Java Collections copy list - I don't understand

Question

I have an ArrayList and I want to copy it exactly. I use util classes when possible ont the assumption that someone spent some time making it correct. So naturally I end up with the Collections class which contains a copy method.

ss I hav the following::

List<String> a = new ArrayList<String>();
a.add("a");
a.add("b");
a.add("c");
List<String> b = new ArrayList<String>(a.size());

Collections.copy(b,a);

this fails because basically it thinks b isn't big enough to hold a. Yes I know b has size 0, but it should be big enough now shouldn't it? If I have to fill b first then Collections.copy becomes a completely useless function in my mind. So except for programming a copy function (which I'm going to do now) is there a proper way to do this?

Answer 1

Just do:

List a = new ArrayList(); 
a.add("a"); 
a.add("b"); 
a.add("c"); 
List b = new ArrayList(a);

ArrayList has a constructor that will accept another Collection to copy the elements from

Answer 2

b has a capacity of 3, but a size of 0. The fact that ArrayList has some sort of buffer capacity is an implementation detail - it's not part of the List interface, so Collections.copy(List, List) doesn't use it. It would be ugly for it to special-case ArrayList.

As MrWiggles has indicated, using the ArrayList constructor which takes a collection is the way to in the example provided.

For more complicated scenarios (which may well include your real code), you may find the Google Java Collections library useful.

Answer 3

the simplest way to copy a List is to pass it to the constructor of the new list:

List<String> b = new ArrayList<String>(a);

b will be a shallow copy of a

Looking at the source of Collections.copy(List,List) (I'd never seen it before) it seems to be for coping the elements index by index. using List.set(int,E) thus element 0 will over write element 0 in the target list etc etc. Not particularly clear from the javadocs I'd have to admit.

Gareth

Answer 4

If you want to copy an ArrayList, copy it by using:

List b = new ArrayList();
b.add("aa");
b.add("bb");

List a = new ArrayList(b);

Answer 5

List b = new ArrayList(a.size())

doesn't set the size. It sets the initial capacity (being how many elements it can fit in before it needs to resize). A simpler way of copying in this case is:

List b = new ArrayList(a);

Answer 6

Copy isn't useless if you imagine the use case to copy some values into an existing collection. I.e. you want to overwrite existing elements instead of inserting.

An example: a = [1,2,3,4,5] b = [2,2,2,2,3,3,3,3,3,4,4,4,] a.copy(b) = [1,2,3,4,5,3,3,3,3,4,4,4]

However I'd expect a copy method that would take additional parameters for the start index of the source and target collection, as well as a parameter for count.

See Java BUG 6350752

Answer 7

Calling

List b = new ArrayList(a);

works fine if you want a shallow copy, but if you want a complete copy (with separate copies of each element) then you can FOLLOW that statement with Collections.copy, ie:

List b = new ArrayList(a);
Collections.copy(b,a);

Then List b will have its own complete copy of the elements that don't reference List A's elements.