Creating ArrayList

[dewitrydan]

Hey there.
I am stuck with creating arraylists.

I have seen them created with reference to List:
List myNewList = new ArrayList();

Also with a reference to ArrayList:
ArrayList myNewList = new ArrayList();


My question is:
Why would you declare an ArrayList with a reference to List?
Are there any advantages over the second method: "ArrayList myNewList = new ArrayList();" ???

I am completely new to java, so this question may seem pretty dumb.
But thanks anyways :D

[shai]

I don't have the understanding to give you a good answer, but I can tell you that if you read about inheritance and polymorphism you will get your answer.

[JosAH]

Suppose your method(s) are only interested in the functionality of a List (the interface) and don't care about the peculiarities of ArrayLists or LinkedLists or whatever (the implementations of the List interface); all they care about is they're dealing with a List. So why not treat a particular implementation as a List? You can do something like this:

List<T> list= new ArrayList<T>();

... and deal with the list object as if it were just a List<T> (as a matter of fact is is a List<T> object). If you want to, or have to use another type of List<T> in the future, all you have to do is change one line, the line above. As a rule of thumb: consider and object of class C as the uppermost superclass B thereof, i.e. B is an ancestor class (or interface) of C.

kind regards,

Jos

[dewitrydan]

thanks allot
I think I understand.
Just to clarify:

(1) When writing: List<T> list= new ArrayList<T>();
Is it true to say:
Data type is: of type "List interface"?
Does this mean it's actually an ArrayList object but only the methods declared in the List interface are visible in the object?



(2) In what situation would it be an advantage to use:

(i) ArrayList<T> list= new ArrayList<T>();
rather than

(ii) List<T> list= new ArrayList<T>()
??

I know you said to use (ii) if your methods don't care about the extra stuff that (i) gives the object.
But why can't I use: "ArrayList<T> list= new ArrayList<T>();" all the time and just use the stuff I need from it?

Sorry if my questions are confusing
I aint sure what I'm talking about
thanks allot.

[JosAH]

Yes, as you define the type of your variable as List<T> the type of the variable is 'interface List<T>'. You can define the type as 'ArrayList<T>' if you want (not much wrong with that) but as soon as you have to change the implementation type, e.g. to 'LinkedList<T>' you have to change it everywhere in your code and recompile everything again. For a small project it doesn't matter much, but what if your code is installed everywhere around the world? Your customers won't appreciate a new version again just because you have changed a bit of it.

kind regards,

Jos

[dewitrydan]

So you can definitely define a variable of type interface?

change the implementation type, e.g. to 'LinkedList<T>'

Can you name a situation where you would have to do that?

you have to change it everywhere in your code and recompile everything again

Won't you just have to change it in the declaration of the variable. (So only once) ?


Thanks again for your time.

[JosAH]

So you can definitely define a variable of type interface?

Yes, that's what I was saying all the time.

Can you name a situation where you would have to do that?

When you have a better implementation available for the job you can easily change to that one without changing one other line of code.

Won't you just have to change it in the declaration of the variable. (So only once) ?

But then again, the variable has to be known by the interface type name, not it's implementing class name.

kind regards,

Jos

[dewitrydan]

ok, I think I finally get it
Thanks a bunch!

[Lil_Aziz1]

JosAH. Are you talking about something like this?

List<String> test = new ArrayList<String>();
...
test = new LinkedList<String>();

[JosAH]

JosAH. Are you talking about something like this?

List<String> test = new ArrayList<String>();
...
test = new LinkedList<String>();

Yep.

kind regards,

Jos