Generic Methods Help

[salih ayan]

in SCJP SCJD 1.6 Kathy Sierra & Bert Bates book page '615', there is a pragraph

(

We still have two questions…how do you get around it and why the heck does
the compiler allow you to take that risk for arrays but not for ArrayList (or any other
generic collection)?

)

it is telling us we have still two question
First "how do you get around it"
my question is exactly this sentence..

What does book want to express by this sentence (how do you get around it)
Please help...

[Adolfo Eloy]

The way to getting around with that is using a List of Animal so you can pass through your method which expects a List of animal.
The point is: you can't deal with polymorphism the same way to ordinary classes.
Base type (List, ArrayList) can be polymorphic as whatever class in java. But generic types can't be treated the same way.
If you really need to use something near to polymorphic generic types you will need to use wildcards with <? extends Something> or <? super Something>.

<? extends Something> you can't use add with collections
<? super Something> you can use add with collections

To see more about it take a look at Generics post from Suresh.

The code bellow is getting around this problem using a list of Animals:

public static abstract class Animal {
		public abstract void checkup();
	}
	
	public static class Dog extends Animal {
		@Override
		public void checkup() {
			System.out.println("Checking up the dog");
		}
	}

	public static class Cat extends Animal {
		@Override
		public void checkup() {
			System.out.println("Checking up the cat");
		}
	}
	
	public static class AnimalDoctorGeneric {
		public void addAnimal(List<Animal> animals) {
			animals.add(new Dog());
			animals.add(new Cat());
		}
	}
	
	public void addAnimal(List<Animal> animals) {
		animals.add(new Dog()); // this is always legal,
		// since Dog can
		// be assigned to an Animal
		// reference
	}

	public static void main(String[] args) {
		List<Animal> animals = new ArrayList<Animal>();
		animals.add(new Dog());
		animals.add(new Dog());
		AnimalDoctorGeneric doc = new AnimalDoctorGeneric();
		doc.addAnimal(animals); // OK, since animals matches
		// the method arg
	}