Generics compiler warning

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Christian Pontesegger - 27 Apr 2006 17:29 GMT

Hi,

I was trying to create a generic compare method for objects of different
type. If both objects are of the same type and implement Comparable then
use the compareTo method. Else use their toString method and compare
their results.

My code looks like this:

private int compare(Object o1, Object o2) {
if (o1 == null)
return -1;
if (o2 == null)
return 11;

if ((o1.getClass().equals(o2.getClass())) && (o1 instanceof
Comparable))
return ((Comparable) o1).compareTo((Comparable) o2);

// compare anything else converted to string
return o1.toString().compareTo(o2.toString());
}

It works as expected, but I get this annoying compiler warning

"Type safety: The method compareTo(Object) belongs to the raw type
Comparable. References to generic type Comparable<T> should be
parameterized"

How should I code my method in a clean way?

thanks
Christian Pontesegger

Reply to this Message

Oliver Wong - 27 Apr 2006 18:15 GMT

> Hi,
>
[quoted text clipped - 26 lines]
>
> How should I code my method in a clean way?

Perhaps you could use method overloading, one for where the two
arguments implement Comparable, and one for where only one, or none of them,
implement Comparable.

<code>
private <T extends Comparable<T>> int compare(T o1, T o2) {
if (o1 == null) {
return -1;
}
if (o2 == null) {
return 11;
}

if (o1.getClass().equals(o2.getClass())) {
return ((T) o1).compareTo((T) o2);
}
return compare((Object) o1, (Object) o2);
}

private int compare(Object o1, Object o2) {
if (o1 == null) {
return -1;
}
if (o2 == null) {
return 11;
}
// compare anything else converted to string
return o1.toString().compareTo(o2.toString());
}
</code>

- Oliver

Reply to this Message

Christian Pontesegger - 02 May 2006 17:48 GMT

> "Christian Pontesegger" <christian.pontesegger@web.de> wrote in message
>> I was trying to create a generic compare method for objects of
>> different type. If both objects are of the same type and implement
>> Comparable then use the compareTo method. Else use their toString
>> method and compare their results.

> Perhaps you could use method overloading, one for where the two
> arguments implement Comparable, and one for where only one, or none of
> them, implement Comparable.

Simple as that. Thanks for opening my eyes :)

Christian

Reply to this Message

Hendrik Maryns - 28 Apr 2006 09:53 GMT

Christian Pontesegger schreef:

> Hi,
>
[quoted text clipped - 14 lines]
> Comparable))
> return ((Comparable) o1).compareTo((Comparable) o2);

Here you use the non-generic Comparable. As you don’t know the classes,
there is no way you can use Comparable<SomeClass>, so the only way out I
see is @SuppressWarnings("unchecked")

> // compare anything else converted to string
> return o1.toString().compareTo(o2.toString());
[quoted text clipped - 7 lines]
>
> How should I code my method in a clean way?

H.
- --
Hendrik Maryns

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Generics compiler warning

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