Problem in Generics

[Poornima Sharma]

Hi friends,

in Generics what does it mean

<X extends Object>

I know what <? extends Object> mean but i cant understand the above.

[ramesh maredu]

X extends Object, is syntax used to define boundaries, this kind of syntax is used in generic classes and methods.

See below code.

 
import com.pkg2.Test;
 
public class Generics<X extends Number> {
X x = null;
public static void main(String[] args) {
Generics<Number> gen1 = new Generics<Number>();
Generics<Integer> gen2 = new Generics<Integer>();
//Generics<Object> gen3 = new Generics<Object>();
test(new Integer(1));
test(new Float(1));
//test("");

}
public static <T extends Number> void test(T t){

}
}
 

 

commented lines don't compile if you remove comment

[Ankit Garg]

Well both <? extends Object> and <X extends Object> mean the same thing to a certain extent. The only difference being this

with <X extends Object> you can declare references of type X.

<X extends Object> void method(List<X> list, X obj)
{
list.add(obj);
}

but you can't do the same with <? extends Object>

void method(List<? extends Object> list, ? obj) //compilation error.