difference with ArrayList and List

Question

Hi Here i am trying to sublist the items from list and print them 5 for each iteration. here in the following code it is printing same items every time

for(int i=0;i<4;i++)
    {
    List<Card> l=a.subList(a.size()-5, a.size());
    System.out.println(l);

 }

But here it is printing different items as if it is removing 5 from the list each time

 for(int i=0;i<4;i++){
     int deckSize = a.size();
     List<Card> handView = a.subList(deckSize-5, deckSize);
     ArrayList<Card> hand = new ArrayList<Card>(handView);
     handView.clear();
     System.out.println(hand);
 }

what is the difference between the above two code snippets

second one is the one i have seen from tutorial ...first one i am trying to do — satheesh
– satheesh, Commented Mar 17, 2011 at 10:16

DaveJohnston · Accepted Answer · 2011-03-17 10:12:14Z

8

You should have a read at the API for List.

The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa.

So in each case the list you are creating is not a new copy of the elements from the original list, but just a view into the original list. In the second example you are calling clear on the new list, which is actually clearing those elements in the original list, hence the behaviour you are seeing.

answered Mar 17, 2011 at 10:12

DaveJohnston

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Comments

Dan D. · Accepted Answer · 2011-03-17 10:11:32Z

1

it seems that using .clear() on the result of .subList() removes the returned items form the original list

answered Mar 17, 2011 at 10:11

Dan D.

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1 Comment

Peter Lawrey Over a year ago

I believe this is expected behaviour

Daniel Rikowski · Accepted Answer · 2011-03-17 10:14:27Z

0

This is the designed behaviour of subList(). See the Javadoc for List.subList().

There's even an example doing almost exactly the same thing. (Removing items from a view)

answered Mar 17, 2011 at 10:14

Daniel Rikowski

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amit · Accepted Answer · 2011-03-17 10:18:30Z

0

Check out the doc for subList. It returns a list backed up by original list and is useful as a view. If you modify the sublist it will modify the main list. Since you cleared elements from it the original list is also modified.

answered Mar 17, 2011 at 10:18

amit

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Mr Lou · Accepted Answer · 2011-04-15 07:20:13Z

0

List is interface.ArrayList is the class of implemention of List interface.

answered Apr 15, 2011 at 7:20

Mr Lou

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