Currently, CopiesList.hashCode() is inherited from AbstractList which:
- calls hashCode() for each element,
- creates a new Iterator every time.
However, for Collections.nCopies():
- All elements are the same. So hashCode() can be called only once.
- An Iterator is unnecessary.
So, I propose overridding hashCode() implementation for CopiesList:
@Override
public int hashCode() {
int hashCode = 1;
final int elementHashCode = (element == null) ? 0 : element.hashCode();
for (int i = 0; i < n; i++) {
hashCode = 31*hashCode + elementHashCode;
}
return hashCode;
}
Benchmark:
List<List<String>> list = Collections.nCopies(10_000, new
ArrayList<>(Collections.nCopies(1_000_000, "a")));
long nano = System.nanoTime();
System.out.println(list.hashCode());
System.out.println((System.nanoTime() - nano) / 1_000_000);
Result:
Old version - ~12 seconds.
New version - ~10 milliseconds.
