Вопрос С чем могут быть связаны такого рода ошибки (Java)?

Ошибка:
Exception in thread "main" java.lang.NullPointerException
at EdgeWeightedDigraph.addEdge(EdgeWeightedDigraph.java:95)
at EdgeWeightedDigraph.(EdgeWeightedDigraph.java:35)
at main.main(main.java:32)

В коде:
public EdgeWeightedDigraph(int V, int E) {

if (E < 0) throw new RuntimeException("Number of edges must be nonnegative");
for (int i = 0; i < E; i++) {
int v = (int) (Math.random() * V);
int w = (int) (Math.random() * V);
double weight = Math.round(100 * Math.random()) / 100.0;
DirectedEdge e = new DirectedEdge(v, w, weight);
addEdge(e);
}
}

public void addEdge(DirectedEdge e) {
int v = e.from();
adj[v].add(e);
E++;
}