Contracts

1
Contracts

• With special reference to RabbitHunt

2
Multi-person projects

• Except for coursework, it is rare for a single
  person to write an entire program
• Each programmer has to know what the other
  programmers are doing
• In addition, each programmer has to let the
  others know what he/she is doing, BUT
• In order to maintain control of code, it is
  necessary to hide details of the implementation

3
Contracts

• A contract specifies the rights and
  responsibilities of each party to the contract
• In programming, a contract spells out what other
  programmers are allowed to depend on in my code
• When I write a contract, I have these
  responsibilities
• I must provide enough information so that others
  can make use of my code (my classes and methods)
• I must not arbitrarily change what I offer
• I also have these rights
• I can change the code however I like, so long as
  it continues to meet the terms of the contract
• I can provide additional functionality

4
javadoc

• In Java, the javadoc comments (or just doc
  comments) typically provide the contract
• In BlueJ, you can read these more easily by
  choosing Interface from the pull-down menu in the
  upper-right corner
• If you have not read The contract portion of
  the RabbitHunt assignment, you should do so
• Rule 89, Program by contract, is also relevant

5
The RabbitHunt contract

• What is most important about the RabbitHunt
  contract is the information I do not provide
• I provide a number of constants, such as Model.N,
  Model.MIN_DIRECTION, and Model.FOX, but I do not
  specify the numerical values of those constants
• I did not specify the size of the field, but in
  the improved version I provide the additional
  constants NUMBER_OF_ROWS and NUMBER_OF_COLUMNS
• If you read the assignment, you already know that
  we may grade your program with different values
  for these constants
• This is relevant to your grade!

6
Magic numbers

• A magic number is a constant (such as 20) that
  appears, sometimes without explanation, in the
  code
• Magic numbers are poor style
• Named constants, such as NUMBER_OF_ROWS, have two
  advantages
• They provide some documentation for the meaning
  of the constant
• If you decide to change the constant, you need
  only do so in one place rather than many
• Zero and one are typically not considered to be
  magic numbers

7
The End