Downcast reference to runtime-known class

Question

Is there any way by which I can cast a reference of type Object, assuming that the reference could point to any class I defined, to said defined class at runtime?

I've been trying to work it out and the code I came out with is:

public class SomeTestBench {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        myEntity a = new myEntity("Hello Code!");
        Receptacle cage = new Receptacle();
        cage.injectYourEntity(a);

        ((cage.itsClass) cage.theEntity).exertExistence();
    }

}

That unfortunately does not work, as the class argument to that cast must be static.

Rest of the code:

public class myEntity extends Object{
    String warcry;
    myEntity(String warcry){
        this.warcry = warcry;
    }
    public void exertExistence(){
        System.out.println(this.warcry);
    }
}

public class Receptacle {
    Object theEntity;
    Class itsClass;
    public void injectYourEntity(Object it){
        this.theEntity = it;
        this.itsClass = it.getClass();
    }
    public void prodIt(){
        System.out.println(theEntity.getClass());
    }

}

Answer 1

To call a no-arg method named exertExistence() on an object of unknown type, you have three choices:

1. Use generics. See answer by WJS.

2. Use reflection:

   Receptacle cage = new Receptacle();
   cage.injectYourEntity(new myEntity("Hello Code!"));
   
   Method method = cage.itsClass.getMethod("exertExistence", null);
   method.invoke(cage.theEntity, null);
   

3. Use an interface (recommended):

   Receptacle cage = new Receptacle();
   cage.injectYourEntity(new myEntity("Hello Code!"));
   cage.theEntity.exertExistence();
   

   interface MyInterface {
       void exertExistence();
   }
   
   class myEntity implements MyInterface {
       String warcry;
       myEntity(String warcry){
           this.warcry = warcry;
       }
       @Override
       public void exertExistence(){
           System.out.println(this.warcry);
       }
   }
   
   class Receptacle {
       MyInterface theEntity;
       public void injectYourEntity(MyInterface it){
           this.theEntity = it;
       }
   }
   

Answer 2

Why don't you just do this using Generics.

    public static void main(String[] args) {
        myEntity a = new myEntity("Hello Code!");
        Receptacle<myEntity> cage = new Receptacle<>();
        cage.injectYourEntity(a);

        cage.theEntity.exertExistence();
    }

//
//That unfortunately does not work, as the class argument to that cast must be static.
//
//Rest of the code:

class myEntity {
    String warcry;
    myEntity(String warcry){
        this.warcry = warcry;
    }
    public void exertExistence(){
        System.out.println(this.warcry);
    }
}

class Receptacle<T> {
    T theEntity;
    public void injectYourEntity(T it){
        this.theEntity = it;
    }
    public void prodIt(){
        System.out.println(theEntity.getClass());
    }

}