Java Reference
In-Depth Information
(resultingfromthe
equals()
comparison)alsoshowsthatthesortedset'snaturalor-
dering is inconsistent with
equals()
, which violates
SortedSet
's contract:
The ordering maintained by a sorted set (whether or not an explicit comparator is
provided) must be consistent with
equals()
if the sorted set is to correctly imple-
ment the
Set
interface. This is so because the
Set
interface is defined in terms of
the
equals()
operation, but a sorted set performs all element comparisons using its
compareTo()
(or
compare()
) method, so two elements that are deemed equal by
this method are, from the standpoint of the sorted set, equal.
Becausetheapplicationworkscorrectly,whyshould
SortedSet
'scontractmatter?
Although the contract does not appear to matter with respect to the
TreeSet
imple-
mentation of
SortedSet
, perhaps it will matter in the context of a third-party class
that implements this interface.
Listing5-12showsyouhowtocorrectthisproblemandmake
Employee
instances
work with any implementation of a sorted set.
Listing 5-12.
A contract-compliant
Employee
class
import java.util.SortedSet;
import java.util.TreeSet;
class CustomClassAndSortedSet
{
public static void main(String[] args)
{
SortedSet<Employee> sse = new TreeSet<>();
sse.add(new Employee("sally doe"));
sse.add(new Employee("bob doe"));
Employee e1 = new Employee("john doe");
Employee e2 = new Employee("john doe");
sse.add(e1);
sse.add(e2);
System.out.println(sse);
System.out.println(e1.equals(e2));
}
}
class Employee implements Comparable<Employee>
{
