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