Example

class Stack

{

Object array[];     //array of Object

:                      //implementing Stack

:

void push(Object o)

 :

 :

Object pop()

 :

 :

}

Stack s;

String x;

s.push(x);  

Stack s = new Stack();

String x;

  :

  :

s.push(x);

  :

  :

x = s.pop(); 

When we remove an object, we may need to

"cast"

it to the right type to let the compiler know what is up.

(String)s.pop()

      target type of cast

x = (String)s.pop();  

String -> Object          "Up" cast         automatic

Object -> String          "Down" cast     requires explicit cast