Re: enumeration - follow up

[Mark]

Paul:

Thanks for your clear response! The question to ask next is... why, if it
was a String in the hashtable do you need to do a "class cast"? By putting
and pulling the data into and outof the Hashtable does it somehow alter it?
In the code, which tries to pull it out of the Hashtable, it is being assigned
to a String variable so why do you need to cast it?

Thanks again,

Mark

"Paul Clapham" <pclapham@core-mark.com> wrote:
>Yes, that's exactly what it's saying. The Java name for this is "class
>casting", and there is a bunch of rules for how it works. You can't just
>cast any class to any other class. So the programmer who wrote this code
>would have known that the objects in the hashtable were strings. Since
the
>hashtable and enumeration are generic and widely usable, they can only deal
>with Objects. And since all classes descend from Object, any object can
be
>cast into an Object variable.
>
>So the phrase "(String)buildingZone.get(buildingKey)" means: "Get the
>Object from the Hashtable buildingZone that belongs to the key buildingKey.
>And I know it was a String when I put it there, so cast it into a String
>variable."
>
>But if it wasn't a String but some other class of Object, say a File or
an
>Integer, this will fail and a ClassCastException will be thrown at the
>program.
>
>Mark <msorteberg@inspec.com> wrote in message news:394a37d2@news.devx.com...
>>
>> Here is the syntax in question:
>>
>> for (int startYear=initYear; startYear<initYear+3; startYear++) {
>> for (Enumeration e=buildingZone.keys(); e.hasMoreElements(); ) {
>>
>> String buildingKey = (String)e.nextElement();
>> String zone = (String)buildingZone.get(buildingKey);
>>
>> generateBuildingSummaryReport(startYear, buildingKey, zone);
>> }
>> }
>>
>> I think I understand what hashtables are and that enumeration provides
a
>> way to loop through a hashtable (in this case buildingZone is a
>hashtable).
>> nextElement() is the way enumeration goes through the keys located within
>> buildingZone. If I've got it pegged this far, then the question is, "Why
>> is (String) in front of e.nextElement() and why is (String) in front of
>buildingZone.get(buildingKey)?
>> Is this saying that whatever e.nextElement() finds we want to make it
>into
>> a string variable? I am confused as to what (String) is being used for
>here.
>> Thanks in advance for you help.
>
>

[Paul Clapham]

When you stored it in the Hashtable, you stored an Object. It's still a
String at heart but the Hashtable doesn't know that. When you retrieved it
from the Hashtable, you used the get() method, which returns an Object. You
may know it's a String, but the Java compiler doesn't know that at this
point. So you have to tell it that. Hashtable was designed this way to
have maximum flexibility, so we didn't have to create one Hashtable-like
class for Strings and another for Files and so on.

Java has specific rules on when you can assign an object of one type to a
variable of another type. In particular, it doesn't always allow you to
just assign x=y, if x and y are different types. You can assign x=y if y's
class is a subclass of x's -- as, for example, String is a subclass of
Object so you can assign a String to and Object variable. But when you want
to do it the other way around, you must cast the assignment. There are
other rules for primitive classes (integer, float, etc.) and for interfaces.
(Those rules are there so that Java can implement the OO concept of
polymorphism.)

