Java static function on generics

Question

Hey I'm trying to write a function that calls a static function based upon its generic arguments. I'm having the following code:

public class Model<T extends Listable>
{
    private Document doc;

    /*
        When the JavaBean is created, a Document object is made using
        the Listable parameter. The request string for the specific
        type is used to pull XML-data from the cloud.
    */
    public Model()
    {
        try
        {
            doc = cloud.request(T.getRequestString());
        }
        catch(Exception e)
        {
        }
    }

    /*
        getMatches (used in JSP as "foo.matches") generates a list
        of objects implementing the Listable interface.
    */
    public List<Listable> getMatches()
    {
        return T.generateMatches(doc);
    }
}

How do I do this, I'm just getting something about static contexts. 'non-static method generateMatches(org.jdom.Document) cannot be referenced from a static context'

Eric Eijkelenboom · Accepted Answer · 2011-03-05 08:39:29Z

4

Turned comment into answer:

You can introduce an instance variable of type T and call generateMatches on that. You cannot call generateMatches on the type T itself.

You could e.g. inject this instance variable via the constructor and store it in a private variable:

private T instanceOfT;

public Model(T instanceOfT){
    this.instanceOfT= instanceOfT;
}

In your getMatches method you can then do this:

return instanceOfT.generateMatches(doc);

answered Mar 5, 2011 at 8:39

Eric Eijkelenboom

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1 Comment

Skeen Over a year ago

Accepted the best, but unacceptable solution. - Changed to C++

templatetypedef · Accepted Answer · 2011-03-05 08:41:22Z

2

Your problem is that you do not have handle to any object of class T. Just saying T.generateMatches(doc) means you are making a static call to static method in class T. You need to have a variable of type T to call instance methods.

edited Mar 5, 2011 at 8:41

templatetypedef

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answered Mar 4, 2011 at 8:54

Zds

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Comments

Rostislav Matl · Accepted Answer · 2011-03-04 08:49:05Z

0

What's the question ?

The reason is clear - the line "T.generateMatches(doc);" calls generateMatches through T, and T is type (class/interface), not instance.

answered Mar 4, 2011 at 8:49

Rostislav Matl

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4 Comments

Skeen Over a year ago

Right, well how should I make my above code work, I guess thats the question

Eric Eijkelenboom Over a year ago

Introduce an instance variable of type T and call generateMatches on that, not on the type T.

Sean Patrick Floyd Over a year ago

@Eric you are the first person making sense on this page. Why not add this as an answer?

Rostislav Matl Over a year ago

Either make both doc and getMatches() static (and set doc using some static method, of course) or go the way Eric suggested. Depends on your needs - Eric's solution looks better for you code.

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