An example of the Java List invariance.

ListInvariance.java

import java.util.LinkedList;
import java.util.List;

interface Powerful {}
class Jedi implements Powerful {}
class Sith implements Powerful {}

public class ListInvariance {
    public static void main(String[] args) {
        List<Jedi> jedi = new LinkedList<>();
        jedi.add(new Jedi());

        List<Powerful> powerfuls = jedi;   // Error
    }
}