Question

I am using adjacency lists to represent a directed weighted graph and based on the example code provided by this SO question, I have created the following:

import java.util.HashMap;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.Map;
import java.util.Set;

public class _Graph {
    private Map<String, LinkedHashSet<HashMap<String, Integer>>> map = new HashMap<String, LinkedHashSet<HashMap<String, Integer>>>();

    public void addEdge(String node1, String node2, int dist) {
        LinkedHashSet<HashMap<String, Integer>> adjacent = map.get(node1);
        HashMap<String, Integer> innerMap = new HashMap<String, Integer>();
        if(adjacent==null) {
            adjacent = new LinkedHashSet<HashMap<String, Integer>>();                       
            map.put(node1, adjacent);
        }
        innerMap.put(node2, dist);
        adjacent.add(innerMap);
    }

    public boolean isConnected(String node1, String node2) {
        Set<HashMap<String, Integer>> adjacent = map.get(node1);
        if(adjacent==null) {
            return false;
        }
        return adjacent.contains(node2);
    }

    public LinkedList<HashMap<String, Integer>> adjacentNodes(String node) {
        LinkedHashSet<HashMap<String, Integer>> adjacent = map.get(node);
        if(adjacent==null) {
            return new LinkedList<HashMap<String, Integer>>();
        }
        return new LinkedList<HashMap<String, Integer>>(adjacent);
    }

}

I am having trouble making the isConnected method to work properly. Am I using a wrong data structure to represent the graph here (Map<String, LinkedHashSet<HashMap<String, Integer>>>)? The hashmap will hold the name of the connected node and the distance to it:

Map<startNode, LinkedHashSet<HashMap<endNode, distanceToEndNode>>>

1. Basically how can I check if a node
   belongs to the adjacency list of a
   given base node? I think the problem
   is reduced to iterating properly
   over the adjacent
Set<HashMap<String, Integer>>
   structure, or is my reasoning wrong?
2. In my second method
   adjacentNodes(String node) I am
   returning a linked list containing
   the maps (in a set structure) of the
   connected nodes and their distances. How could I efficiently iterate to see all connections of a any given node?

Answer 1

I think LinkedHashSet is not needed here, you can represent the graph with just a Map<String, Map<String, Integer>>.

isConnected is basically what you already have:

public boolean isConnected(String node1, String node2) {
    Map<String, Integer> adjacent = map.get(node1);
    if(adjacent==null) {
        return false;
    }
    return adjacent.containsKey(node2);
}

adjacentNodes just needs to pull out the entries in the hash set for the source node

public Collection<Map.Entry<String, Integer>> adjacentNodes(String node) {
    Map<String, Integer> adjacent = map.get(node);
    if(adjacent==null) {
        return new ArrayList<Map.Entry<String, Integer>>();
    }
    return adjacent.entrySet();
}