wildcards.java

public class Lists {
    public static void main(String[] args) {
        // Lower Bounded Wildcards
        m1(new ArrayList<>());  // <> = C
        m1(new ArrayList<B>());
        m1(new ArrayList<D>()); // Error

        // Upper Bounded Wildcards
        m2(new ArrayList<>());  //  <> = C
        m2(new ArrayList<D>());
        m2(new ArrayList<B>());  // Error
    }

    private static void m1(List<? super C> l) {
        l.add(new C());
    }

    private static void m2(List<? extends C> l) {
        System.out.println(l.get(1));
    }
}

class A {}

class B extends A {}

class C extends B {}

class D extends C {}