Generics question

[Nik Arora]

Hi All,
Source : Johns mock exam


public static void main( String args[] )
{
List<? extends Number> type = new ArrayList<Integer>(); // 1
for ( Integer n : type ) // 2
{
System.out.println(n); // 3
}
}
public <T> void seth(List<?> type) // 4
{
type.add("hi"); // 5
}

Output is Line 2 and Line 5 have compiler errors.

I understood line 5 but for line 2 this was the explanation

type in main() has to be referenced by a Number not an Integer. So the for in loop fails to compile.


Can anybody explain me the above line?

Thanks All

[Keith Lynn]

Originally posted by nik arora:
Hi All,
Source : Johns mock exam


public static void main( String args[] )
{
List<? extends Number> type = new ArrayList<Integer>(); // 1
for ( Integer n : type ) // 2
{
System.out.println(n); // 3
}
}
public <T> void seth(List<?> type) // 4
{
type.add("hi"); // 5
}

Output is Line 2 and Line 5 have compiler errors.

I understood line 5 but for line 2 this was the explanation

type in main() has to be referenced by a Number not an Integer. So the for in loop fails to compile.


Can anybody explain me the above line?

Thanks All

In line 2, you define the type in the List to be any type that extends Number.

That means that the List could refer to a List containing Integer, but it could also refer to a List containing Double, Float, or other subclasses of Number.

So the only thing guaranteed about the type in the List is that it is a Number.

[Barry Gaunt]

All you have been given is a list of things of some type "don't know". So how do you know that they are Integers? Any other clues? Yes - these things are all Numbers. Great, so you can at least write:

for (Number n : type) {
    System.out.println(n);
}

[ May 03, 2007: Message edited by: Barry Gaunt ]

[Nik Arora]

Hi Barry and kieth i didnt get

[Barry Gaunt]

Well both Keith and I said basically the same thing.
I suggest that you read though Angelika Langer's Generic FAQ to get a good understanding of Java Generics.

[Abdul Mohsin]

Hi Nik,

In your code "type" contains the elements which extends Number so it can be Integer,Float or Double, so in the same way when you are getting elements out of your collection "type" then also it can be Integer or Float ....
So the only thing that can come out guaranteed from "type" is Number.

Abdul Mohsin

[Nik Arora]

Thanks Barry for the link and Thanks Abdul and Keith i got it