Generics: how to read actual type parameters

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marek.dudek@gmail.com - 30 Sep 2007 20:50 GMT

For example:

public class Pair<S> {

    public Pair(S first, S second) {

        this.first = first;
        this.second = second;
    }

    public String toString() {
Class clas = ??? ;
        return "Pair of " + clas.toString();
    }

    public S first;
    public S second;
}

so that after instantiating

Pair<Double> p = new Pair<Double>( 0.0, 0.0 );

p.toString() gives "Pair of Double"

TIA

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Joshua Cranmer - 30 Sep 2007 21:07 GMT

> For example:
>
[quoted text clipped - 6 lines]
> public S second;
> }

1. Don't use tabs in Usenet posts.
2. What you are probably intending to do is impossible as specified.
There is no possible way at runtime to get the class of S. Java erases
the types of the parameters at runtime.
3. Class is generic. Use Class<?> instead.

The easiest thing you can do is:
Class<?> clas = first.getClass();

A potentially tighter bound is:
Class<?> left = first.getClass();
Class<?> right = second.getClass();
Class<?> clas = left;

while (!clas.isAssignableFrom(right))
clas = clas.getSuperclass();

(This returns the last common ancestor of the classes of first and second)

Signature

Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth

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marek.dudek@gmail.com - 30 Sep 2007 21:10 GMT

Thank You a lot
Sorry for tabs

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Lew - 30 Sep 2007 21:24 GMT

>> For example:
>>
[quoted text clipped - 25 lines]
>
> (This returns the last common ancestor of the classes of first and second)

Another hack in a class you own is to have a Class<?> instance variable to
provide runtime type information.

And here's a recent article I just googled up that delves into the issue (GIYF):
<http://www.artima.com/weblogs/viewpost.jsp?thread=208860>

Signature

Lew

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Daniel Pitts - 01 Oct 2007 04:48 GMT

> > marek.du...@gmail.com wrote:
> >> For example:
[quoted text clipped - 35 lines]
> --
> Lew

Also, since Class is generified, you can do something like this:

class Pair<E> {
E first;
E second;
Class<E> type;

public Pair(Class<E> type) {
this.type = type;
}

public String toString() {
return "A pair of " + type.getName() + " objects: <" + first +
", " + second ">";
}
}

Although, I have to say its been my experience that toString is only
very useful for debug messages, and not for any real textual output
intended for the end user (in most cases). Especially a toString that
gathers runtime information "automagically".

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Generics: how to read actual type parameters

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