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Strange things with java...
Hi all,
i have this piece of code
Suppose that lstA is List of 3 Objects
List<Object> lstA = ...something;
List<Object> lstB = lstA;
...
[block A] //this is a block of code
...
Inside block A i remove one item from lstA, so lstA become a List of 2
Objects. Inside block A i never touch lstB.
Why at the end of block A also lstB is with 2 elements? How is this
possible?
Thanks in advance
R
Roberto Nicastro wrote: > Hi all, > i have this piece of code > Suppose that lstA is List of 3 Objects
> List<Object> lstA = ...something;
> List<Object> lstB = lstA;
> ...
> [block A] //this is a block of code
> ...
> Inside block A i remove one item from lstA, so lstA become a List of 2
> Objects. Inside block A i never touch lstB.
> Why at the end of block A also lstB is with 2 elements? How is this
> possible?
are initialized as pointers to the same List object.
Although block A does not touch lstB, it does remove an element from the
List it points to.
Patricia
On May 11, 10:51 pm, Roberto Nicastro <robnicas@gmail.com> wrote:
> Hi all, > i have this piece of code > Suppose that lstA is List of 3 Objects
> List<Object> lstA = ...something;
> List<Object> lstB = lstA;
> ...
> [block A] //this is a block of code
> ...
> Inside block A i remove one item from lstA, so lstA become a List of 2
> Objects. Inside block A i never touch lstB.
> Why at the end of block A also lstB is with 2 elements? How is this
> possible?
> Thanks in advance
> R
that. So when u modify the list, it is reflected in either of
references pointing to that List.
HTH.
Roberto Nicastro wrote: > Suppose that lstA is List of 3 Objects > List<Object> lstA = ...something;
> List<Object> lstB = lstA;
> ...
> [block A] //this is a block of code
> ...
> Inside block A i remove one item from lstA, so lstA become a List of 2
> Objects. Inside block A i never touch lstB.
> Why at the end of block A also lstB is with 2 elements? How is this
> possible?
lstB = lstA, you make lstB point at the same list; you make lstB ==
lstA. Perhaps what you wanted to do was make lstB a copy of lstA? Or
perhaps you wanted to go further and make lstB a copy of lstA and all
its members copies of the members in lstA? Have you thought about what
happens if you change an object in either list?
--
Regards,
Richard
Roberto Nicastro wrote On 05/11/07 13:51,:
> Hi all, > i have this piece of code > Suppose that lstA is List of 3 Objects
> List<Object> lstA = ...something;
> List<Object> lstB = lstA;
> ...
> [block A] //this is a block of code
> ...
> Inside block A i remove one item from lstA, so lstA become a List of 2
> Objects. Inside block A i never touch lstB.
> Why at the end of block A also lstB is with 2 elements? How is this
> possible?
understand what others have pointed out: lstA is the
number on the debit card in your wallet, and lstB is
the same string of digits recorded in your password-
protected on-line form-filler assistant. In [block A]
you hand the card to a local merchant to pay for your
new jPea, 150 silver splonders including VAT. Then
you go to your computer, use the automatic form-filler
to log in to your bank account, and check the balance.
Are the 150 silver splonders still there to be spent?
One bank account, two copies of the account number.
One List instance, two references to it. Spend money
with the debit card, and the balance change is visible
when you use the copied number. Change the List via
listA, and the change is visible through listB.
--
Eric.Sos@sun.com
On 11 Mag, 20:17, Richard Senior <nos@r-senior.demon.co.uk> wrote:
> lstA. Perhaps what you wanted to do was make lstB a copy of lstA? Or > perhaps you wanted to go further and make lstB a copy of lstA and all > its members copies of the members in lstA? Have you thought about what > happens if you change an object in either list? > --
> Regards,
> Richard
have to create a new List object
List lstA = ...something
List lstB = new ArrayList();
for (Iterator it; it = lstA.iterator(); it.hasNext(); ){
lstB.add(it.next());
}
I think this will work for me.
Thank you all
R
Roberto Nicastro wrote: > I want to create an exact copy of lstA. So, to create a real copy i > have to create a new List object > List lstA = ...something
> List lstB = new ArrayList();
> for (Iterator it; it = lstA.iterator(); it.hasNext(); ){
> lstB.add(it.next());
> }
> I think this will work for me.
lists, contain the same elements. If you change an object in lstA, it
also changes in lstB.
Have a look at this
http://forum.java.sun.com/thread.jspa?threadID=487186&messageID=2281852
and do some searches on "shallow copy" and "deep copy".
--
Regards,
Richard
Roberto Nicastro writes: > I want to create an exact copy of lstA. So, to create a real copy i > have to create a new List object > List lstA = ...something
> List lstB = new ArrayList();
> for (Iterator it; it = lstA.iterator(); it.hasNext(); ){
> lstB.add(it.next());
> }
> I think this will work for me.
List lstB = new ArrayList();
lstB.addAll(lstA);
Look it up in Javadoc.
Jussi Piitulainen wrote: > Roberto Nicastro writes: >> I want to create an exact copy of lstA. So, to create a real copy i
>> have to create a new List object
>> List lstA = ...something
>> List lstB = new ArrayList();
>> for (Iterator it; it = lstA.iterator(); it.hasNext(); ){
>> lstB.add(it.next());
>> }
>> I think this will work for me.
> There's an addAll method you could use instead of a loop of your own:
> List lstB = new ArrayList();
> lstB.addAll(lstA);
> Look it up in Javadoc.
List lstB = new ArrayList(lstA);
Patricia
Roberto Nicastro <robnicas @gmail.com> wrote: > List<Object> lstA = ...something; > List<Object> lstB = lstA;
gotten much by way of how to make it act as you'd like. To do that, you
could change the second line there to:
List<Object> lstB = new ArrayList(lstA); // or other List implementation
That would cause lstB to point to a different List, which has its
CONTENTS initialized from the list that lstA points to. Then you could
modify the list that lstA points to, and the list that lstB points to
would be unchanged.
--
Chris Smith
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