A method contract is a comment that tells precisely what the method accomplishes and how its parameters affect what it accomplishes. A contract is suitable for exporting to library documentation, assuming that the method is intended to be part of a library. It tells someone how to use the method, not how the method works.
Someone who reads a contract is assumed to be able to see the method heading, but not the method body. Therefore:
A contract cannot refer to a method's body. It cannot refer to local variables in the body. It cannot refer to loops, if-statements, or anything else in the method's body.
Here are some do's and don'ts that should help you write clear, precise and concise contracts.
Do make sure to discuss the role of each parameter.
|
It is not enough just
to say that the parameter exists. Say how each parameter affects what
the method accomplishes. If a parameter is an out-parameter,
make that clear; say what is stored in it.
Refer to parameters by their names, not by pronouns or vague phrases such as "the number" or "the graph". |
Do describe what the method returns.
| It is not enough to say that a method returns "an integer". That is obvious from the heading. Say what the returned value means to the method's caller and how the parameters affect it. For methods with a void return type, nothing is returned, so skip this part. |
Do discuss requirements.
| If the method has requirements, be sure to mention them. For example, if parameter x must be a positive number, say that x > 0 is required. The caller needs to know that. If some data structure must have been set up in a particular way, say so. It can be useful to tell the reader that some other method must have been called before calling this one. |
Do mention side-effects.
| If the method has a visible effect (other than what it does with its own local variables) be sure to mention that effect. For example, if a method writes something to the standard output, then the caller needs to know that. It is not necessary to mention side-effects that are only done when tracing is turned on. |
Do provide examples.
| Showing some examples of what the method does can be very helpful to a reader. If a method reads a file in a particular format, show what a sample input file looks like. If it takes a number and yields a number, show some examples. If it takes a list and returns another list, show what it returns on sample lists. |
Do write a contract before you write the method definition.
| Before you try to solve a problem, find out what the problem is. A contract specifies just what problem the method solves. Get it down in writing before you write the method definition. If you do not understand what the problem is well enough to describe it, then you certainly do not understand the problem well enough to write a computer program to solve it. |
Do describe everything that the method does that is relevant to the caller.
| A method should take credit for everything that it accomplishes, including things that are performed by other methods that it calls. |
Do modify the contract if you modify what the method does
| A common mistake is to write a contract and then ignore it during the process of making changes to the program. Keep contracts up to date. |
Do use correct spelling and grammar
| A program with spelling or grammatical errors in contracts looks very unprofessional. Serious errors can make the contract unreadable. Be sure your contract does not look like someone just threw a bucket of words at the screen. |
Don't say how the method works.
| A contract is intended for someone who only wants to use the method. Do not burden that person with information about how it accomplishes what it does. If you only want to drive a car, you do not need to know how the engine works. |
Don't say which other methods this one uses.
| A contract does not tell how a method works. Saying that it uses f, g and h is irrelevant. That is also not an acceptable substitute for saying what it does. |
Don't say which other methods use this method, or where the parameters come from.
|
It is tempting to try to describe just where a method fits into
a program. For example, if you know that a particuar parameter will
be a number that was read from the user, then it is tempting to
say so in the contract. Don't.
Methods are tools that are part of a toolkit for a particular piece of software (or library). You might have a wrench in your toolbox that you acquired in order to turn a particular nut. But don't limit it to turning that nut. It will probably come in handy for some other nut later. Similarly, a method that you use in one place can often come in handy later at a different place. The method's contract should not limit the method's usefulness. If a parameter is an array that was filled in by reading a file, do not call it a file. It is an array. Say what the method does with the array. |
Don't discuss local variables of the body.
| Assume the reader cannot see the body. |
Don't describe the types of the parameters.
|
Type information is clear from the heading. Assume that the reader
can see the heading. If there is any information that a person needs
to know that is not present in the types, then describe that. It is
a requirement.
Do not write a full method heading, with types, in the contract. That is not useful. But you often show the method with parameters, by name. For example, you might say that "factorial(n) returns n!". Notice that you don't say "factorial(int n) returns n!". |
Is the following a sensible contract and heading for method distance?
// distance(x,y,u,v) returns the distance between // points (x,y) and (u,v) in the plane. static double distance(double x, double y, double u, double v)Answer
Is the following a sensible contract and heading for method distance?
// distance(x,y) returns the distance between // points (x,y) and (u,v) in the plane. static double distance(double x, double y)Answer
Is the following a sensible contract and heading for method f?
// f(x,y) takes two integers x and y and returns // an integer. static int f(int x, int y)Answer