Performance of "instanceof"

[Tom Rodriguez]

instanceof is the fastest way of asking the question it's designed to  
ask.  The three code patterns below aren't necessarily asking the same  
question though.  If MyClass has no subclasses then instanceof  
devolves into obj.klass == MyClass.klass, so a load and pointer  
compare against an embedded constant.  If it has subclasses then our  
fast subtype check should trigger which is roughly a couple loads and  
a compare, unless the hierarchy is deep or MyClass is an interface.   
obj.getClass() == mypackage.MyClass.class will produce something  
relatively efficient.  It should be something like  
obj.klass.klass_mirror == MyClass.   
obj.name().equals(objOfMyClass.name()) would be better with == which  
you could rely on if you either explicitly interned the name or used a  
constant string for the name so those are automatically interned for  
you.  If you have to call .equals it's probably the worst.  All of  
this should be true for client and server, with server producing at  
least slightly tighter code and maybe an extra trick or two.

tom

On Jan 13, 2009, at 4:49 AM, Ulf Zibis wrote:

> Hi,
>
> I'm just interested, how "instanceof" works behind the scenes. Does  
> it simply string-compare the names of the classes, and in fail, do  
> the same with the super class in  loop, or is there some  
> sophisticated algorithm.
>
> I'm asking, because I want to know, which is faster:
> -   obj instanceof mypackage.MyClass
> -   obj.getClass() == mypackage.MyClass.class
> -   obj.name().equals(objOfMyClass.name()) (assuming, each MyClass  
> has it's own name, as it is the fact for Charset subclasses.)
>
> - Ulf
>
>