1. CSE 114 – Computer Science I Exceptions
Cape Breton, Nova Scotia

2. How should a method react to improper input?
Exit the program option:
public Person(String initName, int initAge) {
name = initName;
if ((initAge < 0) || (initAge > 100))
System.exit(0);
age = initAge; }
Poor Option
Program cannot recover if error condition occurs
Programmer using this class may not anticipate error
Use default value option:
public Person(String initName, int initAge) {
name = initName;
if ((initAge < 0) || (initAge > 100))
age = 0;
else
age = initAge; }
Poor Option
Calling class not notified of error condition
Prone to logical errors

3. Exception option public Person(String initName, int initAge)
throws InvalidAgeException {
name = initName;
if ( (initAge < 0) || (initAge > 100) )
throw new InvalidAgeException(initAge);
else
age = initAge; }
InvalidAgeException is a class I defined for HW5
Good Option
Program can recover if error condition occurs
Programmer using this class is forced to anticipate a specific type of error
Calling class immediately notified of error condition
Helps to logical errors

4. Exceptions Objects that defines an erroneous condition in a program.
May be thrown in two ways:
by JVM during runtime due to an illegal operation :
ArithmeticException
ClassCastException
ArrayIndexOutOfBoundsException
NumberFormatException
NullPointerException
by a program explicitly using throw
May be caught in several ways:
by an exception handler (you deal with it)
by the operating system (runtime error)

5. Some Exceptions/Errors thrown by the JVM
bash-2.03$ javac Driver.java
bash-2.03$ java Drivver
Exception in thread "main" java.lang.NoClassDefFoundError: Drivver
bash-2.03$
Misspelled class name
Exception in thread "main" java.lang.NullPointerException
This code generates this exception
String dumb = null;
int len = dumb.length();
Exception in thread "main" java.lang.ArithmeticException: / by zero
This code generates this exception
int i = 1, j = 0;
i = i/j;

6. A Program generating and handling an Exception
double radius, area, circ, PI = 3.14;
try
{ System.out.print("Input a circle radius: ");
radius = keyboard.nextDouble();
if (radius <= 0.0)
throw new Exception("ERROR: Invalid radius!");
area = PI * radius * radius;
      circ = 2.0 * PI * radius;
System.out.println("radius: " + radius);
System.out.println("area: " + area);
System.out.println("circumference: " + circ); }
catch (Exception e) {
e.printStackTrace();
Exception is generated here
Exception is handled here

7. Exception Handling
When the throw statement is executed in a try block,
an Exception object is created
a String is passed to the constructor of the Exception object
execution of the try block is suspended
control passes to the catch block that is placed after the try block
If the throw statement is not executed,
execution of the try block is completed
the corresponding catch block is ignored …
try {
throw new Exception(“…”); }
catch (Exception e)

8. Line of error int total = 0, num = 0, avg; PrintStream o = System.out;
o.print("Enter total: ");
total = keyboard.nextInt();
o.print("Enter num: ");
num = keyboard.nextInt();
o.println("Before try");
try
{ o.println("Inside try, pre div");
avg = total/num;
o.println("Inside try, post div");
o.println("Average: " + avg); }
catch(Exception e) {
o.println("Inside catch");
e.printStackTrace();
o.println("After catch");
bash-2.03$ java ExceptionTester
Enter total: 6
Enter num: 3
Before try
Inside try, pre div
Inside try, post div
Average: 2
After catch
Message available via getMessage() method
bash-2.03$ java ExceptionTester
Enter total: 4
Enter num: 0
java.lang.ArithmeticException: / by zero at main(…java:16)
Before try
Inside try, pre div
Inside catch
After catch
Line of error

9. Updated Example int total = 0, num = 0, avg;
PrintStream o = System.out; do {
o.print("Enter total: ");
total = keyboard.nextInt();
o.print("Enter num: ");
num = keyboard.nextInt();
try
avg = total/num;
o.println("Average: " + avg); }
catch(Exception e)
o.println(e.getMessage());
} while (num == 0);
Updated Example
EXAMPLE SESSION:
Enter total: 4
Enter num: 0
/ by zero
Enter total: 6
Enter num: 3
Average: 2

