Difference between Bounded Type parameter (T extends) and Upper Bound Wildcard (? extends)

Question

I know that there was a similar question already posted, although I think mine is somewhat different...

Suppose you have two methods:

// Bounded type         


        

Answer 1

I can think of the below differences :

a) Modifying your list inside the method, consider below code :

private static void processList(List someList)
{


   T t = someList.get(0);
   if ( t.getClass() == Integer.class )
   {
       Integer myNum = new Integer(4);
       someList.add((T) myNum);
   }

}

// Upper bound wildcard
private static void processList2(List someList)
{
    Object o = someList.get(0);
    if ( o instanceof Integer )
    {
        Integer myNum = new Integer(4);
        someList.add(myNum); // Compile time error !!
    }
}

With wildcard you cannot add elements to the list ! Compiler tells you that it doesn't know what is myNum. But in the first method, you could add a Integer by first checking if T is Integer , no compile time error.

b) The first method is called generic method. It follows the syntax that is defined for a generic method. The upper bounds specified in the method definition are used to restrict the parameter types.

The second one is NOT necessarily called a generic method, it is a normal method that happens to accept a generic parameter. The wildcard ? with extends keyword is used as a means of relaxing the types that the method can accept.