The reason for using a zero-returning hashCode

There is one problem which we have not yet considered.
If a client chooses to change a value after it has been put in a collection, the value will no longer be in the right bucket.
So it will not be found if we later search for it.

Rule 1: a hashCode function should not be written in terms of fields that can be altered.
So, if Date provides setYear, setMonth and setDay, we ought not to provide a hashCode written in terms of the iYear, iMonth and/or iDay.
This is the reason why we might want to use:
```
public int hashCode()
{
   return 0;
}
```
A collection class will use one bucket for all the objects we put into the collection.
This means that a method like contains will execute more slowly.
It means we can change values after they have been added to a collection.