Re: hashcode calculation for a Collection of objects
Roedy Green wrote On 08/07/07 20:40,:
On Tue, 07 Aug 2007 12:18:08 -0700, Jimmy <jimmy_please@yahoo.com>
wrote, quoted or indirectly quoted someone who said :
If 2 Collections contain 2 List of objects, and the type of this
object has hashCode() override (hashcode calculation using all fields
in this object), can these 2 different List instances compare by using
list1.hashCode() == list2.hashCode(); without iterate through each
list by comparing each item?
One way to compute a collective hashCode is to xor the individual
hashCodes. ^ is the xor operator. [...]
The Javadoc for List specifies how hashCode()
should be computed. Since List is an interface there
is no language-supported way to enforce what the Javadoc
requires (nor to detect buggy implementations of it),
but the specification is there nonetheless.
Similarly, Set specifies how hashCode() is to be
computed -- and calls for a different algorithm than
List uses. (List's hashCode() depends on the order of
the elements as well as on their hashCodes; Set has no
concept of order.)
So: Any two Lists (of any type) containing equal
elements in the same order will have equal hashCodes if
their hashCode() methods are implemented as called for.
Also, any two Sets (of any type) containing equal elements
should have equal hashCodes. But equality doesn't hold
across different interfaces: A List with elements x,y,z
most likely has a different hashCode than a Set of x,y,z.
--
Eric.Sosman@sun.com