Copying a basic list

Question

You will have to forgive me for asking this but I can't seem to find the exact answer to my question. I need to know how to copy this list:

List<String> jpmcRouting

into a new List (the name doesn't matter).

I have found answers on copying an ArrayList but not a standard list. I've tried a lot of solutions but nothing seems to work. I would appreciate any help!

You know that a 'standard' List is an interface right? That it cannot be instantiated — andrewdleach, Mar 24 '16 at 18:23
What kind of `List` are you trying to copy? Do you want the result to have the same class as the original? — Paul Boddington, Mar 24 '16 at 18:25
Possible duplicate of [How to copy a java.util.List into another java.util.List](http://stackoverflow.com/questions/14319732/how-to-copy-a-java-util-list-into-another-java-util-list) — randers, Mar 24 '16 at 18:26
RAnders00 this is not a duplicate. I saw that answer and did not understand it.. The list has which makes no sense to me what kind of list that is and I tired the method of replacing with and it did not work. — jesric1029, Mar 24 '16 at 18:28

score 2 · Accepted Answer · answered Mar 24 '16 at 18:30

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List<String> myCopy = new ArrayList<String>(jpmcRouting);

does the job. Collections.copy is not what you want; that only works if the destination list already has the right number of elements.

answered Mar 24 '16 at 18:30

Louis Wasserman

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Ján Яabčan · Answer 2 · 2016-03-24T18:28:05.363

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The simplest way for me is:

List<String> copy = jpmcRouting.stream().collect(Collectors.toList());

if your result doesnt depend on order of elements in list, you can use

List<String> copy = jpmcRouting.parallelStream().collect(Collectors.toList());

it will work mutch faster, but it can change order of elements

edited Mar 24 '16 at 18:28

answered Mar 24 '16 at 18:22

Ján Яabčan

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Using a parallel stream means that it may be done using parallelism. It won't change the order of the elements in the answer. – Paul Boddington Mar 24 '16 at 18:28
I got an index out of bounds for this. – jesric1029 Mar 24 '16 at 19:05
are you sure? this is really safe... All methods are internal java methods. – Ján Яabčan Mar 24 '16 at 21:17

score 0 · Answer 3 · answered Mar 24 '16 at 18:23

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I could be wrong, but I think Collections.copy(b, a); will do the trick.

answered Mar 24 '16 at 18:23

RMT

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score 0 · Answer 4 · answered Mar 24 '16 at 18:24

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Do this:

List<String> b = new ArrayList<String>(jpmcRouting.size());
Collections.copy(b, jpmcRouting);

answered Mar 24 '16 at 18:24

Nikolas Charalambidis

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Thanks that's what I was looking for, I'll accept in 5 minutes. – jesric1029 Mar 24 '16 at 18:30
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This will throw an `IndexOutOfBoundsException` at runtime unless the original list is empty, meaning it will basically never work. – Louis Wasserman Mar 24 '16 at 18:31
Yes Louis is correct, I got an error. Any suggestion on how to fix this? – jesric1029 Mar 24 '16 at 19:03

Copying a basic list

4 Answers4