public void addEdge(int vertex1, int vertex2) {
        adjacencyList.get(vertex1).add(vertex2);
        adjacencyList.get(vertex2).add(vertex1);
    }

There is no check here to know if the vertex exists or not.

should be

    public void addEdge(int vertex1, int vertex2) {

   if( !adjacencyList.containsKey(vertex1) ){
        adjacencyList.put(vertex1,new ArrayList());
   }

        adjacencyList.get(vertex1).add(vertex2);

   if( !adjacencyList.containsKey(vertex2) ){
        adjacencyList.put(vertex2,new ArrayList());
   }

        adjacencyList.get(vertex2).add(vertex1);     }