PECS.java

PECS.java

import java.util.ArrayList;
import java.util.List;
import java.util.Optional;

// https://habr.com/ru/post/559268/

class Class0 { }

class Class1 extends Class0 { }

class Class2 extends Class1 { }

class Class3 extends Class2 { }

class Class4 extends Class3 { }

class Class5 extends Class4 { }

class Class6 extends Class5 { }

class Class7 extends Class6 { }

public class PECS {
    public static void someMethodPe(List<? extends Class2> producerList) {
        Class2 class2 = producerList.get(0); // correct producing

        // wrong consuming - not compiles at all
        // producerList.add(new Class2());
    }

    public static void someMethodCs(List<? super Class3> consumerList) {
        consumerList.add(new Class4()); // correct consuming

        // not very wrong producing
        Object object = consumerList.get(0);
    }

    public static void main(String[] args) {
        ArrayList<Class3> class4s = new ArrayList<>();
        class4s.add(new Class4());
        class4s.add(new Class5());
        someMethodPe(class4s);
        System.out.println("Finished PE");

        ArrayList<Class2> forConsuming = new ArrayList<>();
        forConsuming.add(new Class5());
        someMethodCs(forConsuming);
        System.out.println("Finished CS");
    } 
}