How to copy a java.util.List into another java.util.List

I have a List<SomeBean> that is populated from a Web Service. I want to copy/clone the contents of that list into an empty list of the same type. A Google search for copying a list suggested me to use Collections.copy() method. In all the examples I saw, the destination list was supposed to contain the exact number of items for the copying to take place.

As the list I am using is populated through a web service and it contains hundreds of objects, I cannot use the above technique. Or I am using it wrong??!! Anyways, to make it work, I tried to do something like this, but I still got an IndexOutOfBoundsException.

List<SomeBean> wsList = app.allInOne(template);

List<SomeBean> wsListCopy=new ArrayList<SomeBean>(wsList.size());   
Collections.copy(wsListCopy,wsList);
System.out.println(wsListCopy.size());

I tried to use the wsListCopy=wsList.subList(0, wsList.size()) but I got a ConcurrentAccessException later in the code. Hit and trial. :)

Anyways, my question is simple, how can I copy the entire content of my list into another List? Not through iteration, of course.

Solution 1:

Just use this:

List<SomeBean> newList = new ArrayList<SomeBean>(otherList);

Note: still not thread safe, if you modify otherList from another thread, then you may want to make that otherList (and even newList) a CopyOnWriteArrayList, for instance -- or use a lock primitive, such as ReentrantReadWriteLock to serialize read/write access to whatever lists are concurrently accessed.

Solution 2:

This is a really nice Java 8 way to do it:

List<String> list2 = list1.stream().collect(Collectors.toList());

Of course the advantage here is that you can filter and skip to only copy of part of the list.

e.g.

//don't copy the first element 
List<String> list2 = list1.stream().skip(1).collect(Collectors.toList());

Solution 3:

originalArrayList.addAll(copyArrayofList);

Please keep on mind whenever using the addAll() method for copy, the contents of both the array lists (originalArrayList and copyArrayofList) references to the same objects will be added to the list so if you modify any one of them then copyArrayofList also will also reflect the same change.

If you don't want side effect then you need to copy each of element from the originalArrayList to the copyArrayofList, like using a for or while loop. for deep copy you can use below code snippet.

but one more thing you need to do, implement the Cloneable interface and override the clone() method for SomeBean class.

public static List<SomeBean> cloneList(List<SomeBean> originalArrayList) {
    List<SomeBean> copyArrayofList = new ArrayList<SomeBean>(list.size());
    for (SomeBean item : list) copyArrayofList.add(item.clone());
    return clone;
}