Java Generics Type Arguments

Question

I have a quite hard time to get my code running. I have a generic method, that should sort and remove all duplicates from a given List. The items in the List, for example, String, all implement the Comparable interface. Unfortunately, the code is not compiling and I don't understand why...

public static <T> List<T> distinctAndSort(List<T> s) {
    List<T> list = s.stream().distinct().collect(Collectors.toList());
    Collections.sort(list); //Error
    return list;
}

This is what I got so far. Why can't I put my List<T> into sort? (The method sort( List<T extends Comparable<? super T>> ) in the type Collections is not applicable to the arguments ( List<T> ).

In order to make my function work, what am I missing? Thx for every help

Answer 1

Declare your type variable as:

<T extends Comparable<? super T>>

This restricts your method only to be called with a type which implements Comparable , and thus you can call Collections.sort without an explicit Comparator .

Alternatively, you could provide a Comparator<? super T> Comparator<? super T> as an additional parameter to the method, and pass it to Collections.sort .

Additionally, note that you can do the sorting with the stream API:

return s.stream()
    .distinct()
    .sorted()
    .collect(Collectors.toList());

Answer 2

According to the docs, Collections.sort(List<T>) must be a list of Comparable s, or you must supply a Comparator<T> . You can change your signature:

public static <T extends Comparable<? super T>> List<T> distinctAndSort(List<T> s)

https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#sort(java.util.List) https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#sort(java.util.List,%20java.util.Comparator)