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

2. This Week Sokoban: Milestone 3 Team Lab: ArrayLists TA Review Session
First impressions matter!
Comment and style
Team Lab: ArrayLists
TA Review Session
Lecture: Command-line arguments and Exceptions

3. From the 1st Week public class CmdLine {
public static void main(String[] args) { }
//To test command line arguments in Eclipse
//Run>Run Configurations… then select Arguments tab and enter the arguments and click run.
public class CmdLine {
public static void main(String[] args) {
if ( args.length < 1) {
System.out.println("Usage: java CmdLine name");
} else {
System.out.printf("hello %s", args[0]); }
// System.out.println( args); //
// for ( int i = 0; i < args.length; i++) {
// System.out.println( args[i]);
// }

4. Text Interface vs GUI Interface

5. Command-Line Commands
Windows
Linux/Mac
description
dir cd
mkdir
rmdir
del
type
help ls rm
cat
man
show dir. contents
change directory
make directory
remove directory
remove files
show file contents
show help info.

6. 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.

7. 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]);

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

9. Various Kinds and Times for Errors
Editing/Saving errors
Compile time errors
Syntax errors
Runtime errors
Logic errors
Exceptions

10. New This Week Runtime errors Logic errors Exceptions
Unchecked Exceptions
Checked Exceptions
Checked Exceptions are a mechanism where the compiler does some checks to make sure error handling is in place.
These Compile time checks can save some Runtime problems.

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);
public class Sales {
public static double profitMargin( double profit, int numSales) throws Exception {
if ( numSales == 0) {
throw new Exception("Hey, numSales is 0, this is a problem with the calculations"); }
return profit / numSales;
public static void main(String[] args) {
int numSales = 0;
double profit = 0.0;
try {
double profitMargin = profitMargin( profit, numSales);
System.out.printf("sales = %d, profit = %.2f, profitMargin = %.2f\n",
numSales, profit, profitMargin);
} catch ( Exception e) {
e.printStackTrace();
System.out.println("after try-catch block");

12. Question What tool checks whether checked exceptions are handled?
Editor
Compiler
JVM
None - only unchecked exceptions are checked.
compiler is the best answer. In Eclipse errors are shown in the Editor because Eclipse is an Integrated Development Environment but it is compiler functionality that is checking that exceptions are handled and reporting to the Eclipse Editor to show the red line to the user.

13. 3 Categories of Throwables
Unchecked Exceptions
Fix problem, don't wrap with handling code
Example: ArrayIndexOutOfBounds
Checked Exceptions
Require exception handling code
Example: File not found
Errors
System configuration and other VM problems

14. 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
String s2 = null;
System.out.println( s2.charAt(3)); //NullPointerException

15. Exception Handling Hierarchy
Categories
Error - internal system errors
Unchecked Exceptions
Extend from RuntimeException
Checked Exceptions
Others that extend from Exception
JavaDocs for Exceptions
va/lang/RuntimeException.html

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

17. Exception Stack Trace
Exception in thread "main" java.lang.NullPointerException
at TestException.method2(TestException.java:6)
at TestException.method1(TestException.java:3)
at TestException.main(TestException.java:13)
public class TestException {
public static void method1(String str) {
method2( str); }
public static void method2(String str) {
System.out.println( str.charAt(3));
public static void main(String[] args) {
String s2 = null;
if ( args.length > 0) {
s2 = args[0];
method1( s2);

18. Checked Exceptions Compiler requires programmer to handle them.
Must be handled, either:
throws clause
try - catch block

19. Throws clause
In method definition, declare method may throw the exception
public void myMethod() throws FileNotFoundException {
//the following code may throw a FileNotFoundException FileReader reader = new FileReader("filename.txt"); }

20. Try Catch Block In method definition, catch exception
public void myMethod() { try { //the following code may throw a FileNotFoundException FileReader reader = new FileReader("filename.txt"); } catch ( FileNotFoundException e) { e.printStackTrace(); } }

21. Throwing Exceptions
public void myMethod(int count) throws IllegalArgumentException {
if ( count < 0) {
throw new IllegalArgumentException(
“count 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)
P7 & BP2 Eliza

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)
public class Interpret {
public static void main(String[] args) {
methodZ(); }
public static void methodY() {
String str = "";
str.charAt(0);
public static void methodZ() {
methodY();

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
try it

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
try it

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

30. Interpreting an Exception Stack TraceD
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)
public class Interpret2 {
public static void methodA( String [] list) throws Exception {
throw new Exception("message"); }
public static void main(String[] args) throws Exception {
try {
methodA(args);
System.err.println("B");
} catch (Exception e) {
/// May write error to a log file
System.err.println("C");
throw new Exception(e);
} finally {
System.err.println("D");
System.err.println("E");

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
import java.util.Scanner;
public class Process {
public static String reverse(String str) {
String result = "";
for ( int i = str.length() -1; i >=0; i--) {
result += str.substring(i, i+1); }
return result;
public static void main(String[] args) {
if ( args.length <= 0) {
System.out.println("Usage: java Process [-r] [-u]");
System.exit( -1);
boolean reverse = false;
boolean upperCase = false;
for ( String str : args) {
switch ( str) {
case "-r": reverse = true;
break;
case "-u" : upperCase = true;
Scanner input = new Scanner(System.in);
while ( input.hasNextLine()) {
String line = input.nextLine();
if ( reverse) line = reverse(line);
if ( upperCase) line = line.toUpperCase();
System.out.println( line);

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