1. Fall 2007CS 225 Program Correctness and Efficiency Chapter 2

2. Fall 2007CS 225 Chapter Topics Three categories of program errors Exceptions Testing strategies Assertions and loop invariants To analyze an algorithm’s efficiency –big-O notation

3. Fall 2007CS 225 Program Defects and “Bugs” A program may be efficient, but is worthless if it produces a wrong answer Defects often appear in software after it is delivered Testing can never demonstrate the complete absence of defects In some situations it is very difficult to test a software product completely in the environment in which it is used Debugging: the process of removing defects

4. Fall 2007CS 225 Types of Errors Syntax errors Run-time errors Logic errors

5. Fall 2007CS 225 Syntax Errors Syntax errors are mistakes in the grammar of a language The Java compiler detects syntax errors during compilation and requires you to correct them before successfully compiling the program

6. Fall 2007CS 225 Run-time Errors or Exceptions Run-time errors –Occur during program execution –Occur when the JVM detects an operation that it knows to be incorrect –Cause the JVM to throw an exception Examples of run-time errors include –Division by zero –Array index out of bounds –Number format and Input mismatch error –Null pointer exceptions

7. Fall 2007CS 225 Logic Errors A logic error occurs when the programmer or analyst –Made a mistake in the design of a class or method –Implemented an algorithm incorrectly Most logic errors do not cause syntax or run- time errors and are thus difficult to find –Sometimes found through testing –Sometimes found during real-world operation of the program

8. Fall 2007CS 225 The Exception Class Hierarchy When an exception is thrown, one of the Java exception classes is instantiated Exceptions are defined within a class hierarchy that has the class Throwable as its superclass –Classes Error and Exception are subclasses of Throwable –RuntimeException is a subclass of Exception

9. Fall 2007CS 225 Throwable Class Throwable is the superclass of all exceptions All exception classes inherit the methods of throwable

10. Fall 2007CS 225 Throwable Methods String getMessage() returns the detail message void printStackTrace() prints the call stack String toString() returns the name of the Exception followed by the message

11. Fall 2007CS 225 Checked vs Unchecked Exceptions Two categories of exceptions –Unchecked exception may result from Programmer error Serious external conditions that are unrecoverable –Checked exception May be beyond the control of the programmer Must be addressed in the program

12. Fall 2007CS 225 Exception Handling When an exception is thrown, the normal sequence of execution is interrupted –Program stops –JVM displays an error message The programmer may override the default behavior by –Enclosing statements in a try block –Processing the exception in a catch block

13. Fall 2007CS 225 Uncaught Exceptions When an exception occurs that is not caught, the program stops and the JVM displays an error message and a stack trace The stack trace shows the sequence of method calls, starting at the method that threw the exception and ending at main

14. Fall 2007CS 225 The try-catch-finally Sequence Avoid uncaught exceptions –Write a try-catch sequence to catch an exception –Handle it rather than relying on the JVM Catch block is skipped if all statements within the try block execute without error

15. Fall 2007CS 225 Error Recovery Exception handling provides the opportunity to –Recover from errors –Report errors Users are a common source of errors; user errors should be recoverable

16. Fall 2007CS 225 Error Recovery try { /* code */ } catch ( Exception var) { /* recovery */} finally { /* */ } Catch block within the first catch clause having an appropriate exception class executes, others are skipped Compiler displays an error message if it encounters an unreachable catch clause

17. Fall 2007CS 225 The finally Block When an exception is thrown, the flow of execution is suspended and there is no return to the try block There are situations in which allowing a program to continue after an exception could cause problems The code in the finally block is executed either after the try block is exited or after a catch clause is exited The finally block is optional

18. Fall 2007CS 225 Throwing Exceptions Instead of catching an exception in a lower-level method, it can be caught and handled by a higher-level method –Declare that the lower-level method may throw a checked exception by adding a throws clause to the method header –Can explicitly throw the exception in the lower-level method, using a throw statement

19. Fall 2007CS 225 Throwing Exceptions (continued) The throws clause is useful if a higher-level module already contains a catch clause for this exception type Can use a throw statement in a lower-level method to indicate that an error condition has been detected Once the throw statement executes, the lower-level method stops executing immediately

20. Fall 2007CS 225 Catching Exceptions Example

21. Fall 2007CS 225 Guidelines You can always avoid handling exceptions by declaring that they are thrown, or throwing them and letting them be handled farther back in the call chain It is usually best to handle the exception instead of passing it along The following are recommended guidelines: If an exception is recoverable in the current method, handle the exception in the current method If a checked exception is likely to be caught in a higher-level method, declare that it can occur using a throws clause It is not necessary to use a throws clause with unchecked exceptions

22. Fall 2007CS 225 Testing Programs There is no guarantee that a program that is syntax and run-time error free will also be void of logic errors The “best” situation is a logic error that occurs in a part of the program that always executes; otherwise, it may be difficult to find the error The worst kind of logic error is one that occurs in an obscure part of the code (infrequently executed)

