HashMap Java Cannot Display Multiple Values

Question

Let me start by saying I have a file. For more info, you check it here.

I have already added the edges on the hashmap with multiple values. I have check it by adding this piece of code:

        map.put(nodes.get(i), edges);
        System.out.println(nodes.get(i) + " " +  map.get(nodes.get(i)));

After adding each entry to map, I check it right away if it was successfully added. Yes, it worked.

Here's the output:

a0 [acodijkstra.Edge@b1c5fa]
a1 [acodijkstra.Edge@13caecd, acodijkstra.Edge@f84386, acodijkstra.Edge@1194a4e]
a2 [acodijkstra.Edge@15d56d5, acodijkstra.Edge@efd552, acodijkstra.Edge@19dfbff]
a3 [acodijkstra.Edge@10b4b2f, acodijkstra.Edge@750159, acodijkstra.Edge@1abab88]

But when creating another method to display the content of the map, I found out that the map is empty. Here's the code:

public void viewFile() {
    for(int i=0; i<nodes.size();i++) {
        System.out.println(nodes.get(i) + " " + this.map.get(nodes.get(i)));
    }
}

The output of the above code is this:

a0 []
a1 []
a2 []
a3 []

What could be the possible reason for this? I am really confused why this happened.

For the code, here's the simplified version (if complied, this will result to errors since I edited some parts deemed unnecessary out):

class Reader {

    HashMap<String, Vertex> vertexList;
    Map<String, ArrayList<Edge>> map;
    ArrayList<String> nodes;
    ArrayList<Edge> edges;


public Reader(String fileName) {
    vertexList = new HashMap<String, Vertex>();
    map = new HashMap<String, ArrayList<Edge>>();
    nodes = new ArrayList<String>();
    edges = new ArrayList<Edge>();
    readFile(fileName);
}

private void readFile(String fileName) {

    try{
        FileReader file = new FileReader(fileName);
        Scanner sc = new Scanner(file);

        int i = 0;
        while (sc.hasNextLine()) {
          input.add(sc.nextLine());
          i++;
        }

        setNodes();
        setVertices();
        System.out.println();
        file.close();
    } catch(Exception e){
        System.out.println(e);
    }
}
public void setNodes() {

    System.out.println();

    for(int i=0; i<input.size(); i++) {
        line = this.input.get(i);
        nodes.add(line.substring(0,line.indexOf("-")).trim());
        adjLine.add(line.substring(line.indexOf("-")+1).trim());
    }
}

private void setVertices() {

    String[] listEdges;

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

        //if vertex does not exist, create it
        if(vertexList.containsKey(nodes.get(i))) {
            vertexList.put(nodes.get(i), new Vertex(nodes.get(i)));
        }

        line = adjLine.get(i);

        //separate adj edges to *
        if(line.contains("*")) {
            listEdges = line.split("\\*");
        } else {
            listEdges = new String[1];
            listEdges[0] = line;
        }

        //add edges to specified vertex
        for(int j=0; j < listEdges.length; j++ ) {
            String[] word = listEdges[j].split(",");
            edges.add(new Edge(vertexList.get(word[0]),Double.parseDouble(word[1])));
        }

        map.put(nodes.get(i), edges);
        System.out.println(nodes.get(i) + " " +  map.get(nodes.get(i)));
        edges.clear();
    }
}

Answer 1

Here:

    map.put(nodes.get(i), edges);
    System.out.println(nodes.get(i) + " " +  map.get(nodes.get(i)));
    edges.clear();

you're storing a reference to edges in the map, and then clearing edges. This is why, while the elements are still in the map, they are blank.

You need to stop reusing the same edges object, and create a new one for every vertex (why is edges even a member of your class, and not a local variable?)