java.util.HashMap.get method returns 2 values for one key

Question

I made a Graph class which uses a HashMap> to keep nodes as keys and corresponding edges as values.

public class GraphAL {
private HashMap<Integer, ArrayList<Integer>> adjList;
private ArrayList<Integer> vertices;
int numberOfNodes;
boolean visited[];

GraphAL(HashMap<Integer, ArrayList<Integer>> adjList, 
        ArrayList<Integer> vertices, int numberOfNodes){
    this.adjList = adjList;
    this.vertices = vertices;
    this.numberOfNodes = numberOfNodes;
    visited = new boolean[this.numberOfNodes];
}

public HashMap<Integer, ArrayList<Integer>>  getAdjList() {
    return adjList;
}

public ArrayList<Integer>  getVertices() {
    return vertices;
}

public int getNumeberOfNodes()
{
    return numberOfNodes;
}

public boolean[] getVIsitedNodes()
{
    return visited;
}

public void setVisitedNodesToTrue(int node)
{
    visited[node] = true;
}

}

I made then a method to reverse the graph. The problem is that java.util.HashMap.get method returns 2 values instead of one although my keys are unique. This leads to adding an edge to more than the exact node where I want to add an edge.

public static GraphAL reverseGraph(GraphAL g)
{
    HashMap<Integer, ArrayList<Integer>> revAdjList = new HashMap<Integer, ArrayList<Integer>>();

    ArrayList<Integer> revVertices = new ArrayList<Integer>();

    System.out.println("Printing in for loop");
    for(Integer x:g.getVertices())
    {
        System.out.println("x = " + x);
        ArrayList<Integer> edges = new ArrayList<Integer>();
        edges.add(x);
        System.out.println("Edges: ");
        for(Integer m:edges)
        {
        System.out.print(m +  " ");
        }
        for(Integer y:g.getAdjList().get(x))
        {
            System.out.println("y = " + y);

            if(!revVertices.contains(y)) 
            {
                    revVertices.add(y);
                    System.out.println("RevVertices: ");
                    for(Integer n:revVertices)
                    {
                    System.out.print(n + " ");
                    }
            }
            if(revAdjList.containsKey(y))
            {
                for(Integer o:revAdjList.get(5))
                {
                    System.out.println(5 + " contains " + o);
                }
                System.out.println("adding " + x + " to " + y);

                revAdjList.get(y).add(x);

                for(Integer o:revAdjList.get(y))
                {
                    System.out.println(y + " contains " + o);
                }

                for(Integer o:revAdjList.get(5))
                {
                    System.out.println(5 + " contains " + o);
                }

            }
            else
            {
            revAdjList.put(y, edges);
            System.out.println("putting " + x + " at " + y);
            }
            System.out.println("Current AdjList: ");
            for(Integer h:revVertices)
            {
                System.out.print("vertice is: " + h + " ");
                for(Integer j:revAdjList.get(h))
                {
                    System.out.print("edge : " + j + " ");

                }
                System.out.println();
            }
        }

    }
    System.out.println("Done printing in for loop");

    GraphAL revGraph = new GraphAL(revAdjList, revVertices, g.getNumeberOfNodes());
    return revGraph;
}

Sorry for all the print.ln's but I wanted to be sure where the problem is occurring. It looks like revAdjList.get(y).add(x), where y=6 and x=3, is returning the corresponding ArrayList from key 6 like I want but also from key 5. Ofc this leads to adding edge 3 to 6 like I want but it's also adding it to 5 as well. Thoughts?

x = 3
Edges: 
3 y = 6
5 contains 8
adding 3 to 6
6 contains 8
6 contains 3
5 contains 8
5 contains 3
adding 3 to 6
Current AdjList: 
vertice is: 1 edge : 7 
vertice is: 2 edge : 5 
vertice is: 3 edge : 9 
vertice is: 7 edge : 9 
vertice is: 4 edge : 1 
vertice is: 5 edge : 8 edge : 3 
vertice is: 6 edge : 8 edge : 3 

Answer 1

The problem happens on this line

for (Integer j : revAdjList.get(h)) {
    System.out.print("edge : " + j + " ");
}

Lets point out the issue here and why its printing out the "edge" twice

revAdjList.get(h)

returns a collection of Integer not a single Integer so it can have as many integer values as you want since you are defining it as a collection and not a single-ton.

You are adding multiple edges from here

for (Integer m : edges) {
    System.out.print(m + " ");
}

If you just want 1 edge, don't make edges an array.

HashMap<Integer, ArrayList<Integer>> revAdjList = new HashMap<>();

should be defined as

HashMap<Integer, Integer> revAdjList = new HashMap<>();

After you have done that, you can remove the loops and clean up the code to work with the new syntax.