Generics:Wild cards with upper Bounds

[Deepak Lal]

Hi Ranchers,
I have written a java program for testing Generics with Upper Bound.
Please see the below example.
i have a clarification.why doesnt the java api provide feasibility for "Adding Lists" other than null values.???

Please explain me...

import java.util.*;

class Animal{}

class Dog extends Animal{}

class Cat extends Animal{}

public class TestGenericsUpperBound 
{
	public static void main(String[] args) 
	{
		List<? extends Animal> animals = new ArrayList<Animal>();				
		List<Dog> dogs = new ArrayList<Dog>();
		List<Cat> cats = new ArrayList<Cat>();
		
		dogs.add(new Dog());
		dogs.add(new Dog());
		System.out.println("Dogs are .."+dogs.size());
		cats.add(new Cat());
		cats.add(new Cat());		
		System.out.println("cats are .."+cats.size());
		animals=dogs;				
		System.out.println("dogs Size .."+animals.size());
		animals=cats;
		System.out.println("cats Size .."+animals.size());
		[b]animals.add(animals);//why this is not possible.??[/b]		
	}
}

When i ran the above code i got a compile time error


Help provided will be highly appreciated.
Please help me out.

[Henry Wong]

List<? extends Animal> animals = new ArrayList<Animal>();    

The reason you can't add anything other than nulls, is because Java don't know the type -- all it knows is that it is something that extends Animal. This is *not* a language issue. The designers of Java didn't arbitrarly decide to not allow anything to be added... think about it... if all you know is that it is a unknown type that extends Animal, how would you type check it, to safely add items?

Basically, you use these types of references so that you can have more broad code -- in this case, so that you can write code that can work with any type that extends Animal. If you can't stay within the restrictions, then you need to write more specific code, that work with specific types.

Henry

[Deepak Lal]

Hi Henry,
Im extremely thankful for your awesome answer.Thanks once again.



Im understanding the concepts of generics now,Thanks once again.

--
Deepak Lal