Java generics warnings on java.util.Collections

Question

I have a method:

public List sortStuff(List toSort) {
    java.util.Collections.sort(toSort);

    return toSort;
}

T

Answer 1

Collections.sort(List<T>) expects that T must implement Comparable<? super T>. It seems like Stuff does implement Comparable but doesn't provide the generic type argument.

Make sure to declare this:

public class Stuff implements Comparable<Stuff>

Instead of this:

public class Stuff implements Comparable

Answer 2

Do tou use this:

// Bad Code
public class Stuff implements Comparable{

    @Override
    public int compareTo(Object o) {
        // TODO
        return ...
    }

}

or this?

// GoodCode
public class Stuff implements Comparable<Stuff>{

    @Override
    public int compareTo(Stuff o) {
        // TODO
        return ...
    }

}

Answer 3

Sorting Generic Collections

There are two sorting functions defined in this class, shown below:

public static <T extends Comparable<? super T>> void sort(List<T> list);

public static <T> void sort(List<T> list, Comparator<? super T> c);

Neither one of these is exactly easy on the eyes and both include the wildcard (?) operator in their definitions. The first version accepts a List only if T extends Comparable directly or a generic instantiation of Comparable which takes T or a superclass as a generic parameter. The second version takes a List and a Comparator instantiated with T or a supertype.

Answer 4

You will need to change the return type of your method