Problems with Lists

[micha koern]

Hi,
i have got a strange problem with Lists.

Animal is a superclass of Dog- because of this i thought the following would be fine.
Why cant i do this:

public class MyClass {

	public static void main(String[] args) {
		List<? super Dog> l = new ArrayList<Animal>();
		l.add(new Animal());
	}
}

class Animal {}
class Dog extends Animal{}
class Dog2 extends Dog {}
class Dog3 extends Dog2 {}

Why can i do this:

public class MyClass {

	public static void main(String[] args) {
		List<? super Dog> l = new ArrayList<Animal>();
		l.add((Dog) new Animal());
		l.add((Dog2) new Animal());
	}
}

class Animal {}
class Dog extends Animal{}
class Dog2 extends Dog {}
class Dog3 extends Dog2 {}

Thanks for your help!

Micha

[Gaurav. Sachdeva]

public class MyClass {

public static void main(String[] args) {
List<? super Dog> l = new ArrayList<Animal>();
l.add(new Animal());
}
}

class Animal {}
class Dog extends Animal{}
class Dog2 extends Dog {}
class Dog3 extends Dog2 {}

Here you are NOT able to add Animal object as it is not sure what will be reference type. The reference type is ? super Dog, so it can be Dog also. So you cannot add Animal Object to a Dog as ANIMAL IS NOT A DOG.

When you change above all methods invocations to as follows:

l.add((Dog) new Animal());
l.add((Dog2) new Animal());

Here it will compile as you can are casting Animal to a Dog or Dog2 and these can be added even if the reference type is Dog or any of its super class.
ALSO THIS CODE WILL COMPILE BUT WILL THROW A CLASS CAST EXCEPTION AT RUNTIME.

[micha koern]

Thank you now i understand...

The reference type is ? super Dog, so it can be Dog also.

This helps a lot.

Thank you

-Micha

[Dan Drillich]

An interesting related question at Inheritance - Mock Question & Explanation on Dan Chisholm's website.

Regards,
Dan