Confusion with Generics

[shameem Shek]

Hi This is Shameem..
I have littlebit Confusion with Generics..

1. import java.util.*;
2. class TestGen
3. {
4.public static void main(String[] args)
5.{
6.Queue<Integer> q = new LinkedList<Integer>();
7.q.add(2342);
8.q.add(345);
9.q.add(455);
10.q.add(new String("ishmayel"));
11.q.add(new Double(324));
12.q.add(new StringBuilder("ishmayel"));
13.showAll(q);
14.}
15.public static void showAll(Queue q)
16.{
17. q.add(new Float("ishmayel"));
18. q.add(new Double(324));
19. q.add(new StringBuilder("ishmayel"));
20. while (!q.isEmpty())
21 System.out.print(q.remove()+ " ");
22.}
23. }

Why I am getting the compilation error at lines 10,11,12 but there is no compilation errors at line 17,18, 20 what is the reasion.

I think If there is any error at lines 10,11,12 that will be repeat at line 17,18,19 also but I am getting the errors at lines 10,11,12 only why..?
please any one can help me..
Thanks in advance..
Shammem.

[Sheikh Sadiruddin]

Hi shameem ,
Because your showAll(Queue q) method can take any type of Queue object. If you want to make it type safe , then use something like this showAll(Queue<yourType> q).
I think you have got it now.

[ahmed yehia]

When you declare a Collection with type parameter, that means it can only hold objects of that type and its subtypes and nothing else, this check is done at compile time.

    
Queue<Integer> q = new LinkedList<Integer>(); // accepts only Integers
q.add(2342); // Ok adding Integer
q.add(new String("ishmayel")); // Not ok, adding non Integer

In the showAll(Queue q) you are declaring a Queue without type parameter i.e <type>, so you can add objects of various type. Note you will get warning here.
[ October 04, 2007: Message edited by: Ahmed Yehia ]