when do we use generic type and wild cards

[Shashank Sharma]

hello pals,
I am really very confused with the generics type part.
When do we use <?>/<?extends Object> <?> always gives an error as it is an unknown type.so I have to use <T> instead of wild card character.
K& B also says to use <T> instead of <?>.
Then how do i deal with the question using <?>.this part is really tricky.

Please help



thanks
[ August 30, 2008: Message edited by: Bear Bibeault ]

[Mustafa Musaji]

It depends when and where you are using it. If you a creating a generic method then <T> defines the generic type in that method. If you are defining a generic collection then <?>, <String> is used. I've not touched generics since I took the SCJP so correct me if I am wrong.

Post a code snippet of what you are trying to do. It may help answer you question a bit better.

[V K Gupta]

Hi,

The rule is :

1) Question mark works when declaring a reference for a variable, and not for generic class and method declarations.

example of illegal use of ?

class xyz<?> { }

class xyz<? extends Object> { }

public <? extends Number> void disc(T t) // Method declaration inside a class

Legal use of ? is in reference variable like mainly in collections.

public void disc(List<? extends Object> mylist) { } // Here mylist is an reference variable

public void disc(List<? extends Serializable> mylist) { } // here Serializable is interface.

List<?> list = new LinkedList<car>(); // (1)
List<? extends Object> list = new LinkedList<car>(); // (2)

Above statement 1 and 2 both are same in meaning

List<? super PassengerCar> mylist = new Vector<Vehicle>();

[Shashank Sharma]

thanks

[Ankit Garg]

There is a reason for ? not working in class declaration. The rule is that if you are not going to declare any reference of the type of the generic parameter, then you can use ? instead of T(or infact any identifier can be used instead of T). But if you declare a class as MyClass<?> then what is the use??? You cannot say this

class MyClass<?>
{
? obj;//not allowed
}

So you must declare it as

class MyClass<A>
{
A obj;//OK
}

but for method parameters and collections, you can use ? if you don't want to create any reference of the type of the parameter or collection.

void hello(List<?> list)
{
}

you could have also used T instead of ?. But if you want to create a reference of the generic type, you must use an identifier instead of ?

<T> void hello(List<T> list)
{
T obj = list.get(0);
}

you cannot write the above code as

<?> void hello(List<?> list)
{
? obj = list.get(0);//? not allowed like this
}