23. Fall 2007CS 225 Sources of Logic Errors Most logic errors arise during the design phase and are the result of an incorrect algorithm Logic errors may also result from typographical errors that do not cause syntax or run-time errors

24. Fall 2007CS 225 Structured Walkthroughs One form of testing is hand-tracing the algorithm before implementing Structured walkthrough: designer must explain the algorithm to other team members and simulate its execution with other team members looking on

25. Fall 2007CS 225 Testing Testing: exercising a program under controlled conditions and verifying the results Purpose is to detect program defects after all syntax errors have been removed and the program compiles No amount of testing can guarantee the absence of defects in sufficiently complex programs

26. Fall 2007CS 225 Types of Testing Unit testing: checking the smallest testable piece of the software (a method or class) Integration testing: testing the interactions among units System testing: testing the program in context Acceptance testing: system testing designed to show that the program meets its functional requirements

27. Fall 2007CS 225 Approaches to Testing Black-box testing: tests the item based on its interfaces and functional requirements White-box testing: tests the software with the knowledge of its internal structure

28. Fall 2007CS 225 Preparations for Testing A test plan should be developed early in the design phase –Aspects of a test plan include deciding how the software will be tested, when the tests will occur, who will do the testing, and what test data will be used If the test plan is developed early, testing can take place concurrently with the design and coding A good programmer practices defensive programming and includes code to detect unexpected or invalid data

29. Fall 2007CS 225 Testing Methods Most of the time, you will test program systems that contain collections of classes, each with several methods Leave a trace of execution by displaying the method name as you enter it Display values of all input parameters upon entry to a method

30. Fall 2007CS 225 Testing Methods (continued) Display the values of any class attributes that are accessed by this method Display the values of all method outputs after returning from a method Plan for testing as you write each module rather than after the fact

31. Fall 2007CS 225 Developing the Test Data Test data should be specified during the analysis and design phases for the different levels of testing: unit, integration, and system –You should test both good data and bad data –Check special cases called boundary conditions

32. Fall 2007CS 225 White- vs. Black-box Testing In black-box testing, we are concerned with the relationship between the unit inputs and outputs –There should be test data to check for all expected inputs as well as unanticipated data In white-box testing, we are concerned with exercising alternative paths through the code –Test data should ensure that all if statement conditions will evaluate to both true and false

33. Fall 2007CS 225 Why do Testing? Normally testing is done by –The programmer –Other members of the software team who did not code the module being tested –Final users of the software product

34. Fall 2007CS 225 Who does the Testing? Programmers are often blind to their own oversights Companies also have quality assurance organizations that verify that the testing process is performed correctly In extreme programming, programmers work in pairs where one writes the code and the other writes the tests

35. Fall 2007CS 225 When to test? Start testing as early as you can –don't wait until the coding is complete It may be difficult to test a method or class that interacts with other methods or classes

36. Fall 2007CS 225 Stubs A replacement for a method that has not yet been implemented or tested is called a stub A stub has the same header as the method it replaces, but its body only displays a message indicating that the stub was called –It may need a fixed return value

37. Fall 2007CS 225 Drivers A driver program declares any necessary object instances and variables, assigns values to any of the method’s inputs, calls the method, and displays the values of any outputs returned by the method You can put a main method in a class to serve as the test driver for that class’s methods

38. Fall 2007CS 225 Assertions and Loop Invariants Assertions: logical statements about a program that are claimed to be true; generally written as a comment Preconditions and postconditions are assertions A loop invariant is an assertion –Helps prove that a loop meets it specification –True before loop begins, at the beginning of each repetition of the loop body, and just after loop exit

39. Fall 2007CS 225 Assertions and Loop Invariants Example

40. Fall 2007CS 225 Assertions in Java Java has an assert statement which can take two forms 1.assert ; Results in an AssertionError if the boolean expression is ever false 2.assert : ; The expression is printed as part of the AssertionError message.

41. Fall 2007CS 225 Using Assertions in Java Activate assertion checking with a command-line option to the interpreter java -ae

42. Fall 2007CS 225 Efficiency of Algorithms Can characterize a program by how the execution time or memory requirements increase as a function of increasing input size –Big-O notation A simple way to determine the big-O of an algorithm or program is to look at the loops and to see whether the loops are nested

43. Fall 2007CS 225 Algorithm Efficiency Consider: First time through outer loop, inner loop is executed n-1 times; next time n-2, and the last time once. So we have –T(n) = 3(n – 1) + 3(n – 2) + … + 3 or –T(n) = 3(n – 1 + n – 2 + … + 1)

44. Fall 2007CS 225 Algorithm Efficiency We can reduce the expression in parentheses to: –n x (n – 1) / 2 So, T(n) = 1.5n 2 – 1.5n This polynomial is zero when n is 1. For values greater than 1, 1.5n 2 is always greater than 1.5n 2 – 1.5n Therefore, we can use 1 for n 0 and 1.5 for c to conclude that T(n) is O(n 2 )

45. Fall 2007CS 225 Efficiency of Algorithms