Error on Collections.sort : no suitable method found for sort

Question

I have this class name Pair:

public class Pair<K,V> implements Map.Entry<K,V> , Comparable<V>{

    private K key;
    private V value;

    public Pair(){}

    public Pair(K _key, V _value){
        key = _key;
        value = _value;
    }

    //---Map.Entry interface methods implementation
    @Override
    public K getKey() {
        return key;
    }

    @Override
    public V getValue() {
        return value;
    }

    @Override
    public V setValue(V _value) {
        return value = _value;
    }
    ///---Map.Entry interface methods implementation

    @Override
    public int compareTo(V o) {
        return 0;
    }
}

in Program class I have this method:

private static <K,V> void MinMax2(Vector<Pair<K,V>> vector) {

    // Collections.sort() sorts the collection in ascending order
    Iterator iterator = vector.iterator();

    Collections.sort(vector);
}

on this row:

Collections.sort(vector);

I get this error:

Error:(41, 20) java: no suitable method found for sort(java.util.Vector<Pair<K,V>>)
    method java.util.Collections.<T>sort(java.util.List<T>) is not applicable
      (inference variable T has incompatible bounds
        equality constraints: Pair<K,V>
        upper bounds: V,java.lang.Comparable<? super T>)
    method java.util.Collections.<T>sort(java.util.List<T>,java.util.Comparator<? super T>) is not applicable
      (cannot infer type-variable(s) T
        (actual and formal argument lists differ in length))

I implemented Comparable interface. Why do I get the error above?

If you want to sort pairs, Pair must implement Comparable>, not Comparable. Also, don't use Vector, which is obsolete since Java 1.2, and don't use raw types: Iterator>, not Iterator. — JB Nizet, Jul 13 '17 at 21:20
@shmosel I thought that there is might be the problem with implementation of comparable interface.but I can't figure what exactly the problem. — Michael, Jul 13 '17 at 21:21
List (with ArrayList as default implementation). https://docs.oracle.com/javase/tutorial/collections/ — JB Nizet, Jul 13 '17 at 21:28

James Wierzba · Accepted Answer · 2017-07-13T21:28:41.483

4

Your class must implement Comparable<Pair<K,V>> instead of Comparable<V>

Think about it, you are trying to sort a list of Pair<K,V> objects. For the Collections library to know how to sort it, it must know whether one Pair<K,V> object is less than, greater than, or equal to another Pair<K,V> object.

You have instead described a comparison operation for comparing Pair<K,V> to V

edited Jul 13 '17 at 21:28

answered Jul 13 '17 at 21:22

James Wierzba

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Error on Collections.sort : no suitable method found for sort

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