Need Help on Generecs and non-Generics

[swarupa patil]

Hi Ranchers,
I have dobut on Generics and non-Generics
I have written the code that have Generics as well as the non-generics
part.The code below compile fine but gives one warning even though
the list is of Integer genric type and I am trying to add the element
of Integer type but still it gives one warning.

import java.util.*;
public class scjp
{
    public static void main(String[] args) {
        List<Integer> myList = new ArrayList<Integer>();
        myList.add(4);
        myList.add(6);
        Inserter in = new Inserter();
        in.insert(myList); // pass List<Integer> to legacy code
    }
}
class Inserter {
    // method with a non-generic List argument
    void insert(List list) {
        list.add(new Integer(42)); // adds to the incoming list //Line  111
    }
}

If I try to add the element as shown above at Line 111 it gives
compile time waring

I need information why the above code gives the compiletime warnings
[ September 26, 2006: Message edited by: Barry Gaunt ]

[Keith Lynn]

Note that the formal parameter of the insert method is a raw List.

[Barry Gaunt]

Let's trim your example down to only the class Inserter:

import java.util.List;
class Inserter {
    void insert(List list) {
        list.add(new Integer(42));
    }
}    

The compiler warning (use -Xlint to get more info) is:

barry@ours:~/Project1$ javac -Xlint Inserter.java
Inserter.java:4: warning: [unchecked] unchecked call to add(E) as a member of the raw type java.util.List
        list.add(new Integer(42));
                ^
1 warning

What this means is that the compiler is going to allow you to add an Integer to the list even though you have not specified what type of objects the list contains. You may have passed in a list containing only Strings and after performing the add(new Integer(42)) you are going to have a mixture of Strings and Integers. This may cause problems later when you use the list. For example, if you take an object from the list thinking that it is a String but it happens to be that Integer, then you will get a ClassCastException at runtime. Putting it simply, the warning is telling you that the compiler cannot check if you will or will not get a ClassCastException definitely because it does not know how you are intending to use the list in the future.
[ September 26, 2006: Message edited by: Barry Gaunt ]