When trying to compare a generic type in the form
Class<T> implements Comparable<T> {
public int compareTo(T other){
this.T < other
}
}
does not work for me but when using
Class<T extends Comparable<T>>{
T.compareTo(other.T)
}
does work. I have been unable to deciper why I can't compare T directly using the first example
In your first example:
class Foo<T> implements Comparable<T> {
you're saying that Foo objects are comparable. In your second example:
class Foo<T extends Comparable<T>>{
you're saying that whatever T, is, it's comparable.
Then, in the body of your code, you try to compare things of type T -- in the first case, you have no guarantee that they're comparable, in the second, you do.
I hope these two exmaples will cast some light on your problem:
class Foo<T> implements Comparable<T> {
@Override
public int compareTo(T o) {
// We don't know anything about T
return hashCode() - o.hashCode();
}
}
class Boo<T extends Comparable<? super T>> implements Comparable<T> {
private T instance;
@Override
public int compareTo(T o) {
// We know that T implements Comparable<? super T>
return instance.compareTo(o);
}
}
In first case with Foo, you don't know anything about type T, so you can't do much in your compareTo() method.
However, in Boo, T is required to implement Comparable<? super T> (if you don't know what wildcards are, just think there is simply Comparable<T>), so you can call t.compareTo(anotherT). More about bounded type parameters.
EDIT: (wildard explained)
Consider following code:
class Car implements Comparable<Car> { ... }
class SportCar extends Car { ... }
Now call sportCar1.compareTo(SportCar2) is perfectly legal. However, without the wildcard,
Bar<SportCar> is a cpompile error!
Why? Because SportCar doesn't implement Comparable<SportCar>. And you require T to implement Comparable<T>, and in this case T is SportCar.
But SportCar implements Comparable<Car> and Car is a supertype of SportCar. So you want to say something like "T can be compared to T or any supertype of T" (like in this case SportCar can be compared to any Car).
And that what the wildcard is for (among many other things). Hope this helps.
