Can we find the hashcode of a list that contains itself as element?
I know this is a bad practice, but this is what the interviewer asked.
When I ran the following code it throws a StackOverflowError:
public class Main {
public static void main(String args[]) {
ArrayList<ArrayList> a = new ArrayList();
a.add(a);
a.hashCode();
}
}
Now here I have two questions:
- Why is there a
StackOverflowError? - Is it possible to find the hash code in this way?
The hash code for conforming List implementations has been specified in the interface:
Returns the hash code value for this list. The hash code of a list is defined to be the result of the following calculation:
int hashCode = 1; for (E e : list) hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());This ensures that
list1.equals(list2)implies thatlist1.hashCode()==list2.hashCode()for any two lists,list1andlist2, as required by the general contract ofObject.hashCode().
This doesn’t require that the implementation looks exactly like that (see How to compute the hash code for a stream in the same way as List.hashCode() for an alternative), but the correct hash code for a list only containing itself would be a number for which x == 31 + x must be true, in other words, it is impossible to calculate a conforming number.