XOM API for Processing XML with Java

[Remko Popma <remko.popma AT azzurri.jp>]

I understand the problem of the == operator not working as expected.
What I meant was, in situations where you have different classloaders,
not only typesafe enums behave unexpectedly, *every class* suddenly
has problems with their equals() method.

It may be possible to work around this (your solution is an example),
but the standard/recommended equals() implementation (according to Bloch)
will not work:

class Foo {
    // "standard" implementation 
    // (will unexpectedly return false if o is a Foo instance 
    //  loaded by different class loader, even if it has identical field 
values)
    public boolean equals(Object o) { 
       if (o == this) {
           return true;
       }
       if ((o instanceof Foo) == false) {
           return false;
       }
       Foo other = (Foo) o;
       return (mField1 == null ? other.mField1 == null : 
mField1.equals(other.mField1))
           && (mField2 == null ? other.mField2 == null : 
mField2.equals(other.mField2))
           ;
    }
}

In that respect, the classloader issue affects every class
and is not a typesafe enum issue per se.
The question is whether working in an environment with multiple classloaders 
is a design goal, as it will affect design and implementation considerably,
not only whether typesafe enums should be used or not.

Remko


--- John Cowan <jcowan AT reutershealth.com> wrote:
> 
> Remko Popma scripsit:
> 
> > 1) Is this really a typesafe enum issue?
> > Basically, as far as I understand it, neither the == operator nor the
> > equals() method works if you compare two instances loaded by separate
> > classloaders. So eg. someNode.equals(sameNodeLoadedByOtherClassloader)
> > would also return false. 
> 
> Typesafe enums are in competition with plain old integers, which don't care
> about class loaders.  So yes, it's a typesafe enum issue in that you don't
> get the expected results with enums whereas you do with ints.
> 
> Here's a possibility:
> 
>       int theCode;
> 
>       private SomeEnumClass(int code) {theCode = code;}
>       public int hashCode {return theCode;}
>       public boolean equals(Object o) {
>               if 
> (!o.getClass().getName().equals("blah.blah.SomeEnumClass")) {
>                       return false;
>                       }
>               else {
>                       return theCode == o.hashCode();
>                       }
> 
> Note the importance of testing the class *name* for equality, not the
> class object.  That way, we do the hashCode check iff this is the same
> class independent of class loaders.
> 
> -- 
> John Cowan                                <jcowan AT reutershealth.com>     
> http://www.reutershealth.com              http://www.ccil.org/~cowan
> Yakka foob mog.  Grug pubbawup zink wattoom gazork.  Chumble spuzz.
>     -- Calvin, giving Newton's First Law "in his own words"
>