10. Predefined Exception Classes
There are many derived classes from the Exception class for specific types of errors.
Some exception classes:
IOException: A requested I/O operation could not be completed successfully.
NumberFormatException: An operation was attempted using a number in an illegal format.
NullPointerException: A null reference was used where an object reference was needed
ex: method invocation
Die die1 = null;
die1.roll()

11. Example: Parsing ints
public static int readInt(Scanner keyboard) {
String token = keyboard.next();
int value;
try
value = Integer.parseInt (token); }
catch (Exception exception)
System.out.println ("Error reading int data,
MIN_VALUE value returned.");
value = Integer.MIN_VALUE;
return value;
For improper input, the parseInt method throws an NumberFormatException.
Using the Exception class here catches all types of exceptions. Why?
Automatic Type Casting

12. More on Exceptions Why pass exceptions?
An Exception can be passed to the calling method
specifying your method throws specific exceptions means it can be handled (or again may be passed) by the calling method
2 types of exceptions:
(1) Checked: Subclasses of RuntimeException
ex: java.lang.NumberFormatException, from Sun’s JDK
Calling class can call method with or without try-catch block
(2) Unchecked: All other Exception classes
ex: InvalidAgeException, a class I defined for HW5
Calling class must place call to method inside try-catch block
Why pass exceptions?
flexibility

13. Case (1) Examples
Method from Integer class:
public static int parseInt(String s)
throws NumberFormatException
Calling parseInt without try-catch:
System.out.print("Enter text: ");
String text = keyboard.next();
int num = Integer.parseInt(text);
System.out.println(num);
For improper input, the passed exception is not caught. Results in a runtime error & program terminates.
Calling parseInt with try-catch:
try
{ System.out.print("Enter text: ");
String text = keyboard.next();
int num = Integer.parseInt(text);
System.out.println(num); }
catch (NumberFormatException nfe)
{ nfe.printStackTrace(); }
For improper input, the passed exception is caught. Program can continue.

14. Case (2) Example
Constructor from Person class for HW5:
public Person(String initName, int initAge)
throws InvalidAgeException {
name = initName;
if ( (initAge < 0) || (initAge > 100) )
throw new InvalidAgeException(initAge);
else
age = initAge; }
Must be inside try-catch block, else it is a syntax error
Calling the above Person constructor with try-catch:
Person mariano;
try {
mariano = new Person("Rivera, Mariano", 39)); }
catch (InvalidAgeException iae)
{ iae.printStackTrace(); }
System.out.println(mariano);
mariano is declared here so that it may be used outside the try block

15. Example: PassTheException
public class PassTheException {
public static void pass() throws NullPointerException
the(); }
public static void the() throws NullPointerException
exception();
public static void exception()
throws NullPointerException
String dumb = null;
dumb.length();

16. Prints the history of exception passing
public static void main(String[] args) {
try
pass(); }
catch(NullPointerException npe)
{ npe.printStackTrace(); }
Results
bash-2.03$ java PassTheException
java.lang.NullPointerException
at PassTheException.exception(PassTheException.java:16)
at PassTheException.the(PassTheException.java:10)
at PassTheException.pass(PassTheException.java:5)
at PassTheException.main(PassTheException.java:23)
bash-2.03$
Prints the history of exception passing

17. Defining Your Own Exception
When you insert a throw statement in your code
you may use an existing exception class or
you may define your own Exception class.
Define a derived class from Exception or one of its descendants
A constructor is usually all that is required:
public class GradeException extends Exception {
public GradeException()
{ super("ERROR: " + grade + " is an illegal value!“
+ "\n\tGrades must be from "); }
public GradeException(String s)
{ super(s);

