On 2017-09-05 20:08, Steve D'Aprano wrote:
On Tue, 5 Sep 2017 11:47 pm, Gregory Ewing wrote:
Steve D'Aprano wrote:
[quoting Scott Stanchfield]
Figure 7: (Java) Defining a Dog pointer
Dog d;
When you write that definition, you are defining a pointer to a Dog
object, not a Dog object itself.
[end quote]
Here Scott mixes up what the compiler does (creates a pointer to a Dog
object, and what the programmer's Java code does (creates a Dog).
Um, no. The declaration 'Dog d' on its own does NOT create a Dog,
in any way, shape or form. It only declares something that can
*refer* to a Dog created elsewhere, which is what Scott is
quite correctly saying.
That's not what he said. I quoted him: he quite clearly states that d defines a
pointer to a Dog object. He doesn't say that you're declaring an empty slot
that is waiting to be filled with a pointer to a dog. He says it defines a
pointer.
So which Dog object does this pointer point to? Answer: there isn't one. There
may not even be any Dog object existing in the entire program up to this point.
Even allowing that there were a Dog to be pointed to, how do we inspect or
manipulate that pointer? Answer: you can't, because pointers are not meaningful
values in Java.
There's no(?) Java code you can write that allows this:
Dog d;
java.lang.System.out.println( address of variable d );
java.lang.System.out.println( address of the Dog d points to );
Maybe compiler-specific debugging tools? I don't know enough Java to
*categorically* rule it out, but if it exists at all, it would be unsafe to
rely on the addresses you get.
d is initialised to null. You can check for null:
if (d == null)
java.lang.System.out.println("There's no dog.");
else
d.bark();
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