Generic methods doubt # 2

[Andry Dub]

Hi, ranchers! Please explain me why first code compiles fine, but second generates compile time error? What is the main idea on this?

public class Test {
 
public static void main(String[] args) {

List<? super Object> ls1 = new LinkedList<Object>();

testGenericMethod(ls1, new Object());
}
 
public static <T> void testGenericMethod(Collection<T> cs, T t) {}
}

public class Test {
 
public static void main(String[] args) {

List<? extends Object> ls1 = new LinkedList<Object>();  <- Why this is illegal?  :confused:  

testGenericMethod(ls1, new Object());
}
 
public static <T> void testGenericMethod(Collection<T> cs, T t) {}
}

[Meher Parveen]

I don't get an error in the line you mention.

 
public class Test{
public static void main(String[] args) {
List<? extends Object> ls1 = new LinkedList<Object>();
testGenericMethod(ls1, new Object()); //[B]Compilation error[/B]
}
public static <T> void testGenericMethod(Collection<T> cs, T t) {}
}
 

I get an error in the call to testGenericMethod(), but i don't know why your getting this error

[Vidhya Ramaswamy]

I am also getting the compilation error on the line that calls testGenericMethod(). But don't know why.

[Keith Lynn]

Could you post the error you get?

[Vidhya Ramaswamy]

This is the compile error:

The method testGenericMethod(Collection<T>, T) in the type GenericTest is not applicable for the arguments (List<capture-of ? extends Object>, Object)

[Alexsandra Carvalho]

The most interessant is that if you pass only one argument, the code compiles...

[Henry Wong]

Originally posted by Andry Dub:
Hi, ranchers! Please explain me why first code compiles fine, but second generates compile time error? What is the main idea on this?

In the first code, you have a collection of instances that are base classes of Object (which can only be Object) and an Object instance. There is a type that matches the method that you defined (Object).

In the second code, you have a collection of instances that are sub classes of Object (which can be anything) and an Object instance. There isn't a type that matches the method that you defined.

The most interessant is that if you pass only one argument, the code compiles...

It would be more interesting... if you show us this code that compiles.

Henry

[Alexsandra Carvalho]

The code compiles:

  
public static void main(String[] args) {
  List<? extends Object> ls1 = new LinkedList<Object>();
  testGenericMethod(ls1); 
}
public static <T> void testGenericMethod(Collection<T> cs) {}

or if you uses distinct generics, compiles too.

    
  public static void main(String[] args) {
 
List<? extends Object> ls1 = new LinkedList<Object>();
testGenericMethod(ls1, new Object()); 
  }
  public static <T,X> void testGenericMethod(Collection<T> cs, X x) {}
 

[ December 16, 2007: Message edited by: Alexsandra Carvalho ]

[Henry Wong]

Ohhh... That is what you meant by using one parameter. I thought you meant calling it with one parameter -- but keeping the method the same. Now that would have been interesting!


As for these two examples, well, they work because the compiler can find a type (or two types) that matches... which was not true in the original example.

Henry

[Henry Wong]

To further clarify, the reason there is a compile error, is because the compiler can't find a type that matches both parameters. It doesn't mean that the collection is invalid, as you can see by Alexsandra's examples.

Another option to get it to compile is by extracting the value from the collection itself -- hence, it is guarantee to match. (Of course, since the collection is empty, this will generate a runtime exception)

public static void main(String[] args) {
    List<? extends Object> ls1 = new LinkedList<Object>();
    testCaller(ls1);
}
 
public static <T> void testCaller(Collection<T> cs) {
    T t = cs.iterator().next();
    testGenericMethod(cs, t);
}
 
public static <T> void testGenericMethod(Collection<T> cs, T t) {}   

Henry
[ December 16, 2007: Message edited by: Henry Wong ]