Contracts
With special reference to RabbitHunt
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
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
javadoc
• In Java, the javadoc comments (or just “doc
comments”) typically provide the contract
• In BlueJ, you can see the doc comments 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
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, instead I provide
the additional constants NUMBER_OF_ROWS and
NUMBER_OF_COLUMNS
• It is important to know both:
– What you can depend on to remain the same
– What you cannot assume will remain the same
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
• Numbers that “everybody knows” and that “never change”
are not usually considered to be magic
– However, adding or subtracting 1 will make it “magic”
– Example: for(int day = 0; day < 8; day++) { ... }
• Zero and one are typically not considered to be magic
numbers
Summary
• In the “real world,” programmers almost always
work in teams
– You have to let others know how to use your code
– You cannot afford to guarantee you will never change
your implementation
– You need to know how to use other people’s code
– It’s dumb to depend on the details of their
implementation
• It’s important to distinguish interface from
implementation
– Programs that are too interdependent are hard to change
– Programs that cannot be changed are doomed
The End