Java Reference
In-Depth Information
Preconditions and Postconditions
One good way to write a method comment is to break it down into two kinds of informa-
tion, called the
precondition
and the
postcondition.
The
precondition
states what is
assumed to be true when the method is called. The method should not be used and can-
not be expected to perform correctly unless the precondition holds. The
postcondition
describes the effect of the method call; that is, the postcondition tells what will be true
after the method is executed in a situation in which the precondition holds. For a method
that returns a value, the postcondition describes the value returned by the method.
For example, the following is an example of a method heading from Display 4.9
with a precondition and postcondition added:
precondition
postcondition
/**
Precondition: All instance variables of the calling object have values.
Postcondition: The data in the calling object has been written to the screen.
*/
public
void
writeOutput()
You do not need to know the definition of the method
writeOutput
to use this
method. All that you need to know to use this method is given by the precondition
and postcondition. (The importance of this is more dramatic when the definition of
the method is longer than that of
writeOutput
.)
When the only postcondition is a description of the value returned, programmers
usually omit the word
Postcondition
, as in the following example:
/**
Precondition: All instance variables of the calling object have
values.
Returns a string describing the data in the calling object.
*/
public
String toString()
Some programmers choose not to use the words
precondition
and
postcondition
in
their method comments. However, whether you use the words or not, you should
always think in terms of precondition and postcondition when designing a method
and when deciding what to include in the method comment.
Self-Test Exercises
17. List all the accessor methods in the class
DateFifthTry
in Display 4.9.
18. List all the mutator methods in the class
DateFifthTry
in Display 4.9.
(continued)
