Why fill() and copy() of Collections in java is implemented this way
- by Priyank Doshi
According to javadoc... Collections.fill() is written as below :
public static <T> void fill(List<? super T> list, T obj) {
int size = list.size();
if (size < FILL_THRESHOLD || list instanceof RandomAccess) {
for (int i=0; i<size; i++)
list.set(i, obj);
} else {
ListIterator<? super T> itr = list.listIterator();
for (int i=0; i<size; i++) {
itr.next();
itr.set(obj);
}
}
}
Its easy to understand why they didn't use listIterator for
if (size < FILL_THRESHOLD || list instanceof RandomAccess)
condition as of RandomAccess. But whats the use of size < FILL_THRESHOLD in above?
I mean is there any significant performance benefit over using iterator for size>=FILL_THRESHOLD and not for size < FILL_THRESHOLD ?
I see the same approach for Collections.copy() also :
public static <T> void copy(List<? super T> dest, List<? extends T> src) {
int srcSize = src.size();
if (srcSize > dest.size())
throw new IndexOutOfBoundsException("Source does not fit in dest");
if (srcSize < COPY_THRESHOLD ||
(src instanceof RandomAccess && dest instanceof RandomAccess)) {
for (int i=0; i<srcSize; i++)
dest.set(i, src.get(i));
} else {
ListIterator<? super T> di=dest.listIterator();
ListIterator<? extends T> si=src.listIterator();
for (int i=0; i<srcSize; i++) {
di.next();
di.set(si.next());
}
}
}
FYI:
private static final int FILL_THRESHOLD = 25;
private static final int COPY_THRESHOLD = 10;
