Pre and Post Conditions

• Pre and Post conditions are special comments that go at the beginning of EVERY method. 
• The Pre Condition states what must be true before the method is called.
• The Post Condition states what is true after the method is called. (in other words, what the method does).
• These comments are useful when programmers work in teams. Your team member doesn’t want to dig through all of your code to find out what the method does and other information they need to know. You put all of that information in the Pre and Post conditions at the start of the method.
• You can think of these comments as a contract. If whoever calls your method (whichever fellow programmer on your team uses your code) follows what you say in the pre conditions then you guarantee the results in the post conditions.
• If whoever calls your method doesn’t send data that meets the pre conditions, your method makes no promises. It could crash the computer.

Some examples:

public static double density (double    m, double v)
{ //Pre: v is not zero. m and v are positive
  //Post: returns the density for the given mass and volume   

   return m/v; 
} 

public static void pointless (int j)
{ //Pre: none (j should be positive to run the method
  //Post: displays j stars on the screen 

  while (j<0)
  { System.out.println(“*”); 
    j--;
  } 

}