Need help with Creating a Network from the "Random" library

[Petros Sideris]

I have custom classes for Edge and Vertex

public class Edge {

private String id;
private double weight;

public Edge(String id, double weight){
    this.id = id;
    this.weight = weight;
}

public Edge(String id){
    this.id = id;
    this.weight = -1;
}

public String getId() {
    return id;
}

public void setId(String id) {
    this.id = id;
}

public double getWeight() {
    return weight;
}

public void setWeight(double weight) {
    this.weight = weight;
}

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((id == null) ? 0 : id.hashCode());
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    Edge other = (Edge) obj;
    if (id == null) {
        if (other.id != null)
            return false;
    } else if (!id.equals(other.id))
        return false;
    return true;
}

@Override
public String toString(){
    return this.id;
}
}

And Vertex follows along the same lines. I try to implement ErdosRenyi or SmallWorldExample but I always get a null pointer. Should I create a custom Class for my network factory too? Doesn't it handle it directly? For example why can't I insert a Undirected Sparse Graph :/

Can you help me with an example of implementation?