On 8/12/2015 1:39 AM, Rafael Winterhalter wrote:
In this case, one might consider:
if (o instanceof java.util.RandomAccess && (List<?>)o).size() != size())
return false;
You'd also have to check the type of 'this'.
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.
These type of functions are generally already "optimal" for the range of
cases they have to accommodate. Invariably you can't improve your case
of interest without potentially impacting a lot of other cases.
David
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2015-12-07 16:31 GMT+01:00 Cédric Champeau <cedric.champ...@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.lan...@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
