CMSC 202 — Fall 2011

Testing Pre- and Post-Conditions

In many cases, your methods will not work with certain inputs. The primary example is how will your method behave if it is passed a null pointer? Throwing a null pointer exception is fine, but sometimes you can get other unexpected behavior. There are many more elegant ways to handle the passing of a null pointer, so always check to make sure your preconditions are met.

public static void main(String[] args) {
  //Test open cash
  openCashRegister(null);
}

public static void openCashRegister(CashRegister cash) {
  if(cash == null) {
      return;
  }
...
}

Think about your Fraction class and the equals() method. How does it handle 2/3 == 4/6? How could you fix this? Is there a way to handle this issue through the class invariant? What happens when the multiply() method is passed a null reference?
Think about your Point class. What happens when the distance() method is passed a null reference?
Think about your Rectangle class. What happens when the toString() method of the Point class is modified?

1. Modify your code to handle the pre- and post- conditions for all of these methods.