18. Using Your Own Exception
public class GradeExceptionTester {
public static final PrintStream o = System.out;
public static int promptForGrade()
throws GradeException
int grade = 0;
o.print("Please enter a grade: ");
Scanner keyboard = new Scanner(System.in);
grade = keyboard.nextInt();
if ((grade < 0) || (grade > 100))
throw new GradeException(grade);
return grade; }

19. Using Your Own Exception
public static void main(String[] args) {
int[] gradesArray = new int[5];
int counter = 0, total = 0;
while (counter < 5)
try
gradesArray[counter] = promptForGrade();
total += gradesArray[counter];
counter++; }
catch(GradeException ge)
{ o.println(ge.getMessage()); }
o.println("Average grade: " + ((double)total/5));

20. GradeExceptionTester Output
bash-2.03$ java GradeExceptionTester
Please enter a grade: 55
Please enter a grade: 66
Please enter a grade: 77
Please enter a grade: -5
ERROR: -5 is an illegal value!
Grades must be from
Please enter a grade: 88
Please enter a grade: 99
Average grade: 77.0
bash-2.03$

21. Catching Different Exceptions Separately
Sometimes there may be multiple sources of errors inside a single try block, for such cases you have 2 options:
Use a common ancestor class to catch all exceptions
Ex: Exception
catch each type of exception separately (preferred)
This allows your program to react differently to different types of problems

22. Multiple catches example
public static void savePersonName(Person p)
throws NullPointerException {
try
FileOutputStream fos = new FileOutputStream("output.ser");
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(p.getName());
oos.close(); }
catch(NotSerializableException nse)
{ o.println(“Remember to implement Serializable"); }
catch(FileNotFoundException fnfe)
{ fnfe.printStackTrace(); }
catch(IOException ioe)
{ System.exit(0); }

23. Exceptions Ordering
A single try block may have multiple catch blocks but, the order of the caught exceptions is important
Only one catch block may be executed per try block execution
So, descendent exception classes must be listed first
Ex:
try {
number = Double.parseDouble(obj.getText()); }
catch (Exception e)
{ e.printStackTrace(); }
catch (NumberFormatException nfe)
System.out.println(nfe.getMessage());
number = 0.0;
This would produce a syntax error, because of unreachable code

24. Exceptions Ordering - Improvement
This catch block will handle NumberFormatExceptions
try {
number = Double.parseDouble(obj.getText()); }
catch (NumberFormatException nfe)
System.out.println(nfe.getMessage());
number = 0.0;
catch (Exception e)
{ e.printStackTrace(); }
This catch block will handle everything else.

25. What is data storage? The means by which a program stores its data
Can you name data associated with this program?
How about your favorite game?
How is this done?
temporarily
persistently

26. Temporary Data Data loaded into RAM Used for running program
stack, heap, and global segments of program space
Used for running program
primitives
objects
arrays
What happens to this data (in RAM) when a program terminates?
It’s gone
Some other program will be given that region of RAM.

27. Persistent Data After a program ends, this data is not destroyed. Why?
Its not only (if at all) in RAM. Where is it then?
Hard drive, CD, DVD, Server, i.e. permanent storage
What on these devices store this data?
files
database (managed via a group of files)

28. How does temporary data get its values?
Potentially by many means. Ex:
Values hard coded into a program:
int i = 0;
Program calculations:
int i = j + 5;
User input via a device & stream:
int i = keyboard.nextInt();
The Computer’s System:
int i = (int)(Math.random() * 100) + 1;
An input stream from a file or server:
int i = binaryFileReader.readInt();
A database:
int i = resultSet.getInt("SALARY");

29. How does persistent data get its values?
An output stream in your program:
printWriter.println(outputText);
An output stream in some other program:
Notepad
Microsoft Word
Maya
Starcraft Campaign Editor
Anything that can save files

30. Ex: Starcraft Starcraft Campaign Editor Starcraft
for making your own map level
saves your and all necessary data to .scm file
done via output stream
Starcraft
lets player pick map level to play
loads all necessary data from .scm file
done via input stream
Note: this only works if Starcraft knows how the data in scm files is arranged

