ArrayList的多态性，ArrayList多态性

ArrayList的多态性，ArrayList多态性

import java.util.*;

public class TestGenerics3
{
	public static void main(String [] args)
	{
		new TestGenerics3().go();
	}
	public void go()
	{
		ArrayList<Animal> animals=new ArrayList<Animal>();
		animals.add(new Dog());
		animals.add(new Cat());
		animals.add(new Dog());
		takeAnimals(animals);
		
		ArrayList<Dog> dogs=new ArrayList<Dog>();
		dogs.add(new Dog());
		dogs.add(new Dog());
		takeAnimals(dogs);
		
	}
	public void takeAnimals(ArrayList<? extends Animal> animals)
	{
		for(Animal a:animals)
			a.eat();
		//animals.add(new Cat());不可以
	}
}
abstract class Animal
{
	void eat()
	{
		System.out.println("animal eating");
	}
}
class Dog extends Animal
{
	void bark(){}
}
class Cat extends Animal
{
	void meow(){}
}