Testing Pre- and Post-Conditions

Overview

  1. Unit Testing
  2. Class Invariants
  3. Pre- and Post-Conditions
  4. General Cases
  5. Edge Cases
  6. Extra Credit

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.

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