31. Let’s open the Fastest.scm map in Notepad
What happened?
MPQ ã_  à Ã_   s(O쬓ÊÁÝ©ˆ¨ÀPæÒñwA'ŒŠ’'$¶›
„qÚu!ÿA±7m_‘>Îû3®²E¿õµõ4‹‘.B¡]N™.>›ÝØ&ÜB,
>Bži㥣ÂÛ?Ò€€¼âB´dqÝj|¨ç›4ê´Cº3Ñ.ÒÓõ(0J4q¶ñ~¢n‰¿wAŽÚY™×=nââpQ]À&¶‘©1,#õbw}ž}Ú5åNBˆÃYïŠ{sRøö‡Ë¤Vò_V˜i‰£k'CòÑœ]ñRꮟþ’å„à^Ÿ³fJêöďòK}S~c«“¬­$u\诣7<ºƒÚXAi¯LÃúvá·™õÙ^»W;õÄÇq»M˜p#´#ȁÎv …

32. I/O Streams Program I/O must work together
Input Streams must know how the Output Stream sent data:
What data?
What types?
What format are the types? (not an issue in Java)
What type of stream was used?
In what order?
I/O Streams typically use Sequential Access
for output streams
data appears in the file in the order it is written out
for input streams:
data is read from the file in the order it appears in the file

33. Sequential Access & Primitives
Output to file NumbersData.dat (binary)
outputStream.writeInt(x);
outputStream.writeInt(y);
outputStream.writeInt(z);
Input from file NumbersData.dat into variables
int x = inputStream.readInt();
int y = inputStream.readInt();
int z = inputStream.readInt();

34. How about objects? An object is its data (instance variables)
Saving objects to a text/binary file:
write out all of its instance variables
do so in order (sequentially)
Loading objects from a text/binary file:
read in all of its instance variables
use read data to construct an object

35. Sequential Access & Objects
Output to file PersonData.dat (binary)
outputStream.writeUTF(p.getName());
outputStream.writeInt(p.getAge());
Input from file PersonData.dat into variables
String initName = inputStream.readUTF();
int initAge = inputStream.readInt();
Person p = new Person(initName, initAge);

36. Java I/O Java has extensive support for I/O of all sorts
java.io.* package
more than 60 different stream types
Lots of features
I/O to programs/files/Internet (files & servers)
encoding
compression
encryption

37. Readers/Writers

38. Common Java Stream Types
Text
BufferedReader/PrintWriter
reads/writes Unicode
Binary
DataInputStream/DataOutputStream
reads/writes bytes
Object files (using Object Serialization)
ObjectInputStream/ObjectOutputStream
reads/writes Java objects

39. How much memory would be required:
To store the integer value 777
in a text file?
3 chars * 2 bytes per char == 6 bytes
in a binary file?
1 int * 4 bytes per int == 4 bytes
in a Java object file?
lots more
the context (meta data)
and the data itself

40. Text Versus Binary Files
Text files:
are readable (ex: Notepad)
are a common format (think Unicode/ASCII)
good for sharing files between programs
think XML
require data to be converted into text for output
require data to be converted from text for input
Binary files:
can store data in its natural format
typically requires less memory than text
requires no conversion
reading/writing from/to files is faster that text

41. Development vs. Release Formats
During project development:
have program use text files
readable
helps with changing parameters
helps with debugging
When product is released
have program use binary files
less memory
faster
Note that this is an option, not a rule.
XML is common throughout both stages

42. So how do we use Java’s streams?
Output streams:
Open an output stream to a file
While there is more data to write, do steps 3 & 4
Write data to file
Go back to step 2
Close stream
Input streams:
Open an input stream to a file
While there is more data to read, do steps 3-5
Read/write next piece of data
Use read data to initialize proper variables
Close the stream

43. Java’s Streams Streams have counters: When you write, it:
when you open a stream, counter is set to start of file
denotes location in file to read/write from
When you write, it:
writes data to end of file
counter moves to end of data just written
the new end of file
When you read, it:
gets next piece of data
counter moves to next piece

