🚀 go-pugleaf

On Wed, 16 Apr 2003 09:59:41 +0300, Ayhan Molla wrote:

> Hi all,
> The following fragment gives ClassCastException without any reason
> that I can explain.
> <I got Item class extends Object>
> ArrayList aList = new ArrayList() ;
> for(int i=0; i < 10;++i)
>     aList.add( new Item() ) ;
> 
> Item [] list = (Item []) aList.toArray() ; <-- this line give class cast
> exception

The reason is that the object returned by the ArrayList toArray() 
method is of type Object[], which cannot be cast to Item[].  It might 
be that every item in that array is of class Item, but that does not 
mean that you can cast an Object[] to an Item[].

Note that the ArrayList toArray() method is overloaded:

   public Object[] toArray();
   public Object[] toArray(Object[] a);

The first form will construct and return an object of type Object[].

The second form will always return an object that has the same run-time 
type as the argument passed to it.  If the length of that argument 
array is at least as large as the ArrayList, then it will copy the 
elements to that array.  If it is not, it will construct a new array 
with the same type, fill it, and return that array.

For example, lets say you have created an ArrayList containing three
objects of class Item.

   ArrayList aList = new ArrayList();
   aList.add( new Item() );
   aList.add( new Item() );
   aList.add( new Item() );

You could create an Item[] with any of the following.

1)  Item [] list = (Item[]) aList.toArray( new Item[] );

This will first create an Item[] of length 0 for the argument.  The 
toArray() method will then create a new Item[] of length 3 (the size 
of the ArrayList) and then copy the elements from the ArrayList to that 
array.

2)  final static Item[] ITEM_ARRAY_PROTOTYPE = new Item[];
      :
    Item [] list = (Item[]) aList.toArray( ITEM_ARRAY_PROTOTYPE )

This does the same thing, but it reuses the prototype, eliminating the 
extra object creation.

3)  Item [] list = (Item[]) aList.toArray( new Item[ alist.size() ] );

This constructs an Item[] of the appropriate size and passes it to the 
toArray() method.  Since the items in aList will fit into the argument 
array, toArray() will just copy the elements to that array and return 
the array.

-- 
Regards,

John McGrath [TeamB]

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