Extracting a List from a List of lists

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Damo - 08 Feb 2007 21:11 GMT

Hi,
I have an ArrayList of ArrayLists. I want to extract all the lists,
but I dont know how many ArrayLists will be in the ArrayList.
I know I can do it if i know how many lists are there using the
ArrayList get() method.

this is how i'm doing it

List<String> list1 = new ArrayList<String>();
list1 = res.get(0);
List<String> list1 = new ArrayList<String>();
list2 = res.get(1);
List<String> list1 = new ArrayList<String>();
list3 = res.get(2);

But if theres only two lists in the list i get a NullPointerException

Is there any way i can loop through the list and extract the lists

thanks

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Michael Rauscher - 08 Feb 2007 23:28 GMT

Damo schrieb:

> Hi,
> I have an ArrayList of ArrayLists. I want to extract all the lists,
[quoted text clipped - 6 lines]
> List<String> list1 = new ArrayList<String>();
> list1 = res.get(0);

The first line is... nonsense. You don't need to allocate an ArrayList
at this point since you get a List<String>-object from res.get(0);

> Is there any way i can loop through the list and extract the lists

Several. I assume that res is declared something like

List<List<String>> res;

1. for ( int i = 0, n = res.size(); i < n; i++ )
List<String> l = res.get(i);

2. Iterator<List<String>> it = res.iterator();
while ( it.hasNext() )
List<String> l = it.next();

3. for ( List<String> l : res )
...

Bye
Michael

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Oliver Wong - 08 Feb 2007 23:32 GMT

> Hi,
> I have an ArrayList of ArrayLists. I want to extract all the lists,
[quoted text clipped - 14 lines]
>
> Is there any way i can loop through the list and extract the lists

Check the javadocs for ArrayList. There's a method to get its size or
length, which will be the number of inner lists contained by the outer list.

- Oliver

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Damo - 08 Feb 2007 23:52 GMT

List<List<String>> res;

1. for ( int i = 0, n = res.size(); i < n; i++ )
List<String> l = res.get(i);

2. Iterator<List<String>> it = res.iterator();
while ( it.hasNext() )
List<String> l = it.next();

3. for ( List<String> l : res )

But if I use those ways you mentioned , each time around the loop l
will contain a different list, i wont be using the lists in the loop.
so at the end of the loop i will only have one list, ie the last one.

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Lew - 09 Feb 2007 01:02 GMT

It helps other readers if you distinguish who wrote what.

Michael Rauscher wrote:

> List<List<String>> res;
>
[quoted text clipped - 10 lines]
> will contain a different list, i wont be using the lists in the loop.
> so at the end of the loop i will only have one list, ie the last one.

That has nothing to do with using Michael's suggestion but with what you do
with the result each time through the loop. The assumption in Michael's
suggestion is that you will

do something useful with each list

during each iteration.

You cannot expect newsgroup answers to write out every line of code. You
should impute the missing parts. If you "won't be using the lists in the loop"
then you only have yourself, not Michael, to blame.

- Lew

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Extracting a List from a List of lists

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