Sorting Map<Integer, Float> by value using ValueComparator gives error in Java 8

Question

I've declared a map like this:

Map<Integer, Float> errorMap = new HashMap<> ();

and then populate it. Finally I've to sort it by value. I've written this:

ValueComparator vc = new ValueComparator(errorMap);
TreeMap<Integer, Float> sortedMap = new TreeMap<> (vc);
sortedMap.putAll(errorMap);

while ValueComparator class is this:

class ValueComparator implements Comparator<Float> {
 Map<Integer, Float> map;
 public ValueComparator(Map<Integer, Float> base) {
 this.map = base;
 }
 public int compare(Float a, Float b) {
 if(map.get(a) <= map.get(b))
 return -1;
 else
 return 1;
 }
}

The problem is that when I compile this code, it gives me this error:

incompatible types: cannot infer type arguments for TreeMap<>
 reason: inference variable K has incompatible bounds
 equality constraints: Integer
 upper bounds: Float,Object
 where K is a type-variable:
 K extends Object declared in class TreeMap

Can anyone please suggest how to handle this. I'm using Java 8.

• Louis Wasserman: Your ValueComparator is really extremely fragile and prone to bugs. You'd be much better off using something like stackoverflow.com/a/2581754/869736.

Answer 1

In the Map declaration,

TreeMap< Integer, Float> sortedMap = new TreeMap<> (vc);

The key values are of type Integer, but while getting the values in the compare() method, you are using Float values as key. So change the type of a and b to "Integer". It should work fine.