Doubt-Generics and Collections-Kathy Sierrra- SCJP6-Question8

[Richa Sharma]

I have a doubt in Q.8 Chapter-Generics and Collections of Kathy Sierrra book for SCJP 6.

Question:

Given a method declared as:

  
public static <E extends Number> List<E> process(List<E> nums)
A programmer wants to use this method like this:
// INSERT DECLARATIONS HERE
output = process(input);
 

Which pairs of declarations could be placed at // INSERT DECLARATIONS HERE to allow
the code to compile? (Choose all that apply.)

A. ArrayList<Integer> input = null;
ArrayList<Integer> output = null;

B. ArrayList<Integer> input = null;
List<Integer> output = null;

C. ArrayList<Integer> input = null;
List<Number> output = null;

D. List<Number> input = null;
ArrayList<Integer> output = null;

E. List<Number> input = null;
List<Number> output = null;

F. List<Integer> input = null;
List<Integer> output = null;

G. None of the above.

The correct answers are B,E,F

My doubt is why is choice A incorrect?

[vijay saraf]

as per the return type the method would return a List so the 'output' variable must be a List.
How will you assign the List to ArrayList ?

[Richa Sharma]

Thanks Vijay but there are covariant return types in java

[Ankit Garg]

Hint:

Can you do this

ArrayList list = new Object();

or this

String str = new Object();

Thats what the option A is trying to do i.e. assign a List into an ArrayList...

[vijay saraf]

It doesn't mean that you can assign a object to a subtype without cast
see the code

class CovariantReturnType {
	static Object Method1(){
		return new String("Hi");
	}
	public static void main(String[] args){
		System.out.println("Hello World!");
		String a = Method1();
	}
}

This code won't comiple...

as method1() is returnig a String ( Sub class object of the Object Class) in actual,but method signature says it will return a Object type object
so returning value can't be assigned to Sub class object without downcast.

[Ankit Garg]

vijay saraf wrote:It doesn't mean that you can assign a object to a subtype without cast

Was this for me?? Well if yes, then I was just trying to say that direct assignment of a super type object to a sub-type reference will result in a compilation error. The code of option A will fail because the return type of the method is List as opposed to the reference in which it is assigned. Just merge the code and the option and everything will be crystal clear

public static <E extends Number> List<E> process(List<E> nums) {/*some code*/}
public static void main(String[] args) {
    ArrayList<Integer> input = null;
    ArrayList<Integer> output = null; 
    output = process(input);   //compilation error
}

Even if we forget generics, option A wont compile

public static  List process(List nums) {/*some code*/}
public static void main(String[] args) {
    ArrayList input = null;
    ArrayList output = null; 
    output = process(input);  //compilation error
}

[Richa Sharma]

Thanks Ankit

[vijay saraf]

NO...Ankit...i replied to Richa Only.
We both are saying the same thing.

[Ankit Garg]

vijay saraf wrote:NO...Ankit...i replied to Richa Only.
We both are saying the same thing.

Yes we are saying the same thing. That's why I was confused that why are you telling this to me ...