Unable to Iterate through a hashtable using enumeration in java

Question

This is the code where I am trying to print the key but it never gets executed rather it just goes to the the last line and throws null pointer exception.

Enumeration<String> keys = states.keys();
        int max=0;
        String maxClass="";
        while(keys.hasMoreElements())
        {
            String key = keys.nextElement();
            System.out.println(key);
                if(states.get(key)>=max)
                {
                    max=states.get(key);
                    maxClass=key;
                }
                System.out.println(maxClass);
        }
        System.out.println(maxClass);
        Category c = classes.get(maxClass);
        c.add(p);

this is the complete code of the classify function.

static void classify(ArrayList<Point> cities,Hashtable<String,Category> classes)
{
    for(Point p: cities)
     {
        ArrayList<Point> nbours = new ArrayList<Point>();
        nbours.addAll(getNbours( p,cities));
        // System.out.println(p.city);
        Hashtable<String,Integer> states = new  Hashtable<String,Integer>();
        for(Point pt : nbours)
        {
                //System.out.println(pt.city);

                if(states.containsKey(pt.state))
                {
                    int cnt = states.get(pt.state) + 1;
                    states.remove(pt.state);
                    states.put(pt.state,cnt);
                    //states.add(state,);
                }
                else
                {
                    states.put(pt.state,1);
                }
                //System.out.println(states.get(pt.state));
        }

        Enumeration<String> keys = states.keys();
        int max=0;
        String maxClass="";
        System.out.println(keys);
        while(keys.hasMoreElements())
        {
            String key = keys.nextElement();
            System.out.println(key);
                if(states.get(key)>=max)
                {
                    max=states.get(key);
                    maxClass=key;
                }
                System.out.println(maxClass);
        }
        System.out.println(maxClass);
        Category c = classes.get(maxClass);
        c.add(p);
        System.out.println(p.city+ "Classified");
        nbours.clear();

    }

Answer 1

If the execution reaches the last line and exists with NullPointerException, then "c" is null. That means that the call Category c = classes.get(maxClass); returns null. Check if the key in maxClass is available in classes object.