How to copy Java Collections list

user_62388 0
Viewed 270131

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

Suppose I have 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?

copy list collections java
18 Answers

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 tddmonkey 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 collections within Guava useful.

user_226560

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

MrWiggles_515770

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

List<String> b = new ArrayList<>(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.

List<String> a = new ArrayList<>(a);
a.add("foo");
b.add("bar");

List<String> b = new ArrayList<>(a); // shallow copy 'a'

// the following will all hold
assert a.get(0) == b.get(0);
assert a.get(1) == b.get(1);
assert a.equals(b);
assert a != b; // 'a' is not the same object as 'b'
user_314800 
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);
cletus_183930

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);
user_192460

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

ordnungswidrig_90690
Related