Compare HashMap with Objects

Question

I've got a hashmap<CustomObject,Integer> and I'd like to compare the Integers (values) within each entry. So, basically I'd like to sort my values by their Integer value in descending order. I've got a Comparator which consists of the following...

class Compare implements Comparator<Integer>{
    Map<CustomObject,Integer> map;
    /**
     * Constructs our map
     * @param map map to be sorted.
     */
    public Compare(Map<CustomObject,Integer> map){
        this.map = map;
    }
    /**
     * Performs the comparison between two entries.
     */
    public int compare(Integer one, Integer two){
        if(map.get(one) <= map.get(two)){
            return 1;
        }else{
            return 0;
        }
    }
}

I pass my Hashmap into the TreeMap by calling the following line of code.. Tmap.putAll(Hmap);. Where Tmap and Hmap are defined as:

private HashMap<CustomObject,Integer> Hmap;
private TreeMap<CustomObject,Integer> Tmap;

When I run my code I get the error Exception in thread "main" java.lang.ClassCastException: CustomObject cannot be cast to java.lang.Comparable.

The exception appears to be called when I attemp to extract a value from my sorted list. like so...

TreeMap<CustomObject,Integer> sorted = Tmap.putAll(hmap);
sorted.get(o);

where o is a CustomObject.

I think I've misunderstood how comparator works.. what am I doing wrong? How would I compare the two Integer values?

EDIT

Just to clarify what I'm actually trying to do...

I want to compare Integers which are linked to a CustomObject. I can't make the key the Integer because these Integers may not be unique. I was to compare them because I'd like to sort my collection in descending order based on their Integer value.

Answer 1

You need to change your comparator to compare CustomObjects, not Integers:

class Compare implements Comparator<CustomObject>{
    Map<CustomObject,Integer> map;
    /**
     * Constructs our map
     * @param map map to be sorted.
     */
    public Compare(Map<CustomObject,Integer> map){
        this.map = map;
    }
    /**
     * Performs the comparison between two entries.
     */
    public int compare(CustomObject left, CustomObject right){
        return map.get(left).compareTo(map.get(right));
    }
}

Then, you need to tell the TreeMap to use your comparator:

private Map<CustomObject,Integer> Tmap = 
    new TreeMap<CustomObject,Integer>(new Compare(HMap));

Answer 2

new TreeMap<..>(new Compare<..>(map))

You have to specify the comparator when constructing the tree. Otherwise it assumes your keys are comparable (and they aren't)

But check this answer for sorting a map based on the values.