generics wildcard

[ming ming]

in the following code what can we add ?

  
import java.util.*;
public class Test{

 public static void main(String[]args){
List<Number>x=new ArrayList<Number>();
take(x);
}
public static void take(List<? extends Number>x){
//What can we add here?
            //  x.add();
System.out.println(x.get(0));
}
}
  

[babu sharath]

If there is wild card , We can not add any thing .
( unless it is super .. Ex: ? extends super )

[babu sharath]

Sorry ... In previous reply example was wrong...
It is like .. ( ? super Number )
than we can add .

[Vijay Raj]

Let's take into consideration, the following class hierarchy.

  
class A {
}
class B extends A {
}
class C extends A {
}
  

When we say, ? extends A, it means that both B and A can also be taken into consideration. Now if we have,

  
List<B> lb = new ArrayList<B>();
List<? extends A> l = lb;

and, say, add() was possible, then we would be able to add an element of type C also. Just to dis-allow this, its a compile time error.

regards,
vijay.

[Gowher Naik]

Check code below


import java.util.*;
public class Test {
public static void main (String args []) {
List<Number>x=new ArrayList<Number>();
x.add(new Integer("10"));
take(x);
take1(x);
}
public static void take(List<? extends Number>x){
//In this method nothing can be added to List x
//This method becomes read only method
System.out.println(x.get(0));
}
public static void take1(List<? super Number>x){
//In this method anything extends Number can
//be added to List x
x.add(new Integer("20"));
x.add(new Long("20"));
x.add(new Float("20.120"));
for(Object n:x){
System.out.println(n);
}
}
}