Potential performance improvement for java.util.AbstractList?

[Rafael Winterhalter]

In this case, one might consider:

if (o instanceof java.util.RandomAccess && (List<?>)o).size() != size())
  return false;

Type checks are cheap, so the overhead of this additional statement should
not be too big. Yet, maybe many list comparisons in practice involve lists
of equal size. Otherwise, the contract of the List::equals method would
allow for such a short-wiring.

2015-12-07 16:31 GMT+01:00 Cédric Champeau <cedric.champeau at gmail.com>:

> I assume one reason it's done this way is that depending on the list
> implementation you are using, getting the size might involve iterating over
> all elements, so you would be iterating twice at worst case.
>
> 2015-12-07 16:28 GMT+01:00 Langer, Christoph <christoph.langer at sap.com>:
>
> > Hi all,
> >
> > a Java application developer of our company has indicated that it might
> > yield some performance benefit to modify the coding of
> > java.util.AbstractList.equals() that it would first compare the size of
> the
> > lists before iterating the elements. It would for sure be better in cases
> > where one compares lists which don't have the same size. In case of
> > comparing "equal" lists it would add some minor cost, though.
> >
> > Currently the implementation is like this:
> >
> > public boolean equals(Object o) {
> >         if (o == this)
> >             return true;
> >         if (!(o instanceof List))
> >             return false;
> >
> >         ListIterator<E> e1 = listIterator();
> >         ListIterator<?> e2 = ((List<?>) o).listIterator();
> >         while (e1.hasNext() && e2.hasNext()) {
> >             E o1 = e1.next();
> >             Object o2 = e2.next();
> >             if (!(o1==null ? o2==null : o1.equals(o2)))
> >                 return false;
> >         }
> >         return !(e1.hasNext() || e2.hasNext());
> >     }
> >
> > One could do for instance:
> >
> >
> > public boolean equals(Object o) {
> >
> >         if (o == this)
> >
> >             return true;
> >
> >         if (!(o instanceof List))
> >
> >             return false;
> >
> >         if ((List<?>)o).size() != size())
> >
> >             return false;
> >
> >
> >
> >         ListIterator<E> e1 = listIterator();
> >
> >         ListIterator<?> e2 = ((List<?>) o).listIterator();
> >
> >         while (e1.hasNext() && e2.hasNext()) {
> >
> >             E o1 = e1.next();
> >
> >             Object o2 = e2.next();
> >
> >             if (!(o1==null ? o2==null : o1.equals(o2)))
> >
> >                 return false;
> >
> >         }
> >
> >         return !(e1.hasNext() || e2.hasNext());
> >
> >     }
> >
> > How would you assess this idea? Are there other drawbacks/showstoppers to
> > this which I don't see?
> >
> > Thanks in advance for comments.
> >
> > Best regards
> > Christoph
> >
> >
>