CODEWITHSUNDEEP
javaobjectgenericsinheritanceinstantiation
TCulos
TCulos

Reputation: 183

Instantiation of generic type to support custom objects

While constructing a random graph I am trying to support 2 different edge types

Each of which extend the generic Edge class

public class TimeStampEdge extends Edge<DiffusionVertex,Integer> {

//timestamp edge specific methods

}

public class DiffusionEdge extends Edge<DiffusionVertex,Integer> {

    public DiffusionEdge(String name, DiffusionVertex v, DiffusionVertex v1) {
        super(name, v, v1);
    }
}

V is a self referential variable so that a vertex can maintain a list of its neighbours, and K just some value for an edge to have

public class Edge<V extends Vertex<?,V>,K> implements Edges<V,K> {
    //Generic edge methods and values
}

What I want to do is be able to create a graph of both edge types from the same graph model, here's an example for random K-Trees. Where E should be either TimeStampEdge or a DiffusionEdge, diffusion vertex is the vertices used for all graph models, and GraphClass is the abstract class all graph models inherit from.

public class KTree<E extends Edge<DiffusionVertex, Integer>> extends GraphClass<DiffusionVertex,E> 
//ktree methods

   public boolean addEdge(DiffusionVertex v, DiffusionVertex v1){
        if(this.findEdgeSet(v,v1).isEmpty()){

            v.setDegree(v.getDegree()+1);
            v1.setDegree(v1.getDegree()+1);

            E edge = (E) new Edge(v.getLabel() +"-"+v1.getLabel() ,v,v1);


            this.Edges.put(edge.getHashCode(), edge);

            return true;

        }else{

            return false;

        }
    }

//more ktree methods
}

I keep running into the java.lang.ClassCastException where class Edge cannot be cast to TimeStampEdge or DiffusionEdge, I believe the problem comes from my instantiation of edges in the shown addEdge method always being of the Edge class. However I'm not sure how to make it where i create edges of type E. Any help on how to go about doing this would be much appreciated!

Upvotes: 1

Views: 171

Answers (1)

Jack Ammo
Jack Ammo

Reputation: 280

calling new Edge(...) will not create any subclass of Edge represented as E unless E really does represent Edge. Java does not change the actual type of an object when you cast it from one type to another, let alone when you cast it to a generic type. For a cast to a specific type to work, the object itself must already be that type.

if you want to create a new E, maybe you can pass it a Supplier<E> or use a factory pattern to create a new E properly.

Upvotes: 3

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