44. Writing to a text file in Java
To use java.io.PrintWriter
Construct (open) the stream
Write all text one line a time using println(…)
Close stream when done (i.e. end of file)

45. Text File Writing Example
try { File file = new File("Output.txt"); Writer writer = new FileWriter(file); PrintWriter out = new PrintWriter(writer); out.println("Janie Jones"); out.println("" + 25); out.close(); } catch(IOException ioe) System.out.println("File not Found");

46. Reading from a text file in Java
To use java.io.BufferedReader
Construct (open) the stream
Read text one line at a time using readLine()
readLine returns null if no more text to read
Close stream when done (i.e. end of file)

47. Text File Reading Example
Person p; try { File file = new File("Output.txt"); Reader reader = new FileReader(file); BufferedReader in = new BufferedReader(reader); String name = in.readLine(); String ageText = in.readLine(); int age = Integer.parseInt(ageText); p = new Person(name, age); System.out.print(p); } catch(IOException ioe) System.out.println("File not Found");
Output?
Janie Jones, 25

48. How much data does the file have?
Many times we don’t know
A Web browser doesn’t know how much text is in Web page until it reads in the entire thing
2. Read text one line at a time using readLine()
readLine returns null if no more text to read

49. I/O Example We have a file called grades.txt
It stores exam grades for entire class
One grade per line. Ex: 90 77 …
Read in all grade and calculate average
Write N and average to stats.txt

50. Example import java.io.*; public class TextFileDemo {
 public static void main(String[] args)
  try
{ BufferedReader inputStream = new BufferedReader
(new FileReader("grades.txt"));
    PrintWriter outputStream = new PrintWriter
(new FileOutputStream("stats.txt"));
int count = 0, sum = 0;
String line;
line = inputStream.readLine();
while (line != null) // at EOF?   {
count++;
sum += Integer.valueOf(line).intValue(); }
Example

51. Example (cont’d) if (count != 0) { int average = sum / count;
outputStream.println("Number of students: "
+ count);
outputStream.println("Class average: "
+ average); }
else
outputStream.println("No grades to average.");
inputStream.close();
outputStream.close();
catch (FileNotFoundException e)
{ System.out.println(e.getMessage()
+ "\nFile not found.");} catch (IOException e)
+ "\nError accessing file");}
} // main
} // class

52. What’s the delimiter here?
Delimited Text
How about saving Person objects to one text file?
One line per variable is not very readable:
Joe 16
Jane 26
George 13
Using delimited data is better:
Joe|16
Jane|26
George|13
What’s the delimiter here? |

53. Choosing Delimiters We might want each line to represent an object
Common delimiters:
‘|’, ‘:’, ‘-’
What about ‘\t’?
What about ‘,’?
Example file for Employee objects:
Wayland Smithers|38|100000
Ned Flanders|37|60000
Selma Bouvier|45|40000
Tab-separated values
Comma-separated values (csv)

54. Writing Delimited Output
How would we save an array of Employees?
Construct PrintWriter stream to file
For each student in array:
build String with all data on one delimited line. Ex:
Wayland Smithers|38|100000
Write line to file
Close stream

55. Reading Delimited Output
Requires parsing of delimiters
Construct BufferedReader stream to file
For each line in the file:
break line into pieces (parse)
construct object from pieces
assign new object to variable or entry in array
Close stream

56. How might we parse our line of text?
String methods
we could look for delimiter indices (indexOf)
extract substrings using indices (substring)
Use split method
also from String class
returns array of text divided by delimiters
delimiter as regular expression
use ‘-’ instead of ‘|’
Use java.util.StringTokenizer

57. Splitting a String Example
String test = "Wayland Smithers ";
String[] array = test.split("-");
System.out.println(array.length);
for (int i = 0; i < array.length; i++)
System.out.println(array[i]);
Note that I have hard coded test, but we could also have read it as a line from a text file 3
Wayland Smithers 38
100000
What’s the output?