Preconditions/Postconditions

1. What are we assuming to be true before the function is called? (Preconditions)
2. What are we promising to deliver when we are done writing the function? (Postconditions)

// Pre: Array contains at least one element
// Post: Returns arithmetic mean of array elements (sum / # elements)
double average(double[] grades) {
  // Code goes here
}

// Pre: Book is not checked out
// Post: Book is recorded as checked out; a due date is present
void checkOut(Book b) {
  // code goes here
}