1. CS Week 10
Jim Williams, PhD

2. This Week MineSweeper, Milestone 3 Team Lab: ArrayLists
First impressions matter!
Comment and style as you go
Having difficulty reading partner's code?
Formatting
printMap
Team Lab: ArrayLists
Lecture: Command-line arguments, paths and Exceptions

3. Text Interface vs GUI Interface

4. Current Working Directory
A specific path in the file system
Relative paths:
relative to current working directory
images/file.jpg
Absolute paths:
specified from root directory
C:\Users\jimw\workspace\images
/Users/jimw

5. Which kind of path? jim/pictures absolute relative other
B: relative to current working directory

6. Forward / or Backward \ Windows: C:\Users\Fred In Java program:
Or "C:/Users/Fred"
Linux/Unix/Mac
/users/fred

7. Command-Line Commands
Windows
Unix/Linux/Mac cd
mkdir
rmdir
del
type
help rm
cat
man

8. What are command-line arguments?
an array of Strings
information passed into the program
the main method parameter
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]); }
All are true
The main method parameter is named args that is an array of Strings that contains information passed into the program.

9. Usage Message public class CmdLine {
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]);

10. Command Line Arguments
Passing to program from within Eclipse
Passing to program from Command-Line

11. 3 Categories of Throwables
Checked Exceptions
Require exception handling code
Unchecked Exceptions
Programmer error, in general fix, don't wrap with handling code
Errors
System configuration and other VM problems

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

13. 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)

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

15. ArrayIndexOutOfBounds is
a checked exception
an unchecked exception
other
B: an unchecked exception

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

17. Throws clause //declare method throws them
public void myMethod() throws IOException { }

18. Catching Exceptions try { some statements; } catch ( Exception1 e) {

19. Throwing Exceptions
public void myMethod(int i) throws IllegalArgumentException {
if ( i < 0) {
throw new IllegalArgumentException(
“i cannot be negative”); }
//IllegalArgumentException is a RuntimeException

20. 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?

21. What happens when... int methodA() throws Exception5 { try {
statement1;
statement2;
statement3;
} catch( Exception1 e) {
statementE;
throw new Exception5(“some message”);
} finally {
statement4; }
Statement5; return 1;

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

23. If methodA doesn't throw an exception then output is?
public static void main(String[] args) throws MyException {
try {
methodA( args);
System.out.print("B");
} catch( MyException e) {
throw e;
System.out.print("C");
} finally {
System.out.print("D"); }
System.out.print("E");
BCDE
CDE CD
BDE
try it

24. If methodA throws MyException then output is?
public static void main(String[] args) throws MyException {
try {
methodA( args);
System.out.print("B");
} catch( MyException e) {
//e.printStackTrace();
System.out.print("C");
throw e;
} finally {
System.out.print("D"); }
System.out.print("E");
BCDE
CDE CD
BDE
try it

25. 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
ExceptionQ is an unchecked exception.
D: BDE is printed.

26. If methodA throws MyException2 then output is?
public static void main(String[] args) throws MyException {
try {
methodA( 3);
System.out.print("B");
} catch( MyException2 e) {
System.out.print("C");
} catch( MyException e) {
throw e;
System.out.print("D"); }
System.out.print("E");
BCE
CDE CE BD
try it

27. How would you access "Hi."? responses[1][1][1]
Error
String [ ][ ][ ] responses = {
{{"hello"},
{ "How do you do.", "Hi."}},
{{"always"},
{ "When?", "Really, always?"}} };
try it

28. Handling Command Line Arguments