Question about declaring type as interface

Hi
I am trying to get my head around declaring an object as the interface type (I think this is right correct me if I am wrong). Ok so I understand if I declare my object reference as the interface I will only get the methods of the interface defined in the objects class..? ie
List myList = new ArrayList();I think any method particular to ArrayList will not be there. But I dont actually understand wha is happening here, I have a ref to a List but the object is actually an ArrayList?
So if I passed ths list to a session object and got it out I could cast it back to the interface. I dont understand why or what is happening here.
I hope someone understands my question enough that they can help me out, this has been nugging me for a while.
Thanks

I am trying to get my head around declaring an object
as the interface type (I think this is right correct
me if I am wrong). Ok so I understand if I declare my
object reference as the interface I will only get the
methods of the interface defined in the objects
class..? ie
List myList = new ArrayList();I think any method particular to ArrayList will not
be there. If you use myList without casting, then, yes, only List's methods will be available, because all the compiler knows is that it's a reference to a List. It doesn't know that at runtime it will point to an ArrayList object.
You could cast myList to ArrayList to get ArrayList-specific things (if there are any), but a) if you're not careful, you can end up with a ClassCastException at runtime (you won't here, but in general you can) and b) if you're going to do that, you have to question why you declared it as a List in the first place. There are cases where casting is appropriate, but you have to be careful.
But I dont actually understand wha is
happening here, I have a ref to a List but the object
is actually an ArrayList?Yes.
So if I passed ths list to a session object and got
it out I could cast it back to the interface. I dont
understand why or what is happening here.You could cast it to anything that the object actually is, that is, any of its superclasses and superinterfaces. If it's an ArrayList, you could cast it to any of the following: ArrayList, AbstractList, AbstractCollection, Object, Serializable, Cloneable, Iterable, Collection, List, RandomAccess.

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Squall867 wrote:
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There are 2 classes and one is subclass of another like below:
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Edited by: newbie on Oct 25, 2010 11:03 PM
Edited by: newbie on Oct 25, 2010 11:26 PM

newbie wrote:
There are 2 classes and one is subclass of another like below:
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Hello. I have a a few quick questions about declaring and defining variables and about their size and so forth. The book I'm reading at the moment, "Learn C on the Mac", says the following in reference to the difference between declaring and defining a variable:
A variable declaration is any statement that specifies a variables name and type. The line *int myInt;* certainly does that. A variable definition is a declaration that causes memory to be allocated for the variable. Since the previous statement does cause memory to be allocated for myInt, it does qualify as a definition.
I always thought a definition of a variable was a statement that assigned a value to a variable. If a basic declaration like "int myInt;" does allocate memory for the variable and therefore is a definition, can anyone give me an example of a declaration that does not allocate memory for the variable and therefore is not a definition?
The book goes on, a page or so late, to say this:
Since myInt was declared to be of type int, and since Xcode is currently set to use 4-byte ints, 4 bytes of memory were reserved for myInt. Since we haven't placed a value in those 4 bytes yet, they could contain any value at all. Some compilers place a value of 0 in a newly allocated variable, but others do not. The key is not to depend on a variable being preset to some specific value. If you want a variable to contain a specific value, assign the value to the variable yourself.
First, I know that an int can be different sizes (either 4 bytes or 8 bytes, I think), but what does this depend on? I thought it depended on the compiler, but the above quote makes it sound like it depends on the IDE, Xcode. Which is it?
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Third, for the part about some compilers giving a newly allocated variable a value of 0, does this apply to Xcode or any of its compilers? I assume not, but I wanted to check.
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Tron55555 wrote:
I always thought a definition of a variable was a statement that assigned a value to a variable. If a basic declaration like "int myInt;" does allocate memory for the variable and therefore is a definition, can anyone give me an example of a declaration that does not allocate memory for the variable and therefore is not a definition?
I always like to think of a "declaration" to be something that makes no changes to the actual code, but just provides visibility so that compilation and/or linking will succeed. The "definition" allocates space.
You can declare a function to establish it in the namespace for the compiler to find but the linker needs an actual definition somewhere to link against. With a variable, you could also declare a variable as "extern int myvar;". The actual definition "int myvar;" would be somewhere else.
According to that book, both "extern int myvar;" and "int myvar;" are declarations, but only the latter is a definition. That is a valid way to look at it. Both statements 'delcare' something to the compiler, but on the second one 'define's some actual data.
First, I know that an int can be different sizes (either 4 bytes or 8 bytes, I think), but what does this depend on? I thought it depended on the compiler, but the above quote makes it sound like it depends on the IDE, Xcode. Which is it?
An "int" is supposed to be a processor's "native" size and the most efficient data type to use. A compiler may or may not be able to change that, depending on the target and the compiler. If a compiler supports that option and Xcode supports that compiler and that option, then Xcode can control it, via the compiler.
Second, it said that Xcode is currently set to use 4-byte ints. Does this mean that there is a setting that the user can change to make ints a different size (like 8 bytes), or does it mean that the creators of Xcode currently have it set to use 4-byte ints?
I think that "setting" is just not specifying any option to explicitly set the size. You can use "-m32" or "-m64" to control this, but I wouldn't recommend it. Let Xcode handle those low-level details.
Third, for the part about some compilers giving a newly allocated variable a value of 0, does this apply to Xcode or any of its compilers? I assume not, but I wanted to check.
I don't know for sure. Why would you ask? Are you thinking of including 45 lines of macro declarations 3 levels deep to initialize values based on whether or not a particular compiler/target supports automatic initialization? Xcode current supports GCC 3.3, GCC 4.0, GCC 4.2, LLVM GCC, CLang, and Intel's compiler for building PPC, i386, and x86_64 code in both debug and release, with a large number of optimization options. It doesn't matter what compiler you use or what it's behavior is - initialize your variables in C.

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