Quoting from http://sites.google.com/site/gson/gson-design-document:
Why are most classes in Gson marked as
final?While Gson provides a fairly
extensible architecture by providing
pluggable serializers and
deserializers, Gson classes were not
specifically designed to be
extensible. Providing non-final
classes would have allowed a user to
legitimately extend Gson classes, and
then expect that behavior to work in
all subsequent revisions. We chose to
limit such use-cases by marking
classes as final, and waiting until a
good use-case emerges to allow
extensibility. Marking a class final
also has a minor benefit of providing
additional optimization opportunities
to Java compiler and virtual machine.
Why is this the case? [If I would guess: of JVM knows class is final it does not maintain method override tables? Are there any other reasons?]
What is the benefit in performance?
Does this applies to classes that are frequency instantiated (POJO?) or perhaps to class that are holders static methods (Utility classes) ?
Are methods defined as final also can theoretically improve performance?
Are there any implications?
Thank you,
Maxim.
Virtual (overridden) methods generally are implemented via some sort of table (vtable) that is ultimately a function pointer. Each method call has the overhead of having to go through that pointer. When classes are marked final then all of the methods cannot be overridden and the use of a table is not needed anymore – this it is faster.
Some VMs (like HotSpot) may do things more intelligently and know when methods are/are not overridden and generate faster code as appropriate.
Here is some more specific info on HotSpot. And some general info too.