Re: Newbie question: Can a parameterized container hold objects
derived from the parameter?
On May 23, 12:53 pm, mrstephengross <mrstevegr...@gmail.com> wrote:
The ClassCastException here has nothing to do with the relationship
between Base and Derived (and as written the error would be a compile-
time error about incorrect types; see previous remark about
'extends' :). TreeSet requires that either the elements themselves
implement Comparable or that you provide a Comparator at construction
time. If you don't provide a Comparator, it tries to cast elements to=
Comparable when inserting them -- and since neither Base nor Derived
implements Comparable, this causes the CCE you're encountering.
Unless you need a set that's sorted, try another Set implementation
like HashSet that doesn't impose extra constraints on the contents.
If you do need a sorted set, make sure there's something to sort on!
Either make Base implement Comparable (correctly) or provide an
implementation of Comparator for Base when constructing the TreeSet.
Ok, I changed it around to implement Comparator. However, I'm still
getting a ClassCastException:
class Foo implements Comparator {
public int compare(Object o1, Object o2) { return 0; } }
class Bar extends Foo {}
SortedSet<Foo> set = new TreeSet<Foo> ();
set.add(new Bar()); // <-- Causes exception!
So Bar extends Foo, which implements Comparator. However, the
exception still occurs. Any ideas?
Yes: you've confused Comparator and Comparable.
Instances of Comparable classes can be compared to other instances of
the same Comparable class.
Instances of Comparator can be used to compare two instances of some
other class.
You probably want Foo to implement Comparable<Foo>.
-o