Trying to sort a map where the value is an object and I can sort by multiple values in the object

Question

I'm trying to sort a map based on different possible values in the Student object. This doesn't seem to be working for me and I can't seem to figure out why. Any help would be very appreciated. Thanks!

protected static Map<Integer, Student> sort(Map<Integer, Student> unsorterMap, String field)
{
    Comparator<Integer> valueComparator =  
            new Comparator<Integer>() 
            {
                public int compare(Integer k1, Integer k2) 
                {
                    int compare;
                    if(field.equalsIgnoreCase("name"))
                    {
                        compare = unsorterMap.get(k1).getName().compareTo(unsorterMap.get(k2).getName());
                    }
                    else
                    {
                        compare = Integer.compare(unsorterMap.get(k1).getIndex(), unsorterMap.get(k2).getIndex());
                    }


                    if (compare == 0)
                        return 1;
                    else
                        return compare;
                }
            };

    Map<Integer, Student> sortedMap = new TreeMap <Integer, Student>(valueComparator);
    sortedMap.putAll(unsorterMap);
    return sortedMap;
}

Answer 1

Your comparator violates the Comparator contract. From javadoc :

The implementor must ensure that sgn(compare(x, y)) == -sgn(compare(y, x)) for all x and y

But in your code, you have

if (compare == 0)
    return 1;
else
    return compare;

If two students have the same value for the field you use to do the comparison, both compare(x,y) and compare(y,x) will return 1.