PECS

MainGenerics.java

class A{}
class B extends A{}
class C extends B{}

public class MainGenerics {
    public static void main(String[] args) {
        List<? extends B> producer = new ArrayList<C>();
        for (B b : producer){}
        for (A a : producer){}
        
        List<? super B> consumer = new ArrayList<A>();
        consumer.add(new B());
        consumer.add(new C());
    }
}