Program Correctness and Efficiency

1. Program Correctness and Efficiency Chapter 2

2. Chapter Objectives • To understand the differences between the three categories of program errors • To understand the effect of an uncaught exception and why you should catch exceptions • To become familiar with the Exception hierarchy and the difference between checked and unchecked exceptions • To learn how to use the try-catch-finally sequence to catch and process exceptions • To understand what it means to throw an exception and how to throw an exception in a method

3. Chapter Objectives (continued) • To understand the different testing strategies and when and how they are performed • To learn how to write special methods to test other methods and classes • To become familiar with debugging techniques and debugger programs • To be introduced to the process of program verification and the use of assertions and loop invariants • To understand the meaning of big-O notation and how it is used to analyze an algorithm’s efficiency

4. 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: removing defects

5. 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 • Some common syntax errors include: • Omitting or misplacing braces • Performing an incorrect type of operation on a primitive type value • Invoking an instance method not defined • Not declaring a variable before using it • Providing multiple declarations of a variable

6. 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. Run-time Errors or Exceptions (continued)

8. 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

9. 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

10. The Class Throwable • Throwable is the superclass of all exceptions • All exception classes inherit the methods of throwable

11. The Class Throwable (continued)

12. Checked and Unchecked Exceptions • Two categories of exceptions: checked and unchecked • Checked exception normally not due to programmer error and is beyond the control of the programmer • Unchecked exception may result from • Programmer error • Serious external conditions that are unrecoverable

13. Checked and Unchecked Exceptions

14. Catching and Handling Exceptions • When an exception is thrown, the normal sequence of execution is interrupted • Default behavior • 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

15. 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

16. 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

17. Handling Exceptions to Recover from Errors • Exceptions provide the opportunity to • Recover from errors • Report errors • User error is a common source of error and should be recoverable • 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

18. 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

19. 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 throw the exception in the lower-level method, using a throw statement • The throws clause is useful if a higher-level module already contains a catch clause for this exception type

20. Throwing Exceptions (continued) • 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

21. Catching Exceptions Example

22. Programming Style • 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

23. 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)

24. Structured Walkthroughs • 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 • 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. Levels and Types of 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 • Unit testing: checking the smallest testable piece of the software (a method or class) • Integration testing: testing the interactions among units

26. Levels and Types of Testing (continued) • System testing: testing the program in context • Acceptance testing: system testing designed to show that the program meets its functional requirements • 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

27. 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

28. Testing Tips for Program Systems • Most of the time, you will test program systems that contain collections of classes, each with several methods • If a method implements an interface, its specification should document input parameters and expected results • Carefully document each method parameter and class attribute using comments as you write the code • 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

29. Testing Tips for Program Systems (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

30. 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 • 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

31. Testing Boundary Conditions • When hand-tracing through an algorithm or performing white-box testing, you must exercise all paths • Check special cases called boundary conditions

32. 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 • Do not rely on programmers for testing as they 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

33. Stubs • It may be difficult to test a method or class that interacts with other methods or classes • The replacement of 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

34. 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

35. Using a Test Framework • A test framework is a software product that facilitates writing test cases, organizing the test cases into test suites, running the test suites, and reporting the results • A test framework often used for Java products is JUnit, an open-source product that can be used in a stand-alone mode and is available from junit.org

36. “BUGS” • IMPORTANT NOTICE! • YOU PUT EVERY BUG THE PROGRAM YOU WROTE! • They DID NOT “Crawl In” from outside.

37. LOOK FOR BUGS IN CORNERS • Boundary conditions.

38. Debugging a Program • Debugging is the major activity performed by programmers during the testing phase • Testing determines if there is an error, debugging determines the cause of it • Debugging is like detective work • Inspect carefully the information displayed by your program • Insert additional diagnostic output statements in the method to determine more information

39. Using a Debugger • Debuggers often are included with IDEs • A debugger can execute your program incrementally rather than all at once • Single-step execution executes in increments as small as one program statement • Breakpoints are used to traverse large portions of code before stopping • The actual mechanics of using a debugger depend on the IDE that you are using

40. Using a Debugger (continued)

41. Reasoning about Programs: 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

42. Assertions and Loop Invariants Example

43. Efficiency of Algorithms • Difficult to get a precise measure of the performance of an algorithm or program • 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

44. Efficiency of Algorithms (continued) • 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)

45. Efficiency of Algorithms (continued) • We can reduce the expression in parentheses to: • n x (n – 1) 2 • So, T(n) = 1.5n2 – 1.5n • This polynomial is zero when n is 1. For values greater than 1, 1.5n2 is always greater than 1.5n2 – 1.5n • Therefore, we can use 1 for n0 and 1.5 for c to conclude that T(n) is O(n2)

46. Efficiency of Algorithms (continued)

47. Chapter Review • Three kinds of defects can occur in programs • Syntax errors • Run-time errors • Logic errors • All exceptions in the Exception class hierarchy are derived from a common superclass called Throwable • The default behavior for exceptions is for the JVM to catch them by printing an error message and a call stack trace and then terminating the program

48. Chapter Review (continued) • Two categories of exceptions: checked and unchecked • A method that can throw a checked exception must either catch it or declare that it is thrown • “throw” statement throws an unchecked exception • Program testing is done at several levels starting with the smallest testable piece of a program called a unit • Integration testing: once units are individually tested, they can then be tested together • System testing: once the whole program is put together, it is tested as a whole

49. Chapter Review (continued) • Acceptance programming involves testing in an operational manner demonstrating its functionality • Black-box testing tests the item based on its functional requirements without knowledge of its internal structure • White-box testing tests the item using knowledge of its internal structure • Test drivers and stubs are tools used in testing • Test drivers exercise a method or class • Stubs stand in for called methods • Big-O notation determines the efficiency of a program