java <? super Fruit>与<? extends Fruit>

package Test2016;

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

public class Test2016 {

	public static void main(String[] args) {

		List<? super Fruit> first = new ArrayList<Fruit>();	//Fruit -> Apple

		first.add(new Apple());
		first.add(new Fruit());
		first.add(new RedApple());

//		first.add(new Food());	//error

		System.out.println(first.get(0));
		System.out.println(first.get(1));

		List<? extends Apple> second = new ArrayList<Apple>();

		second.add(null);
	}

}

class Food {
}

class Fruit extends Food {
}

class Apple extends Fruit {
}

class RedApple extends Apple {
}