1	Preconditions and Postconditions An important topic: preconditions and postconditions. They are a method of specifying what a method accomplishes.
2	Preconditions and Postconditions Frequently a programmer must communicate precisely what a method accomplishes, without any indication of how the method does its work.
3	Example You are the head of a programming team and you want one of your programmers to write a method for part of a project.
4	What are Preconditions and Postconditions? One way to specify such requirements is with a pair of statements about the method. The precondition statement indicates what must be true before the method is called. The postcondition statement indicates what will be true when the method finishes its work.
5	Example
6	Example The precondition and postcondition appear as comments in your program. They are usually placed before the method’s implementation.
7	Example In this example, the precondition requires that x >= 0 be true whenever the method is called.
8	Example
9	Example
10	Example
11	Example The postcondition always indicates what work the method has accomplished. In this case, when the method returns the square root of x has been written.
12	Another Example
13	Another Example
14	Another Example
15	Another Example
16	Always make sure the precondition is valid . . . The programmer who calls the method is responsible for ensuring that the precondition is valid when the method is called.
17	. . . so the postcondition becomes true at the method’s end. The programmer who writes the method counts on the precondition being valid, and ensures that the postcondition becomes true at the method’s end.
18	A Quiz Suppose that you call a method, and you neglect to make sure that the precondition is valid. Who is responsible if this inadvertently causes a 40-day flood or other disaster? You The programmer who wrote that torrential method Noah
19	A Quiz Suppose that you call a method, and you neglect to make sure that the precondition is valid. Who is responsible if this inadvertently causes a 40-day flood or other disaster? You The programmer who calls a method is responsible for ensuring that the precondition is valid.
20	On the other hand, careful programmers also follow these rules: When you write a method, you should make every effort to detect when a precondition has been violated. If you detect that a precondition has been violated, then print an error message and halt the program.
21	On the other hand, careful programmers also follow these rules: When you write a method, you should make every effort to detect when a precondition has been violated. If you detect that a precondition has been violated, then print an error message and halt the program... ...rather than causing a disaster.
22	Example Throwing an exception(described in Section 1.1) is useful.
23	Advantages of Using Preconditions and Postconditions Succinctly describes the behavior of a method... ... without cluttering up your thinking with details of how the method works. At a later point, you may reimplement the method in a new way ... ... but programs (which only depend on the precondition/postcondition) will still work with no changes.
24	Summary Precondition The programmer who calls a method ensures that the precondition is valid. The programmer who writes a method can bank on the precondition being true when the method begins execution. Postcondition The programmer who writes a method ensures that the postcondition is true when the method finishes executing.
25	THE END