Java ArrayList copy

Question

Yes, assignment will just copy the value of l1 (which is a reference) to l2. They will both refer to the same object.

Creating a shallow copy is pretty easy though:

List<Integer> newList = new ArrayList<>(oldList);

(Just as one example.)

Try to use Collections.copy(destination, source);

Yes l1 and l2 will point to the same reference, same object.

If you want to create a new ArrayList based on the other ArrayList you do this:

List<String> l1 = new ArrayList<String>();
l1.add("Hello");
l1.add("World");
List<String> l2 = new ArrayList<String>(l1); //A new arrayList.
l2.add("Everybody");

The result will be l1 will still have 2 elements and l2 will have 3 elements.

Another convenient way to copy the values from src ArrayList to dest Arraylist is as follows:

ArrayList<String> src = new ArrayList<String>();
src.add("test string1");
src.add("test string2");
ArrayList<String> dest= new ArrayList<String>();
dest.addAll(src);

This is actual copying of values and not just copying of reference.

There is a method addAll() which will serve the purpose of copying One ArrayList to another.

For example you have two Array Lists: sourceList and targetList, use below code.

targetList.addAll(sourceList);

Java doesn't pass objects, it passes references (pointers) to objects. So yes, l2 and l1 are two pointers to the same object.

You have to make an explicit copy if you need two different list with the same contents.