Exercising Covariance

CovarianceExample.java

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

public class CovarianceExample {
    public static class  Father {
        int x = 5;
        public Father(int x) {
            this.x = x;
        }
    }

    public static class Children extends Father {
        int newProperty  = 5;

        public Children(int x, int newProperty) {
            super(x);
            this.newProperty = newProperty;
        }
    }


    public static class Wrapper<T extends Father> {
        private List<T> catalog;

        @SafeVarargs
        public Wrapper(T... values) {
            this.catalog = new ArrayList<>();
            for (T v : values) {
                this.catalog.add(v);
            }
        }

        public List<T> getCatalog() {
            return this.catalog;
        }
    }

    public static void format(String s, Object... args) {
        System.out.println(String.format(s, args));
    }

    public static void printFather(List<Father> l) {
        for (Father f : l ) {
            format("**************************************************");
            format("Father Value for X: %d", f.x);
            format("**************************************************");
        }
    }

    public static void printChildren(List<Children> l) {
        for (Children c : l ) {
            format("**************************************************");
            format("Children Value for X: %d", c.x);
            format("Children Value for NewProperty: %d", c.newProperty);
            format("**************************************************");
        }
    }

    @SuppressWarnings("unchecked")
    public static void main(String[] args) {
        Wrapper<Father> wf = new Wrapper<>(
                new Father(1),
                new Father(10)
        );

        Wrapper<Children> wc = new Wrapper<>(
                new Children(1, 2),
                new Children(10, 20)
        );

        format("-------------------------------------");
        format(" FATHER ");
        format("-------------------------------------");
        printFather(wf.getCatalog());


        format("\n\n\n");
        format("-------------------------------------");
        format(" CHILDREN ");
        format("-------------------------------------");

        printFather((List<Father>) (List<?>) wc.getCatalog());
        format("-------------------------------------");
        printChildren(wc.getCatalog());
        format("-------------------------------------");
    }
}