Welcome to CIS 068 !

1. Welcome to CIS 068 ! Algorithm Correctness And Efficiency CIS 068

2. Overview • Subjects: • Program Defects • Exceptions • Testing Strategies • Formal Methods of Verification • Efficiency of Algorithms CIS 068

3. Errors • Three kinds of errors: • Syntax errors • Runtime errors • Logical errors CIS 068

4. Syntax Errors • Mistakes in the use of language‘s grammar (or syntax) • Usually not critical • Usually discovered by compiler • ...but sometimes hard to find ! CIS 068

5. Runtime Errors • Occuring during program execution • Critical if appearing in special cases (see Example ‘AT&T breakdown’, last lesson) • Not discovered by compiler • Forces the computer to exit program (if no recovery code is written) CIS 068

6. Runtime Error Example • The next slide will show a JAVA program, assigning the series 1,1/2, 1/3, 1, 1/2, 1/3,... to a 60-element array. • Please answer the following questions: • Will the compiler accept the code ? • What will happen executing the program ? CIS 068

7. Runtime Error Example • public class Class1 • { • public static void main(String[] args) • { • int myArray[] = new int[60]; • // Assign 1, ½, 1/3, 1, ½, 1/3... to myArray • for (int i =1; i <= 60; i++){ • myArray[i] = i/(i%3); • } • } • } Index out of bounds (60) Expression (i%3) is 1,2,0,1,2,0.. Division by Zero ! CIS 068

8. Runtime Error Example • Will the compiler accept the code ? • What will happen executing the program ? Yes (Multiple) Runtime Errors will occur, starting with a division by zero. CIS 068

9. Common Runtime Errors CIS 068

10. NullPointerError • Occurs when trying to attempt a non-existing object • For C++ - programmers: don’t be confused, there are really no pointers in JAVA ! Example: Object testObject ; // init Object (to null ) If (expression) { // testObject = new Object (); // create Object } testObject.testMethod(); // will result in error if expression was false CIS 068

11. Exceptions How to handle errors Without exceptions: • program entered undefined state or crashes • possible errors can be guarded by if – statements • better use exceptions ! CIS 068

12. Exceptions Are The Rule Runtime Error Create instance of class Exception (‘throw’ exception) Program ‘catches’ the exception, i.e. appropriate codeblock is entered CIS 068

13. Try – Catch mechanism Syntax: try { // Statements that may throw exceptions … } catch(Exception1 e1){ //statements to execute for exceptions type Exception1 } catch(Exception2 e2){ //statements to execute for exceptions type Exception2 } … finally{ // Statements to execute after try-catch } CIS 068

14. Try – Catch Example try{ int n = 4 / 0; } catch(ArithmeticException ae){ ae.printStackTrace(); } Some Exceptions provided by JAVA: CIS 068

15. Exceptions Exceptions are Objects Since Exceptions are Objects, you can derive your own Exception. myOwnException CIS 068

16. Exceptions What to do with exceptions ? • every exception has the methods • getMessage(), returning a detailed message of the exception • printStackTrace(), printing the exception and its backtrace • getMessage() and printStackTrace() are inherited from class Throwable CIS 068

17. Exceptions Example: try{ … } catch(Exception e){ System.out.println(e.getMessage()); e.printStackTrace(); } … Example for Stack-Trace: CIS 068

18. Exceptions try – catch – finally • finally {} is ALWAYS executed independent from execution of try{} and/or catch {} – body • ALWAYS means: even after a possible return statement in either try or catch block ! This is especially useful if there are multiple exit (return) points. • Only System.exit() overrides that rule • A good place to clean up ! CIS 068

19. Throwing Exceptions Using the throw – command, Exceptions can directly be triggered (rather than waiting for the JVM) CIS 068

20. Logic Errors Most Critical Errors Occur in the design – phase Can’t be detected by computer, not at compile-time, not at run-time What can be done ? CIS 068

21. Logic Error Example • The next slide will show a JAVA program. It is supposed to show all numbers n divisible by 7 without remainder, 0<n<1000 • Please answer the following questions: • Will the compiler accept the code ? • What is the program‘s result ? • Does a compiler always find syntax-errors ? CIS 068

22. Syntax Error Example • public class Class1 • { • public static void main(String[] args) • { • /* Detect if number is multiple of 7 */ • for (int i=1; i<1000; i++){ • if ((i%7) == 0){ • System.out.println(i+""); • } • // Exit Program --------------------------------- • System.out.println("That's it. Goodbye."); • System.exit(0); • } • } • } The loop is defined by these brackets ! The program will exit INSIDE the loop CIS 068

23. Logic Error Example • Will the compiler accept the code ? • What is the program‘s result ? YES “That’s it. Goodbye” CIS 068

24. Logic Errors carefully check the algorithm single step tracing explain and simulate execution with other team members (structured walkthrough) use program testing strategies CIS 068

25. Testing Strategies Develop test-plan early in the design stage CIS 068

26. Testing Strategies • Use defensive programming, i.e. include code for every unexpected or invalid data values ! CIS 068

27. Testing Strategies Testing questions: • Who does the testing ? • Blackbox or Whitebox – Testing ? CIS 068

28. Testing Strategies Top – Down Testing • Entire logical flow is implemented • Usage of stubs instead of completed Methods • Easy to write • provide defined results • easy simulation of unexpected data CIS 068

29. Testing Strategies Bottom – Up Testing • implementation of single methods • test of each method separately by driver - programs CIS 068

30. Debugging Tips Carefully document each method parameter and local variable using comments as you write the code. Also describe the method’s purpose. Name methods and variables meaningful Create an execution-trace by printing out the method’s name when executed Display the values of all arguments upon entry to a method Display the values of all results after returning from a method Verify results by hand – computation ! CIS 068

31. Efficiency • Measurement of number of program – steps performed • Usually a precise measure can’t be given • Approximation dependent on preconditions • Big – O (Order of Magnitude) Notation CIS 068

32. Big – O Notation Definition: Algorithm has order of magnitude f(n) [=O(f(n))] means: There exists a constant C such that the actual running-time T(N) is less than C * f(n) for N towards infinity f(n) can therefore be determined by the fastest growing term of the algorithm. CIS 068

33. Big – O Notation Example: An algorithm performing n*n + 4 steps (depending on precondition n) has the order of magnitude n*n, O(n*n) Question: Is an O(n) algorithm A1 necessarily always (i.e. for all n) faster than an O(n*n) algorithm A2 ? CIS 068

34. Review Subjects: • Different Types of Errors • Exceptions, try-catch-finally • Testing Strategies • Efficiency of Algorithms • Big O CIS 068

35. Good Bye The Subjects of this lesson are covered in chapter 2 of Software Design & Data Structures in Java By Elliot B. Koffman + Paul A. T. Wolfgang CIS 068