1. E x c e p t i o n s
Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.
— Martin Golding

2. Outline for Today What is an Exception? How to Catch Exceptions
checked v. unchecked
How to Catch Exceptions
examples of good and bad try-catch code
Creating your own types of Exception
declaring your new exception
throwing your new exception

3. Definition
An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions during the execution of a program.

4. Sample Exceptions ArithmeticException ArrayStoreException
when a divide by zero is attempted
ArrayStoreException
trying to put an int into a float array
IndexOutOfBoundsException
accessing invalid array or string element
ArrayIndexOutOfBoundsException
subclass of IndexOutofBoundsException
accessing invalid array element
NegativeArraySizeException
obvious
NullPointerException
when you try to dereference a null object reference

5. Part of the Exception Class Hierarchy
Throwable
Error
Exception
ClassNotFoundException
InstantiationException
IOException
FileNotFoundException
EOFException
RuntimeException
ArithmeticException
ClassCastException
IllegalArgumentException
IllegalThreadStateException
NumberFormatException
NullPointerException

6. Types of Exceptions Error Exception checked unchecked
example - OutOfMemoryError
Exception
checked
example - FileNotFoundException
unchecked
example - ArrayIndexOutOfBoundsException
"checked" must be handled
"unchecked" are usually not dealt with directly
all "errors" are "unchecked"

7. How Exceptions are Propagated
When a statement produces an exception (checked or unchecked) and the statement is not inside a try statement, then the exception is thrown up to whoever called this piece of code.
If that calling function does not catch the exception, the exception is passed up to whoever called this function.
etc etc etc
If main throws an exception, then the JVM prints the calling stack and exits the program.

8. The throws Statement
"Checked" exceptions must be dealt with. If a statement can produced a checked exception that statement must either be inside a try statement or the method must pass the possible exception up to the calling method via throws.
public static void main(String[] args) throws IOException {
code for reading or writing
that is not inside a try statement }

9. Catching Exceptions try { code that might throw an exception }
catch (ExceptionName param)
code used when this error occurs
catch (AnotherExceptionName param)
{ yadda yadda }
finally
do this code, no matter if error occurs

10. Example 1 - File Opening // open the data file for reading try {
FileReader freader = new FileReader(fname);
inputFile = new BufferedReader(freader); }
catch (FileNotFoundException e)
System.out.println("Unknown File: " + fname);
return;
catch (IOException e)
System.out.println("Error Opening " + fname);
Note: the order of these
two exceptions is important

11. Example 2.1 - Converting String to Integer
Version One - Crashes on Bad Input
// read until eof and sum the values
dataline = inputFile.readLine();
while (dataline != null) {
nextint = Integer.parseInt(dataline);
sum += nextint; }

12. Example 2.2 - Converting String to Integer
Version Two - Sum is Wrong
// read until eof and sum the values
dataline = inputFile.readLine();
while (dataline != null) {
try
nextint = Integer.parseInt(dataline); }
catch (NumberFormatException nfe)
System.out.println("unable to ...
sum += nextint;

13. Example 2.3 - Converting String to Integer
Version Three - Infinite Loop
// read until eof and sum the values
dataline = inputFile.readLine();
while (dataline != null) {
try
nextint = Integer.parseInt(dataline);
sum += nextint; }
catch (NumberFormatException nfe)
System.out.println("unable to ...

14. Example 2.4 - Converting String to Integer
// read until eof and sum the values
dataline = inputFile.readLine();
while (dataline != null) {
try
nextint = Integer.parseInt(dataline);
sum += nextint; }
catch (NumberFormatException nfe)
System.out.println("unable to convert \"" +
nfe.getMessage() +
"\" into an integer");

15. Creating Your Own Exceptions
Suppose you are writing a stack class. Then you will want to create your own exceptions.
StackException
StackOverflowException
StackUnderflowException
StackTypeMismatchException
Which existing exception class do you build your exception off of?
best idea is to use Exception
RuntimeException is okay

16. Example - new checked exception
declaration of your stack class:
public class StackClass {
public class StackException
extends Exception; {...}
public class StackOverflowException
extends StackException; {...}
public int pop() {...}
inside a method that uses your stack class:
try
mystack.push(8); }
catch (StackOverflowException e)
see code on next slide

17. Example - declaration details
public class StackException extends Exception {
StackException ()
super (); }
StackException ( String description )
super ( description )
"super" = use my mom's constructor

18. How to throw an exception
public class StackClass {
...
// the pop method
public int pop ()
if (stacksize == 0)
throw StackUnderflowException;
return list [ --stacksize ]; }

19. Next Classes
writing classes
writing classes that extend other classes