Mark <msorteberg@inspec.com> wrote in message
news:394a50b9$1@news.devx.com...
>
> Paul:
>
> Thanks for your clear response! The question to ask next is... why, if it
> was a String in the hashtable do you need to do a "class cast"? By
putting
> and pulling the data into and outof the Hashtable does it somehow alter
it?
> In the code, which tries to pull it out of the Hashtable, it is being
assigned
> to a String variable so why do you need to cast it?
>
> Thanks again,
>
> Mark
>
> "Paul Clapham" <pclapham@core-mark.com> wrote:
> >Yes, that's exactly what it's saying. The Java name for this is "class
> >casting", and there is a bunch of rules for how it works. You can't just
> >cast any class to any other class. So the programmer who wrote this code
> >would have known that the objects in the hashtable were strings. Since
> the
> >hashtable and enumeration are generic and widely usable, they can only
deal
> >with Objects. And since all classes descend from Object, any object can
> be
> >cast into an Object variable.
> >
> >So the phrase "(String)buildingZone.get(buildingKey)" means: "Get the
> >Object from the Hashtable buildingZone that belongs to the key
buildingKey.
> >And I know it was a String when I put it there, so cast it into a String
> >variable."
> >
> >But if it wasn't a String but some other class of Object, say a File or
> an
> >Integer, this will fail and a ClassCastException will be thrown at the
> >program.
> >
> >Mark <msorteberg@inspec.com> wrote in message
news:394a37d2@news.devx.com...
> >>
> >> Here is the syntax in question:
> >>
> >> for (int startYear=initYear; startYear<initYear+3; startYear++) {
> >> for (Enumeration e=buildingZone.keys(); e.hasMoreElements(); ) {
> >>
> >> String buildingKey = (String)e.nextElement();
> >> String zone = (String)buildingZone.get(buildingKey);
> >>
> >> generateBuildingSummaryReport(startYear, buildingKey, zone);
> >> }
> >> }
> >>
> >> I think I understand what hashtables are and that enumeration provides
> a
> >> way to loop through a hashtable (in this case buildingZone is a
> >hashtable).
> >> nextElement() is the way enumeration goes through the keys located
within
> >> buildingZone. If I've got it pegged this far, then the question is,
"Why
> >> is (String) in front of e.nextElement() and why is (String) in front of
> >buildingZone.get(buildingKey)?
> >> Is this saying that whatever e.nextElement() finds we want to make it
> >into
> >> a string variable? I am confused as to what (String) is being used for
> >here.
> >> Thanks in advance for you help.
> >
> >
>

[John]

Mark,

The answer to your most recent question is that Enumeration.nextElement()
is declared to return Object. The Java compiler won't accept assignment
of the return from this function to a more specific type (e.g. String) without
casting.

In the case of String, that isn't completely true. Object, which as Paul
said is the base type for all objects in Java, has a method called toString().
It can be overridden for any class (since all classes inherit from Object)
so that you can customize how any particular class is converted to a string
(although in many cases this doesn't make any sense). But the upshot is
that any object can be converted to a String.

John

"Mark" <msorteberg@inspec.com> wrote:
>
>Paul:
>
>Thanks for your clear response! The question to ask next is... why, if
it
>was a String in the hashtable do you need to do a "class cast"? By putting
>and pulling the data into and outof the Hashtable does it somehow alter
it?
> In the code, which tries to pull it out of the Hashtable, it is being assigned
>to a String variable so why do you need to cast it?
>
>Thanks again,
>
>Mark
>
>"Paul Clapham" <pclapham@core-mark.com> wrote:
>>Yes, that's exactly what it's saying. The Java name for this is "class
>>casting", and there is a bunch of rules for how it works. You can't just
>>cast any class to any other class. So the programmer who wrote this code
>>would have known that the objects in the hashtable were strings. Since
>the
>>hashtable and enumeration are generic and widely usable, they can only
deal
>>with Objects. And since all classes descend from Object, any object can
>be
>>cast into an Object variable.
>>
>>So the phrase "(String)buildingZone.get(buildingKey)" means: "Get the
>>Object from the Hashtable buildingZone that belongs to the key buildingKey.
>>And I know it was a String when I put it there, so cast it into a String
>>variable."
>>
>>But if it wasn't a String but some other class of Object, say a File or
>an
>>Integer, this will fail and a ClassCastException will be thrown at the
>>program.
>>
>>Mark <msorteberg@inspec.com> wrote in message news:394a37d2@news.devx.com...
>>>
>>> Here is the syntax in question:
>>>
>>> for (int startYear=initYear; startYear<initYear+3; startYear++) {
>>> for (Enumeration e=buildingZone.keys(); e.hasMoreElements(); ) {
>>>
>>> String buildingKey = (String)e.nextElement();
>>> String zone = (String)buildingZone.get(buildingKey);
>>>
>>> generateBuildingSummaryReport(startYear, buildingKey, zone);
>>> }
>>> }
>>>
>>> I think I understand what hashtables are and that enumeration provides
>a
>>> way to loop through a hashtable (in this case buildingZone is a
>>hashtable).
>>> nextElement() is the way enumeration goes through the keys located within
>>> buildingZone. If I've got it pegged this far, then the question is,
"Why
>>> is (String) in front of e.nextElement() and why is (String) in front
of
>>buildingZone.get(buildingKey)?
>>> Is this saying that whatever e.nextElement() finds we want to make it
>>into
>>> a string variable? I am confused as to what (String) is being used for
>>here.
>>> Thanks in advance for you help.
>>
>>
>