List Assignment vs. List.addAll()

[Tom Henricksen]

What is the difference between these two methods of getting a list.

    
List localList = new ArrayList();
localList = object.getAList();

And this code.

List localList = new ArrayList();
localList.addAll(object.getAList());

Is there any difference in these. I ask as I tried the first approach with problems. When I tried the second it worked.

Thanks,
Tom

[Ernest Friedman-Hill]

The list you create with "new" in the first version is simply discarded when you make the assignment. Assigning to a Java variable in Java always completely discards any existing value without affecting it in any way. In this first version, you end up with localList pointing to the exact same List that object.getAList() returned -- meaning that if "object" still has a reference to that List, code in "object" could modify that list, and the changed would be visible via localList, and vice-versa, because they reference the same object.

In the second version, the elements of "object"'s list are put into a new list. Now either object can chnage their own list independently, without affecting the other one.

[Ilja Preuss]

The difference in pictures:

localList     object
    |           |
    +---------+ |
              V V
+-----+      +-----+  
+-----+      | foo |
             | bar |
             +-----+

localList     object
    |           |
    |           |
    V           V
+-----+      +-----+  
| foo |      | foo |
| bar |      | bar |
+-----+      +-----+