile time or run time error, to be the most difficult bit. Probably just me.

Thanks again.