Collection of object -> Array of another object??

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qazmlp1209@rediffmail.com - 04 Mar 2006 20:17 GMT

I have a case where it is required to convert the Collection of objects
of a class into an array of objects of an another class.

I have come up with the following sample code so far. I would like to
know whether there is any other better/shorter way of doing such a
conversion.

------------------------
// Object1Coll is of type Collection<Class1>
final int collectionSize = Object1Coll.size() ;
Class2[] class2Arr = new Class2[ collectionSize ] ;

int index = 0 ;

Class1 class1Obj = new Class1() ;

Iterator Object1CollIter = Object1Coll.iterator();
while ( Object1CollIter.hasNext() )
{
    class1Obj = Object1CollIter.next() ;

    class2Arr[ index ] = new Class2( class1Obj.getMemVal1()
,    class1Obj.getMemVal2() ) ;

    ++index ;
}
------------------------

Any suggestion for improvements in the above code?

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VisionSet - 04 Mar 2006 21:27 GMT

...

> I would like to
> know whether there is any other better/shorter way of doing such a
> conversion.

...

> Iterator Object1CollIter = Object1Coll.iterator();
> while ( Object1CollIter.hasNext() )
[quoted text clipped - 5 lines]
>
> Any suggestion for improvements in the above code?

If you really do want to convert between 2 polymorphically distinct types,
then no.
Other than using the Java1.5 syntactic short cut for iteration:

> Iterator Object1CollIter = Object1Coll.iterator();
> while ( Object1CollIter.hasNext() )
> {
> class1Obj = Object1CollIter.next() ;

is equivalent to:

for (Class1 obj : object1Coll.iterator()) {}

neat huh?!

> Class1 class1Obj = new Class1() ;

Oh, and that line is pointless. What are you doing with that object you just
instantiated? nothing!
a/ set it to null, or
b/ better reduce the scope by keeping the reference inside the loop

--
Mike W

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qazmlp1209@rediffmail.com - 05 Mar 2006 01:21 GMT

Once the Collection -> Array conversion is completed, I have a case
later to do the reverse conversion i.e. converting from an array of a
object to Collection of an another object.
During this conversion, I just noticed that it is not possible to
initialize the Collection to a certain size(equivalent to
array.length). Is that really the case?

Reply to this Message

Roedy Green - 05 Mar 2006 03:18 GMT

>During this conversion, I just noticed that it is not possible to
>initialize the Collection to a certain size(equivalent to
>array.length). Is that really the case

does this answer your concern?

public ArrayList(Collection<? extends E> c) {
size = c.size();
// Allow 10% room for growth
int capacity = (int) Math.min((size*110L)/100,
Integer.MAX_VALUE);
elementData = (E[]) c.toArray(new Object[capacity]);

The constructor picks the right size automatically.

Signature

Canadian Mind Products, Roedy Green.
http://mindprod.com Java custom programming, consulting and coaching.

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IchBin - 05 Mar 2006 03:20 GMT

> Once the Collection -> Array conversion is completed, I have a case
> later to do the reverse conversion i.e. converting from an array of a
> object to Collection of an another object.
> During this conversion, I just noticed that it is not possible to
> initialize the Collection to a certain size(equivalent to
> array.length). Is that really the case?

I had to take 1.6 off my system because of space. There is an article
"Using the Desktop API in Java SE 6 (Mustang)" that touches on that API
and the author supplies a source link at the end of the article. Maybe
this could help?

http://java.sun.com/developer/technicalArticles/J2SE/Desktop/mustang/desktop_api/

Thanks in Advance...
IchBin, Pocono Lake, Pa, USA
http://weconsultants.servebeer.com/JHackerAppManager
__________________________________________________________________________

'If there is one, Knowledge is the "Fountain of Youth"'
-William E. Taylor, Regular Guy (1952-)

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qazmlp1209@rediffmail.com - 13 Mar 2006 15:19 GMT

"object1Coll.iterator()" does not seem to work.
I have a method like this:
private otherClass[] convertFromMineToOthers( Collection<myClass>
myObjColl )
{
for ( myClass myObj : myObjColl.iterator() )
{
// processing
}
}

"Can only iterate over an array or an instance of java.lang.Iterable"
error is reported for "myObjColl.iterator()". Why does it occur? As I
checked Collection<> is an iterable type. So, I expected it to work.
But, unfortunately the above-mentioned error is reported.

Reply to this Message

Jacques-Olivier Haenni - 13 Mar 2006 15:27 GMT

Hi,

The correct syntax is:

for (MyClass myObj : myObjColl) {
...
}

Note the '.iterator()' was wrong.

Cheers,

Jacques-Olivier

>"object1Coll.iterator()" does not seem to work.
>I have a method like this:
[quoted text clipped - 13 lines]
>
>

Reply to this Message

Greg R. Broderick - 04 Mar 2006 21:35 GMT

qazmlp1209@rediffmail.com wrote in news:1141503457.827727.98670
@e56g2000cwe.googlegroups.com:

> I have a case where it is required to convert the Collection of objects
> of a class into an array of objects of an another class.
[quoted text clipped - 25 lines]
>
> Any suggestion for improvements in the above code?

Implement a method on Class1, that returns an appropriately-constructed
reference to an object of class2. This is more object-oriented, and allows
Class1 to hide more of its implementation-specific details.

Use Collection.toArray(new Class1[collectionSize]), then iterate this array
using a for loop. This should be faster than using an Iterator.

If Class2 is a superclass of Class1, you should be able to directly create
the array using Collection.toArray(new Class2[collectionSize]);

Cheers
GRB

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Collection of object -> Array of another object??

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