public class Pet {	
	public void sayHello(){
		System.out.println(this+" sayHello");
	}
}

public class Cat extends Pet {}




		List<Cat> cats = new ArrayList<Cat>(); 
		cats.add(new Cat());
		
		// Type mismatch: cannot convert from List<Cat> to List<Pet>
		// List<Pet> pets = cats;  
		
		// pets 指向 cats 
		List<? extends Pet> pets = cats;    
		pets.get(0).sayHello();
		cats.get(0).sayHello();
		
		// 编译错误 
		// pets.add(new Cat());
		
		// 数组 
		Cat[] catarr = {new Cat()};
		
		Pet[] petarr = catarr;
		petarr[0].sayHello();    
		catarr[0].sayHello();
		
		//  java.lang.ArrayStoreException: dyyx.generic.Pet
		// petarr[0] = new Pet();