comparator for Map.Entry

Question

I have a Map with an enumeration type as the key and Double as the value. I want to sort this based on the Double values. So I got the entry set and want to use Collection

Answer 1

You probably want this:

// Declare K and V as generic type parameters to ScoreComparator
class ScoreComparator> 

// Let your class implement Comparator, binding Map.Entry to T
implements Comparator> {
    public int compare(Map.Entry o1, Map.Entry o2) {

        // Call compareTo() on V, which is known to be a Comparable
        return o1.getValue().compareTo(o2.getValue());
    }   
}

ScoreComparator takes two generic type arguments K and V. Map.Entry is not a valid generic type definition, but you may well use it to bind to Comparator's T type.

Note that V must extend Comparable, in order to be able to call compareTo() on o1.getValue().

You can now use the above ScoreComparator as such:

new ScoreComparator();
new ScoreComparator();
// etc...

Note, from your current implementation, you probably don't even need the K parameter. An alternative:

class ScoreComparator> 
implements Comparator> {
    public int compare(Map.Entry o1, Map.Entry o2) {

        // Call compareTo() on V, which is known to be a Comparable
        return o1.getValue().compareTo(o2.getValue());
    }   
}