Potential performance improvement for java.util.AbstractList?

[Cédric Champeau]

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
>
>