Solution:

class SortedVector 
{ 

Vector vector; // The contained vector. 

boolean sorted; // Whether the vector is now sorted. 

SortedVector() 
  { 
  vector = new Vector(); // Construct contained vector. 
  sorted = true; // It is sorted because it is empty. 
  } 

void AddElement(Object X) 
  { 
  vector.addElement(X); 
  sorted = false; // Adding element generally destroys order 
  } 

Object ElementAt(int I) 
  { 
  if( !sorted ) // Don't sort unnecessarily
    { 
    Sort(); // Sort if and only if not already sorted 
    sorted = true; 
    } 
  return vector.elementAt(I); 
  } 
}