On Tue, Apr 28, 2009 at 11:07 PM, pravin karne <[email protected]>wrote:
> hi,
> i want some explanation for following code
>
> object sessionObj extends SessionVar[HashMap[String, Int]](
> new HashMap[String, Int]
> {
> override def default(key: String): Int = 0
> }
> )
>
Changing a few things that Tim said.
>
> 1. how object extends class in scala.?
object creates a singleton... an instance of the class that can be shared
within the scope that it was created. If object is used at the top-level
scope, then it is a singleton shared by all classes in that classloader
(your Lift app.)
In Scala everything is either an instance of an object or a method. This
means there are no statics, no primitives, etc. In order to have a
singleton instance shared within a classloader, use the object mechanism.
>
> 2. are we going to create object of SessionVar class?
Yes, we create a singleton instance of the SessionVar class. The SessionVar
class uses a little machinery to access the backing store of your session.
So, the SessionVar doesn't actually store a value. It is a proxy that gets
the current session based on the context of your thread, it has a globally
unique name and uses this name to look up the underlying value in a hashmap
in the session and then casts it to the type of the specific SessionVar...
in this case HashMap[String, Int]. In this way, you have type-safe access
to session data. This is a material advance over the way session data is
stored in a Java session... there's no casting on the user's part.
>
> 3. are we calling constructor of given class?
Yes. However, if you take a look at the SessionVar's constructor, it's as
follows:
class SessionVar[T](dflt: => T)
This is a "call-by-name" parameter. So, it's a function rather than a
value. Why do we do this? This allows us to create a new instance of the
default value of the SessionVar each time it's needed. When you call the is
method on the SessionVar, Lift looks to see if the SessioVar has been
defined. If it has not been defined, then the default value creator
function is applied and we get a new default value, the SessionVar for the
current session is set to that default value and the freshly minted value is
returned.
>
> 4. is it similar to java class which extends other class ,and then create
> object of subclass?
Scala creates a subclass of SessionVar for your singleton. If you do
sessionObj.getClass.getName, it will not be SessionVar, but some
compiler-generated name. See the answer to #1.
>
> I am beginner to scala and lift.
>
> Welcome to the community.
Thanks,
David
>
> On Tue, Apr 28, 2009 at 7:28 PM, Timothy Perrett
> <[email protected]>wrote:
>
>>
>>
>> Try:
>>
>> // this gets you whatever is in the session object so add to it here
>> SessionObj.is
>>
>> Do you specifically need to use Java HashMap? If not, seems like
>> List[(String,Int)] would be more lift-esq.
>>
>> Cheers, Tim
>>
>> On 28/04/2009 13:30, "pravin" <[email protected]> wrote:
>>
>> >
>> > Hi guys,
>> > I want to add objects into session scope.
>> >
>> > i am using following code :
>> >
>> > object sessionObj extends SessionVar[HashMap[String, Int]](
>> > new HashMap[String, Int]
>> > {
>> > override def default(key: String): Int = 0
>> > }
>> > )
>> >
>> > So as per my understanding :-
>> >
>> > 1. Session object is of HashMap[String, Int] type.
>> > 2. I want to add no of string object into above map so i can
>> > access them during my session
>> >
>> > correct me if i am wrong
>> >
>> >
>> > So please let me know how can i add/remove different String object
>> > from session scope with above code snippet
>> >
>> > Thanks in advance
>> >
>> > >
>> >
>>
>>
>>
>>
>>
>
> >
>
--
Lift, the simply functional web framework http://liftweb.net
Beginning Scala http://www.apress.com/book/view/1430219890
Follow me: http://twitter.com/dpp
Git some: http://github.com/dpp
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