Mixing Legacy with Generic Code

[Abhi vijay]

  import java.util.*;
public class LegacyCode
{
public static void main(String[] args)
{
	List<String> s =new ArrayList<String>();
	warning(s);


}
public static void warning(List o)
	{
	}
}
  

Here even though I have mixed Generic+Legacey, NO warning is generated as the legacy code does not add anything to the ArrayList.

But when I change the program,

  public class LegacyCode
{
public static void main(String[] args)
{
	List s =new ArrayList();
	warning(s);


}
public static void warning(List<String> o)
	{
	}
}  

Here there is a Compiler Warning generated, even though I am not trying to add anything to the Arraylist. Can anyone explain?

[Ruben Soto]

The first case is similar to:

List l = new ArrayList<String>();

This is OK as long as you don't add elements through l.

The second case is equivalent to:

List l = new ArrayList();
...
List<String> ls = l; // 1

The problem in this case is that you don't know what type of elements are already in the object pointed to by the non-generic List reference l. You could have added something that is not a String prior to the assignment in line // 1, and the compiler is warning you about that possibility.

[Abhi vijay]

Thanks Ruben, I got it!

[Ruben Soto]

Great!