Returning list of keys from a hashmap

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1

I have a hashmap in the form of HashMap<Vertex, HashSet<Edge>>
and want to be able to return a set of keys from this.


public Set<Vertex> getVertices()
    {
          return edgeLists.keySet();
    }

This is the method that i use to do that.

However when i run this method for example.


        HashSet<Vertex> blah = (HashSet<Vertex>)graph.getVertices();

        for (Vertex v : blah)
        {
            System.out.println(v);
        }

i get the following error

java.lang.ClassCastException: java.util.HashMap$KeySet cannot be cast to java.util.HashSet

Any ideas on how to solve this issue.

Thanks in advance

2

The key to your quandary is in the cast exception…

java.lang.ClassCastException: java.util.HashMap$KeySet cannot be cast to java.util.HashSet

The Set “type” in the HashMap is of type KeySet. You’re trying to cast it to a HashSet which isn’t allowed. (HashSet extends AbstractSet and it is unclear what KeySet extends because I think it is an Inner Class of HashMap. However, they don’t seem to be in the same class hierarchy.)

Try casting to the generic Set interface instead.

Best of Luck.

3

Cheers for the advice.


        Set<Vertex> ver = edgeLists.keySet();

        HashSet<Vertex> output = new HashSet<Vertex>();
        for (Vertex v : ver)
        {
            output.add(v);
        }

        return output;

The following code worked - although probably not the best it could be it suffices for my needs.

4

Do you have to use a HashSet? It is typically best to just use the interface Set. That way you really don’t care what the implementation of the Set is behind the scenes.

Best of Luck.

5

I dont have to use that, Sorry - i’m not that good at programming, only a first year computer science student. I see your logic there though and why i should change it to that.

Cheers for the advice!

6

You’re welcome. In my opinion it is almost always best to program to the interface instead of a specific data type if an interface exists.

BTW, no reason to apologize. We all started out at some point. :slight_smile: