Sorting list with unknown type in Java

Question

I have this list in a method:

List<? extends Object> objects;

or it can be also:

List<?> objects;

I wanted to sort it this way, with Java 8 lambda:

objects.sort((p1, p2) -> p1.compareTo(p2));

But it says:

The method compareTo(capture#6-of ?) is undefined for the type capture#6-of ?

I also tried with generic type:

List<O extends Object> objects;

But then it says:

The method compareTo(O) is undefined for the type O

How could I sort a list like this way? I need it in my abstract class, where it recognizes the list field by reflection, and it can sort it.

Answer 1

compareTo is available for object that implements Comparable interface. If you want to do that you should use

List<? extends Comparable> objects;

For a clean code check @Holger answer