Question

While String is a sub-type of Object, it is not true that Bag<String> is a sub-type of Bag<Object>. Explain why?

(Hint: If A "is a" B, then everything you can do with B you can also do with A. What can you do with a Bag<Object> than you cannot do with Bag<String>?)

Answer 1

Bag<String> cannot be a subtype of Bag<Object> unless and until it is specifically inherited.

However, the advantage in having Bag<Object> is that it allows to use classes that are derived from Object.

i.e we can add an Integer and String into the Bag<Object> for the same instance.

For example:

class Bag<E>
{
   E data;
   Bag(E data)
   {
       //do something
   }
}
class sample
{
   public static void main (String[] args) throws java.lang.Exception
   {
       //both string and integer is possible
       Bag<Object> bag = new Bag<Object>("Hello");
       bag = new Bag<Object>(23);
   }
}