A question from the Master Exam CD (generics)

[Nikhil Pujari]

This question is regarding generics.

In the below program, the return type (alist2 and alist3) meet the conditions set in the return portion of the method signature (which is List<? extends T>). Yet, I get an error. Thanks in advance. The program follows:

import java.util.*;

class BackLister
{
	public static <T> List <? extends T> backwards(List<T> input)
	{
		List<T> output = new LinkedList<T>();
		for (T t : input)
			output.add(0,t);
		return output;
	}
}

public class Test{
public static void main(String[] args){
List<Number> alist=new ArrayList<Number>();
List<Integer> a2list=BackLister.backwards(alist);//error here
List<Number> a3list=BackLister.backwards(alist);//error here too
}}

[Kunal Lakhani]

Can you please post the error?

[ravi suthar]

try this

List<Integer> a2list=(List<Integer>) BackLister.backwards(alist);
   List<Number> a3list=(List<Number>) BackLister.backwards(alist);

[Nikhil Pujari]

Sure, here it is:

[Nikhil Pujari]

@ravi: Explicit casting works, but I wonder why I need it. It seems redundant.

Edit: I think I get it. Please tell me if I am wrong. if T is "Number", List<? extends T> can return List<Short> or List<Float>. These cannot be caught in a List<Integer>, hence the cast.

You need a cast for List<Number> because, the only thing a List<Number> variable can point to, is a List<Number>. It cannot point to a List<Integer>, even though Integer is-a Number.

Is this right?

[Tim McGuire]

Nikhil Pujari wrote:@ravi: Explicit casting works, but I wonder why I need it. It seems redundant.

Edit: I think I get it. Please tell me if I am wrong. if T is "Number", List<? extends T> can return List<Short> or List<Float>. These cannot be caught in a List<Integer>, hence the cast.

You need a cast for List<Number> because, the only thing a List<Number> variable can point to, is a List<Number>. It cannot point to a List<Integer>, even though Integer is-a Number.

Is this right?

pretty much.
A List<Number> can be assigned to a List<? extends Number> but a List<? extends Number> cannot be assigned to a List<Number> (which is what you are trying to do in line 18)
A List<Integer> can be assigned to a List<? extends Number> but a List<? extends Number> cannot be assigned to a List<Integer> (which you are trying to do in line 17)

[Nikhil Pujari]

Thanks to all.

[Henry Wong]

Nikhil Pujari wrote:@ravi: Explicit casting works, but I wonder why I need it. It seems redundant.

In my opinion, if you are using generics, you should not be explicitly casting. It is not redundant. If the compiler is complaining, it is because they compoler considers it not safe. If you override the compiler by explicitly casting, at best, you are not letting the compiler do its job. And at worst, you are wrong (and the compiler is right) and you may have broken something in your program.


As for your example....

List<Integer> a2list=BackLister.backwards(alist);//error here   

A List<? extends Number> is *NOT* assignable to a List<Integer>, hence, the compiler error.

List<Number> a3list=BackLister.backwards(alist);//error here too   

A List<? extends Number> is *NOT* assignable to a List<Number>, hence, the compiler error.




Now again, for your example, your method is returning a List<Number>, so with your explicit casting...

List<Integer> a2list=(List<Integer>) BackLister.backwards(alist);   
List<Number> a3list=(List<Number>) BackLister.backwards(alist);  

The first line of the example is actually broken, it is trying to assign a List<Number> to a List<Integer>, which is *NOT* type safe. From within the method, it is possible to put Number types that is not Integer into the list, which you then explicit cast to List<Integer>. So, in the first cast, you were wrong, and the compiler right, and you could have broken something.


Henry