CS 200 Command-Line Arguments & Exceptions Jim Williams, PhD

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1 CS 200 Command-Line Arguments & Exceptions Jim Williams, PhD

2 This Week 1. Battleship: Milestone 3 a. First impressions matter! b. Comment and style 2. Team Lab: ArrayLists 3. BP2, Milestone 1 next Wednesday a. shorter than typical, no Challenge Activities this weekend 4. Lecture: Command-line arguments and Exceptions

3 BP2 Wa-Tor Planet of Wa-Tor Sharks and Fish population simulation Image:

4 From the 1st Week public class CmdLine { public static void main(string[] args) {

5 Text Interface vs GUI Interface

6 Command-Line Commands Windows dir cd mkdir rmdir del type help Unix/Linux/Mac ls cd mkdir rmdir rm cat man

7 What are command-line arguments? public class CmdLine { public static void main(string[] args) { System.out.println( args.length); for ( int i = 0; i < args.length; i++) { System.out.println( args[i]); an array of Strings information passed into the program the main method parameter

8 Usage Message public class UseCmd{ public static void main(string[] args) { if ( args.length!= 1) { System.out.println( "Usage: java CmdLine filename"); System.exit( 1); //0 success, non-zero error System.out.println( "Filename: " + args[0]);

9 Command Line Arguments Passing to program from within Eclipse Passing to program from Command-Line

10 Various Kinds and Times for Errors Editing/Saving errors Compile time errors Runtime errors New this week: Checked Exceptions are a mechanism where the compiler does some checks to make some errors visible at Compile time rather than Runtime.

11 Demo Unchecked vs Checked Exceptions public static double profitmargin( double profit, int numsales){ return profit / numsales; public static void main(string[] args) { int numsales = 10; double profit = 100.0; double profitmargin = profitmargin( profit, numsales); System.out.printf("sales = %d, profit = %.2f, profitmargin = %.2f\n", numsales, profit, profitmargin);

12 What tool checks whether checked exceptions are handled? Editor Compiler JVM None - only unchecked exceptions are checked.

13 3 Categories of Throwables 1. Checked Exceptions Require exception handling code 2. Unchecked Exceptions Programmer error, in general fix, don't wrap with handling code 3. Errors System configuration and other VM problems

14 Unchecked Exceptions - Programming Errors System.out.println( 1 / 0); int[] list = new int[4]; System.out.println( list[4]); //ArithmeticException //ArrayIndexOutOfBoundsException String s = abc ; System.out.println( s.charat(3)); //StringIndexOutOfBoundsException Object o = new Object(); String d = (String)o; Object o = null; o.tostring(); //ClassCastException //NullPointerException

15 Javadocs Exception in thread "main" java.lang.classcastexception: java.lang.object cannot be cast to java.lang.string at Test.method2(Test.java:5) at Test.method1(Test.java:9) at Test.main(Test.java:27)

16 Exception Handling Runtime Exceptions Unchecked Exceptions Error - internal system errors RuntimeException Checked Exceptions Exception

17 ArrayIndexOutOfBounds is a checked exception an unchecked exception other

18 Checked Exceptions Compiler forces programmer to handle them. Must be handled, either: catch declare the method throws

19 Throws clause //declare method throws them public void mymethod() throws IOException {

20 Catching Exceptions try { some statements; catch ( Exception1 e) { catch ( Exception2 e) {

21 Throwing Exceptions public void mymethod(int i) throws IllegalArgumentException { if ( i < 0) { throw new IllegalArgumentException( i cannot be negative ); //IllegalArgumentException is a RuntimeException

22 Notes BP1 Milestone 3 Due today Graded: Highest scoring, last submission by deadline. For teams, we choose one partner's work to grade and assign both the same grade. (both must turn in) Grading Big Programs Exam 2 Next Thursday Exam Conflict Alternatives have been ed (from Marc Renault)

23 Interpreting an Exception Stack Trace Exception in thread "main" java.lang.stringindexoutofboundsexception: String index out of range: 0 at java.lang.string.charat(unknown Source) at Interpret.methodY(Interpret.java:7) at Interpret.methodZ(Interpret.java:10) at Interpret.main(Interpret.java:3)

24 What happens when... try { statement1; statement2; //throws an exception statement3; catch( Exception1 e) { catch( Exception2 e) { statement4; //will statement 3 be executed? //if exception is not caught will statement 4 be executed? //if the exception is caught will statement 4 be executed?

25 What happens when... static int methoda() throws Exception5 { try { statement1; statement2; statement3; catch( Exception1 e) { statemente; throw new Exception5( some message ); finally { statement4; Statement5; return 1;

26 More Exceptions Exception Message What to do when you have to catch an exception and aren't sure yet how to handle? e.printstacktrace()

27 If methoda doesn't throw an exception then output is? public static void main(string[] args) throws Exception { try { methoda( args); System.out.print("B"); catch( Exception e) { ///write error to a log file System.out.print("C"); throw e; finally { System.out.print("D"); System.out.print("E"); BCDE CDE CD BDE

28 If methoda throws Exception then output is? public static void main(string[] args) throws Exception { try { methoda( args); System.out.print("B"); catch( Exception e) { //e.printstacktrace(); System.out.print("C"); //throw e; finally { System.out.print("D"); System.out.print("E"); BCDE CDE CD BDE

29 If this compiles, what is true? static void methodb(int i) { if ( i < 3) throw new ExceptionQ(); public static void main(string[] args) { try { methodb( 3); System.out.print("B"); catch( ExceptionQ e) { System.out.print("C"); finally { System.out.print("D"); System.out.print("E"); ExceptionQ is an checked exception CDE is printed D only is printed BDE is printed

30 Interpreting an Exception Stack Trace C D Exception in thread "main" java.lang.exception: java.lang.exception: message at Interpret2.main(Interpret2.java:13) Caused by: java.lang.exception: message at Interpret2.methodA(Interpret2.java:4) at Interpret2.main(Interpre t2.java:8)

31 Handling Command Line Arguments A program that reverses and/or upper cases whatever is typed in. Usage: java Process [-r] [-u] java Process -r -u This is some text TXET EMOS SI SIHT And more EROM DNA

32 Walk Through Testing with a Debugger Set breakpoints on each branch path Run program in Debugger stepping through each statement Verify execution by predicting and then verifying values in variables Remove breakpoint when segment verified Repeat creating conditions until all branch paths are executed and all breakpoints removed

CS 200 File Input and Output Jim Williams, PhD

CS 200 File Input and Output Jim Williams, PhD This Week 1. WaTor Change Log 2. Monday Appts - may be interrupted. 3. Optional Lab: Create a Personal Webpage a. demonstrate to TA for same credit as other