Implementing a Graph using a HashMap

Question

I'm new to Java, so please keep that in mind before getting trigger happy down-voting. I'm familiar with the popular implementation of a graph of integers using an array.

Graph{
 int vertices;
 LinkedList<Integer>[] adj;

Graph(int vertices){
   this.vertices = vertices;
   adj = new LinkedList<>();
   for(int i= 0; i <vertices; i++){
      adj[i] = new LinkedList(); 
   }
}

However this implementation is best suited to a graph of integers. In case I want to implmentation graph of Characters, this implemetation doesn't render itself directly usable. So I tried implementing using a HashMap.

public class Graph {
    int vertices;
    HashMap<Character, LinkedList<Character>> adj;


    Graph(int item){

       this.vertices = item;
       adj = new HashMap<>();

    }
}

Where I'm a little stuck syntactically with Java is adding keys and values to this HashTable. What I'm trying to do is implement this method.

public void add(Character a, Character b){

            if (adj.containsKey(a)){

               //get a reference to the existing linked list
               //add to the existing LinkedList
            }else{

               //create a new LinkedList and add to it.
            }

        }
    }

I could use some help with the incomplete add method as also how to iterate through the this adj HashMap once the graph is constructed.

Answer 1

Since your question is only about syntax, you can do something like this:

public void add(Character a, Character b){

        if (adj.containsKey(a)){

           //get a reference to the existing linked list
           LinkedList<Character> l = adj.get(a);

           //add to the existing LinkedList
           //You need to do a null check here to make sure (l != null)
           l.add(b)
        }else{

           //create a new LinkedList and add to it.
           LinkedList<Character> l = new LinkedList<Character>();
           l.add(b);
           adj.put(a, l);
        }

    }
}