clone(): ArrayList.clone() I thought does a shallow copy

Question

ArrayList<Integer> a=new ArrayList<Integer>(); a.add(5); ArrayList<Integer> b=(ArrayList<Integer>)a.clone(); a.add(6); System.out.println(b.toString()); 

In the above piece of code, i think clone() does a shallow copy. So, b and a should point to the same memory location. However, when i do b.toString(), the answer is only 5. Why is 6 also not displayed if clone() does a shallow copy?

Answer 1

Shallow copy does not mean that they point to the same memory location. That would be just an assignment:List b = a;.

Cloning creates a new instance, holding the same elements. This means you have 2 different lists, but their contents are the same. If you change the state of an object inside the first list, it will change in the second list. (Since you are using an immutable type - Integer - you can't observe this)

However, you should consider not using clone(). It works fine with collections, but generally it's considered broken. Use the copy-constructors - new ArrayList(originalList)

Answer 2

If it was like you thought, then the clone method would be completely useless, because in that case, the following lines would be equivalent:

ArrayList<Integer> b = (ArrayList<Integer>)a.clone(); ArrayList<Integer> b = a; 

Cloning is - like in real world scenarios - a process of creating two entities with exactly the same properties (at the time of the cloning operation).

And as Bozho mentioned - avoid the Java clone() concept. Even it's author mentioned, that it is broken.

This question and it's answers are quite valuable and provide a link to Josh Blochs own comments on his piece of work ;-)