Is it possible to convert ArrayList into ArrayList?

Question

In my application I want to convert an ArrayList of Integer objects into an ArrayList of Long objects. Is it possible?

You can't do it directly. Maybe this post could help you: http://stackoverflow.com/questions/6690745/converting-integer-to-long — MikO, Mar 18 '13 at 18:23
It's not clear what you are exactly asking. You have to demonstrate it with some code. — Bhesh Gurung, Mar 18 '13 at 18:26
The question is pretty clear, however it does not demonstrate any effort. With common questions such as this, you will typically get less down votes if you include *what* you have tried - and the results. — Leigh, Mar 18 '13 at 18:27

score 14 · Accepted Answer · edited Nov 19 '19 at 07:49

14

Not in a 1 liner.

List<Integer> ints = Arrays.asList(1,2,3,4,5,6,7,8,9);
int nInts = ints.size();
List<Long> longs = new ArrayList<Long>(nInts);
for (int i=0;i<nInts;++i) {
    longs.add(ints.get(i).longValue());
}

// Or you can use Lambda expression in Java 8

List<Integer> ints = Arrays.asList(1,2,3,4,5,6,7,8,9);
List<Long> longs = ints.stream()
        .mapToLong(Integer::longValue)
        .boxed().collect(Collectors.toList());

edited Nov 19 '19 at 07:49

lagivan

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answered Mar 18 '13 at 18:26

Tom

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Brilliant!!! I was not able to convert it without your answer! Thank you very much! – Shajeel Afzal Mar 18 '13 at 18:37
Could anybody please give a hint, why is the boxed() method used in the second example? Since it's been worked here not with primitive 'int' values... – montie Aug 03 '23 at 19:41

PermGenError · Answer 2 · 2013-03-18T18:40:40.793

6

No, you can't because, generics are not polymorphic.I.e., ArrayList<Integer> is not a subtype of ArrayList<Long>, thus the cast will fail. Thus, the only way is to Iterate over your List<Integer> and add it in List<Long>.

List<Long> longList = new ArrayList<Long>();
for(Integer i: intList){
longList.add(i.longValue());
}

edited Mar 18 '13 at 18:40

answered Mar 18 '13 at 18:22

PermGenError

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Thanks! But this is not working! – Shajeel Afzal Mar 18 '13 at 18:34
Sorry! now i have selected the correct answer – Shajeel Afzal Mar 18 '13 at 18:41

score 0 · Answer 3 · answered Mar 18 '13 at 18:26

0

ArrayList<Long> longList = new ArrayList<Long>();

Iterator<Integer> it = intList.iterator();
while(it.hasNext())
{
    Integer obj = it.next();
    longList.add(obj); //will automatically convert Int to Long
}

Done....

answered Mar 18 '13 at 18:26

VishalDevgire

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This doesn't even compile because these object types don't convert automatically. `longList.add((long)obj);` or any of the other solutions would work. – T Tse May 10 '23 at 20:13

Is it possible to convert ArrayList into ArrayList?

3 Answers3