Re: Cloning and Distinct Copies of Objects
Hal Vaughan wrote On 08/15/06 14:40,:
Eric Sosman wrote:
You could *copy* (not *clone*) the String objects by using
String's copy constructor, or by a few other routes. But it's
a dumb thing to do: Why do you think you need all those copies?
I'm using a "restore to last saved" option in the menu so if someone makes
mistakes, they can revert to the last saved data. I need to store several
String[] structures as they are when the data is saved so they can be used
later if needed.
You may need a copy of the String[], which is an array of
references to String objects, but there's no need to make copies
of the String objects those references point to.
Here's the original array, before anybody edits anything:
oldarray[0] -> "Duke"
oldarray[1] -> "rulez!"
Now make a new copy of the array and the references it contains,
which point to the original String objects:
oldarray[0] -> "Duke" <- newarray[0]
oldarray[1] -> "rulez!" <- newarray[1]
Now edit the new array, changing one of its references to point
to a different String:
oldarray[0] -> "Duke" newarray[0] -> "Elvis"
oldarray[1] -> "rulez!" <- newarray[1]
There is no need for extra copies of "rulez!" or of "Duke".
All the String objects are still alive, still referred to,
still hanging on to their immutable content. If you want
to commit the edit, keep newarray and let oldarray drop on
the floor. If you want to revert, keep oldarray and abandon
newarray to the garbage collector.
I think the distinction between an object reference and
an object instance is not yet quite clear to you. When the
light eventually dawns, things will make sense quite suddenly.
Objects are telephones; references are telephone numbers.
If you want to get calls from a hundred people, give out a
hundred copies of your telephone number -- a hundred copies
of the reference value -- but don't burden yourself by trying
to carry a hundred phones on your belt.
--
Eric.Sosman@sun.com