Generics - wildcard?

[Alexsandra Carvalho]

Hello,
I don't understand that...
Can I use wildcard on type of class definition???

public class Teste<T,X extends Number>{

public static void main (String args) {

Teste<String, ? super Integer> teste = new Teste<String, Number>();


}
}

[Alexsandra Carvalho]

I'm so cofused.

What's the meaning of to define a class like

public class Teste<T,X extends Number>{

and later use like this:
Teste<String, ? super Integer> teste = new Teste<String, Number>();

What I can and I can't to do with the second argument?

[Peter Ricke]

Try to think about the two thinks completly separated one from another:
One is defining a class Tester, the other defining a variable of type Tester

When defining a class like

  public class Teste<T,X extends Number>{}
  

you declair that every instance of this class will have to have 2 parameters (of cause you can leave the parameters at all, but if paramters are used...).
The first has to be whatever the implementer wants.
The second has to extend Number , so could be Integer or Long or...

This guarantees that Methods of tester are acting only on Number-Instances, not e.g. on Strings..


If than someone instantiates your class

  Teste<String, ? super Integer> teste = new Teste<String, Number>();
  

she is doing 3 things:
a) declair a variable named teste.
This can hold every Teste-object which has parameters String ans Something superior to number.
b) create a new Teste-Object with Parameters String and Number
c) assign the object to the variable

Due to your class-definition the object could also be a
new Teste<Integer,Integer>()
but you would not be able to assign to the variable.

On the other hand, later inb the program, you could instantiate another tester-object eg (new Tester<String,Integer>()) and assign to your variabe tester.
This is ok cause Integer is "super" to Integer.

Other Instance-types are not possibel to assign to tester,because
var 1 has to be a String and
var 2 has to be super to Integer and extending Number and this is true only for Number and Integer

Hope this helps