JAVA generic class method not applicable

Question

public class IdfMap<K, V> extends HashMap<K, V>{

    public IdfMap() {
        super();
    }

    public IdfMap(int initialCapacity){
        super(initialCapacity);
    }

    public IdfMap(int initialCapacity, float loadFactor){
        super(initialCapacity, loadFactor);
    }

    public <K, V extends Comparable<? super V>> SortedSet<Map.Entry<K, V>> entriesSortedByValues(){
        SortedSet<Map.Entry<K, V>> sortedEntries = new TreeSet<>(
                new Comparator<Map.Entry<K, V>>() {
                    @Override
                    public int compare(Map.Entry<K, V> e1, Map.Entry<K, V> e2){
                        return e2.getValue().compareTo(e1.getValue());
                    }
                }
                );
        sortedEntries.addAll(this.entrySet());                      
        return sortedEntries; 
    }
}

The line

sortedEntries.addAll(this.entrySet());

does not work. Why? It tells me that the method is not applicable for the given argument, which is a pretty vague error statement to understand. I would except this.entrySet() to return the set, which should in theory be usable for the addAll method.

Why does your method have its own type parameters?

user2357112
– user2357112

2014-07-08 05:15:53 +00:00
Commented Jul 8, 2014 at 5:15 — user2357112
– user2357112, Commented Jul 8, 2014 at 5:15
Java noob, thanks for the help!

devmax
– devmax

2014-07-08 06:09:31 +00:00
Commented Jul 8, 2014 at 6:09 — devmax
– devmax, Commented Jul 8, 2014 at 6:09

Thilo · Accepted Answer · 2014-07-08 05:18:28Z

2

Your method introduces its own generic type parameters, also called K and V, but completely different from the ones defined by the class.

As a result, within the method, K and V refer to something different from (hence incompatible with) the "real" types.

This is like local variables shadowing member variables. And since they have the same name, the error message becomes hard to understand.

Remove the type parameter declaration from the method, should be

public SortedSet<Map.Entry<K, V>> entriesSortedByValues(){

answered Jul 8, 2014 at 5:18

Thilo

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1 Comment

user2357112 Over a year ago

Additionally, the Comparable requirement will have to be moved to the class's type parameters, or a cast involving Comparable will need to be introduced somewhere.

Alexandre Santos · Accepted Answer · 2014-07-08 05:21:45Z

0

Read the error message. It says The method addAll(Collection<? extends Map.Entry<K,V>>) in the type Set<Map.Entry<K,V>> is not applicable for the arguments (Set<Map.Entry<K,V>>)

This is because you are mixing Maps and Sets.

answered Jul 8, 2014 at 5:21

Alexandre Santos

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1 Comment

user2357112 Over a year ago

No, that's not the problem.

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