java - How do I add a null object to an ArrayList inside an ArrayList -

i trying represent ad hoc network using adjacency matrix structure. this, creating arraylist inside arraylist.

when add new vertex graph, create new arraylist (inside super arraylist) , have loop add new null object each arraylist, size of arraylists not increase correctly , can't figure out why.

here code:

public class matrix {  public arraylist<arraylist<edge>> graph; public arraylist<vertex> verticies; public arraylist<edge> edges;  public matrix() {     graph = new arraylist();     verticies = new arraylist();     edges = new arraylist(); }  public matrix(arraylist<vertex> verticies, arraylist<edge> edges) {      this.verticies = verticies;     this.edges = edges;       }  public void addvertex(vertex v) {     verticies.add(v);     graph.add(new arraylist());      for(int i=0; i<graph.size()-1; i++ ) {         graph.get(i).add(null);     } } 

any appreciated.

the initial size of graph 0, for loop in addvertex() runs 1 time less should:

public void addvertex(vertex v) {     verticies.add(v);     graph.add(new arraylist()); // graph has size 1      (int = 0; < graph.size() - 1; i++) { // = 0, 0 < 0 false                graph.get(i).add(null); // not executed last added list     } } 

the next time call addvertex() add null previous arraylists, not 1 added.

so should do:

for (int = 0; < graph.size(); i++) 

even fix though, note if call addvertex() 5 times have this:

index             arraylist   0      [null, null, null, null, null]   1      [null, null, null, null]   2      [null, null, null]   3      [null, null]   4      [null] 

this not want. better approach add vertices first:

public void addvertex(vertex v) {     this.vertices.add(v); } 

and create arraylists adjacency matrix appropriate size:

public void initializeadjacencymatrix() {     int n = this.vertices.size();     (int = 0; < n; i++) {         list<edge> edges = new arraylist<>(collections.ncopies(n, null));         graph.add(edges);     } } 

also, you're using raw types when instantiating arraylists. not practice. should use diamond operator instead. example:

graph = new arraylist<>(); graph.add(new arraylist<>());