Generics in java.util.ArrayList

Question

I've opened java.util.ArrayList source code, and I can't understand one thing: why elementData[] array is of type Object if ArrayList is parametrized?

public class ArrayList<E> extends ... {
  .........
  private transient Object[] elementData;
  .........
  public boolean add(E e) {/*More code*/}
}

Question: Why don't define elementData as:

private transient E[] elementData

*what advantages and disadvantages?

score 1 · Accepted Answer · edited May 23 '17 at 12:28

1

Everytime that you create a List with raw type, like :

List<MyObject> list = new ArrayList<MyObject>();

The contructor converts all data into a array that must be an Object[] array, on:

public ArrayList(Collection<? extends E> c) { elementData = c.toArray(); ...

I think this happens because ArrayList can be initialized with no raw types, like on:

List list = new ArrayList();

    list.add(new String("VALUE"));
    list.add(new Integer("1"));

    for (Object c : list) {
        System.out.println(c.toString());
    }

and you can put more than one type of object inside.

Also, ArrayList uses the

Arrays.copyOf(elementData, size);

to manage some operations, which will return an Object[].

You may also take a look here and here like Paul said.

edited May 23 '17 at 12:28

Community

1
1

answered Apr 16 '13 at 20:56

Deividi Cavarzan

10,034
13
66
80

this does not explain why it can't be `E[]` – newacct Apr 17 '13 at 00:40
I put this explanation on one of the links. http://stackoverflow.com/questions/13776576/why-arraylist-implementation-uses-object – Deividi Cavarzan Apr 17 '13 at 01:41
I know you linked to it at the end as "you may also take a look". But otherwise none of this is relevant to the question, which is, why not `E[]` – newacct Apr 17 '13 at 05:32

Generics in java.util.ArrayList

1 Answers1