Wildcards

The definition

public class SortedList<E extends Comparable> {
    public void insert(E item);
    ...
}

is legitimate in Java 5 (because of compatibility with legacy code) but nonetheless is not fully in the spirit of the new generics. The reason is that the Comparable interface now takes a type parameter:

public interface Comparable<T> {
    public int compareTo(T o);
}

so that compareTo only has to compare this with another T and not with an arbitrary Object as before. Thus the Integer class, for example, is now defined by

public final class Integer ... implements Comparable<Integer> {
    ...
    public int compareTo(Integer anotherInteger) {
    ...
    }
}

Similar definitions are used for Float, String etc. As a consequence, the use of the raw Comparable interface in

public class SortedList<E extends Comparable> {
    ...
}

is inadequately typed from the new perspective.

A simple modification to the SortedList class definition, to make it compliant with Java 5 generics, would be

public class SortedList<E extends Comparable<E>> {
    public void insert(E item);
    ...
}

which requires that E must implement the Comparable<E> interface. The following will then all be legitimate:

SortedList<Integer> ilist = new SortedList<Integer>();
SortedList<Float> flist = new SortedList<Float>();
SortedList<String> slist = new SortedList<String>();