/** A quick example of the covariance and the use of the ?
 * in generic types in Java. 
 * As said in class, you cannot change the contents of the
 * ArrayList in the add2Ai method, however, you can change the
 * contents of the items contained therein, so long as the
 * contents are a part of A
 * 
 * The actual actions taken by the add2Ai are irrelevant. What
 * is important is the weird <? extends A> rather than just <A>. 
 * 
 * @author gtowell
 * Created Oct 6, 2021
 */
import java.util.ArrayList;

public class Cov {
    private abstract class A {
        int ai;

        public void add2Ai(int amt, ArrayList<? extends A> arrL) {
            for (A aa : arrL) {
                aa.ai += amt;
            }
        }
    }

    private class B extends A {
        int bi;

        public void dooB() {
            ArrayList<B> arrB = new ArrayList<>();
            for (int i = 0; i < 5; i++) {
                arrB.add(new B());
                arrB.get(i).bi = i;
                arrB.get(i).ai = i - 5;
            }
            System.out.println("Before " + arrB);
            arrB.get(0).add2Ai(4, arrB);
            System.out.println("After " + arrB);
        }

        public String toString() {
            return "<<ai " + ai + " bi " + bi + ">>";
        }
    }

    private class C extends A {
        int ci;

        public void dooC() {
            ArrayList<C> arrC = new ArrayList<>();
            for (int i = 0; i < 5; i++) {
                arrC.add(new C());
                arrC.get(i).ci = i * i * i;
                arrC.get(i).ai = i - 5;
            }
            System.out.println("Before " + arrC);
            arrC.get(0).add2Ai(4, arrC);
            System.out.println("After " + arrC);
        }

        public String toString() {
            return "<<ai " + ai + " ci " + ci + ">>";
        }
    }

    public static void main(String[] args) {
        new Cov().doo();
    }

    public void doo() {
        Cov cov = new Cov();
        B bi = new B();
        C ci = new C();
        bi.dooB();
        ci.dooC();
    }
}