generics question

[geeta vemula]

this is from javabeat.net

  39)What is the effect of the following program?
class Test
{
public static void m1(List<Object> l1,List<String> l2) { } //Method1
public static void m1(List<Object> l1,List<Object> l2) { } //Method2
public static void m1(List<Number> l1,List<String> l2) { } //Method3
public static void m1(List<Number> l1,List<Object> l2) { } //Method4
public static void m1(List<?> l1,List<?> l2) { }
//Method5
public static void main(String[] args)
{
List<Number> ls1 = new ArrayList<Number>();
List<String> ls2 = new ArrayList<String>();
m1(ls1,ls2);
}
}
a only method 3
b method 1 and method 3
c method 3,method 4 and method 5
d compile time error
e Ambiguous method call
  

Ans d
Can anyone exlain me why it is giving compiler error?

I think because two methods hold valid for the given call --> method3 and method5. Is it the reason it is giving compiler error?
thanks,
Geeta V

[M Srilatha]

Hi,

# public static void m1(List<Object> l1,List<String> l2) { } //Method1  
# public static void m1(List<Object> l1,List<Object> l2) { } //Method2  
# public static void m1(List<Number> l1,List<String> l2) { } //Method3  
# public static void m1(List<Number> l1,List<Object> l2) { } //Method4  
# public static void m1(List<?> l1,List<?> l2) { } 

all of the statements above will result as the following statement after removing generic type information:

  public static void m1(List l1,List l2) { }  

hence the compiler will report them as duplicate method declaration!

Always remember that generic type information is only valid at compile time. when the code is compiled, all generic type information is removed from the code.

Hope this helps!

[Stephen Davies]

Yes 'M' is right, the methods are not correctly overloaded, as the basic requirement in overloaded methods is to change the argument list.

The generics tag does not fulfill this requirement as the compiler simply sees a List Object in all the Methods called l1 and l2: