generic casting with object.getClass() ?

[nimo frey]

I have something a generic class:

public class MyObject<T>
{
public MyObject(){}

private Map<String, T> myObjects = new HashMap<String, T>();
//getter/setter

}

In MyBean, I instantiate this generic class to a object of type 'Hobby':

public class MyBean{


MyObject<?> o1 = new MyObect<Hobby>();

// fill the map with a object of type hobby
o1.getMyObjects().put("all", this.getHobby());


// generic method: get object of of key 'all' and cast it to Hobby 
public void countEntities(MyObject <?> o){
// way 1: this explicit cast works:
Hobby h = (Hobby) o.getMyObjects("all");


// way 2: this cast does NOT work:
// the debugger says: The method getMyObjects() is undefined for the type Class<capture#82-of ?>
Hobby h = (o.getClass()) o.getMyObjects("all");

// way 3: this cast does also NOT work:
// the debugger says: The method getMyObjects() is undefined for the type Class<capture#82-of ?>
Class<Hobby> hobbyClass;
Hobby h = (hobbyClass) o.getMyObjects("all");
}

How can I solve way 2 or way 3 ?
I cannot use way 1, as I have a generic method in which I need to put the cast explicitly by its given object-instance.

[Wouter Oet]

Just change the reference from MyObject<?> o1 to MyObject<Hobby> 01 or MyObject<? super extends Hobby>

[nimo frey]

Just change the reference from MyObject<?> o1 to MyObject<Hobby> 01 or MyObject<? super Hobby>

Ineed, I can do that - But when doing this:

public void countEntities(MyObject <Hobby> o){  .. }

I can use this method only for objects of type Hobby and this is what I want to avoid !

I want to use this method for any type of MyObject.

I want to have something like an abstract generic Method which can be invoked from any type of MyObject.

[Wouter Oet]

In my previous post I used super when I meant extends. I edited my post. When you use ? extends Hobby then you can use any class as long as it extends Hobby.

[Rob Spoor]

nimo frey wrote:I can use this method only for objects of type Hobby and this is what I want to avoid !

I want to use this method for any type of MyObject.

Apparently not, or you wouldn't have the cast to Hobby in there.

[nimo frey]

Apparently not, or you wouldn't have the cast to Hobby in there.

Yes, I want to cast it to Hobby, but not always.

In other cases, I want cast it to, for example, Object of type 'Work'.

Imagine, I have this method and would cast the object 'o' to 'object_type':

public void getMyObject(MyObject <?> o, Class object_type){  

// way 1: the explicit cast works:  
if(object_type.equals(Hobby.class)
Hobby h = (Hobby) o.getMyObjects("all");

if(object_type.equals(Work.class)
Work h = (Work) o.getMyObjects("all");


// but the implicit cast does NOT work:
Object h = (object_type) o.getMyObjects("all");

}

So you see,
I want to avoid the if-clauses and cast the object directly as I have the 'object_type'-property, which indicates which type of class my object 'o' is.

Is this possible? Is there another better solution?

[Rob Spoor]

Class has a method called cast but that will only make sense if you know the Class type. For instance, with a Class<Hobby> object "cls", cls.cast(x) will either return x cast to Hobby or throw an exception.

[nimo frey]

Indeed,

I can do the cast via:

Object o;
Hobby.class.cast(o);

but that will only make sense if you know the Class type

Unfortunately, I cannot cast something like:

public void getMyObject(MyObject <?> o, Class myClazz){  

myClazz.cast(o);

}

So the only solution is to use the if-clauses.

By the way, where lies the difference of

Hobby.class.cast(o)

and

(Hobby) o

(faster?)

[nimo frey]

Okay, I found the solution:


According to this:


http://www.velocityreviews.com/forums/t145694-p2-class-cast-object-what-benefit.html

I am able to do the cast this way:

    public <S> S getElementAs(Class<S> cls, int i)
    {
    return cls.cast(list.get(i)); // no warning
    // return (S)(list.get(i)); // warning
    }
    }

So Bob, you are right, cls.cast will do the cast.

[Rob Spoor]

And check if the object is an instance (or null) at the same time, throwing a class cast exception if it isn't.