Java Generics Bound mismatch

Question

Here is a implementation of a generic search algorithm:

The interface:

public interface Comparable<T extends Comparable<T>> {
    int compare(T arg); 
}

CompareSort.java

public abstract class CompareSort<T extends Comparable<T>> {

protected boolean less(T v, T w) {
    return (v.compare(w) < 0);
}

protected void swap(T[] args, int i, int j) {
        T swap = args[i];
        args[i] = args[j];
        args[j] = swap;
}

public abstract void sort(T[] args);

}

One of the algorithm:

public class SelectionSort <T extends Comparable<T>> extends CompareSort<T>  {

@Override
public void sort(T[] args) {
    int N = args.length;
    for (int i = 0; i < N; i++) {
        int min = i;
        for (int j = i + 1; j < N; j++) {
            if (less(args[j], args[min])) {
                min = j;
            }
        }
        swap(args, i, min);
    }
}

}

And finally a main method to sort Strings.

public class StringSorter {
    public static  <T extends Comparable<T>> void main(String[] args) throws IOException {

        ArrayList<String> list = new ArrayList<String>();

        int i = 0;
        while (i < 10) { 
            Scanner s = new Scanner(System.in); 
            String str = s.nextLine(); 
            list.add(str);
            i++;
        }
        String[] a = list.toArray(new String[list.size()]);

        // Create a sort object, use it on a, and print the sorted array.
        SelectionSort<String> selection = new SelectionSort<String>();
        selection.sort(a);
        for (i = 0; i < a.length; i++) {
            System.out.println(a[i]);
        }
    }
}

Here is the problem:

SelectionSort<String> selection = new SelectionSort<String>();

Bound mismatch: The type String is not a valid substitute for the bounded parameter (T extends Comparable(T)) of the type SelectionSort(T)

(box brackets = curved brackets)

Where is the problem? I can not figure it out... the generic parameter T is extended as well.

Answer 1

Instead of creating your own Comparable, which String does not implement, use Java's java.lang.Comparable, which String does implement.