CODEWITHSUNDEEP
javagraphnullpointerexceptionhashmaphashset
thanos
thanos

Reputation: 3341

Adding to a HashSet when it is the value of a HashMap

I'm doing an ''generic class'' exercise from my textbook right now, trying to implement a Graph data structure using a HashMap as the underlying structure, with keys being nodes in the Graph, and the values of the keys being a HashSet of the nodes adjacent to that node. The beginning of the code looks like this:

   public class Graph <T> {

       public HashMap<T, HashSet<T>> foo_graph;
       //... Code ...

I must mention, both the keys and values in the HashSet must be the same type, this is the reason for the T placeholder. With all my methods written out, I'm trying to test this with a static void main block, but Eclipse keeps giving me NullPointerExceptions whenever I try to print the Graph (which has a working ToString method already given to me):

    public static void main(String[] args) {

Graph<Integer> g = new Graph<>();
        g.addNode(5, [5,3,7]); //Trying to add 5,3,7 to the HashSet, but Eclipse is saying I can't do this.
        System.out.println(g); 
    }

This is my addNode method by the way, I can't seem to add anything the Graph either.

public void addNode(T node_key, HashSet<T> node_value ) {
            main_graph.put(node_key, node_value);

Eclipse is telling me at the addNode line in my static void testing block that: 

 The method addNode(Integer, HashSet<Integer>) in the type Graph<Integer> is not applicable for the arguments (int, int, int, int)

Does anyone know why this is? I just can't seem to get it working, I'm stumped. Neither creating a

Upvotes: 0

Views: 2059

Answers (2)

Dawood ibn Kareem
Dawood ibn Kareem

Reputation: 79877

Sure. The syntax of putting stuff in square brackets like you've done just doesn't exist. I have found that the easiest way to initialise a HashSet is with the Arrays.asList static method. I would write something like this.

g.addNode(5, new HashSet<Integer>(Arrays.asList(5, 3, 7)));

Upvotes: 1

iluxa
iluxa

Reputation: 6969

I have to agree with Eclipse. You can't do this:

g.addNode(5, [5,3,7]);

Java won't know how [5,3,7] is magically turning into a HashSet.

What you can do is this:

HashSet<Integer> set = new HashSet<>();
set.add(5);
set.add(3);
set.add(7);
g.addNode(5, set);

Upvotes: 0

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