1. EXCEPTIONS

2. What’s an exception?? Change the flow of control when something important happens ideally - we catch errors at compile time doesn’t happen data error someone pulls a plug hardware error software error (blame it on the machine)

3. Exceptions & Errors Exceptions are actual objects They are instances of classes that inherit from the class Throwable an instance of a Throwable class is created when an exception is thrown Throwable has two sub classes Error - internal runtime error (no hope) Exception - we can do something here

4. Error Class These Exceptions are internal errors in the JAVA runtime environment (such as) LinkageError NoClassDefFoundError NoSuchMethodError OutOfMemoryError VirtualMachineError

5. Exception class is divided into three major categories: >Runtime exception classes >I/O exception classes >other classes that are directly derived from the Exception class

6. RunTimeException We know these guys (they are all classes) NullPointerException IllegalArgumentException NumberFormatException IllegalThreadStateException IndexOutOfBoundsException StringIndexOutOfBoundsException Array IndexOutOfBoundsException

7. I/O Exceptions General I/O failures EOFException FileNOtFoundException MalformedURLException SocketException

8. other classes of Exceptions These exceptions are their own classes derived from the Exception class (I think) ClassNotFoundException AWTException Instantiationexception InterruptedException

9. Some terms exception - an error condition at run time throwing - cause an exception to occur catching - capturing an exception and executing some statements to try and resolve the error catch clause - the block of code that does this (the catching) stack trace - sequence of method calls that brought control to this point

10. What causes an exception? Two ways explicitly - the programmer throws an exception with the throw statement EX throw ExceptionObject implicitly - carry out some illegal action EX divide by zero array index out of bounds (we know that one)

11. errors that might be thrown We are using a JAVA provided method and get the following compiler error blah blah: Exception blah blah must be caught or it must be declared in the Throws clause of this method what’s going on???

12. try/catch blocks a JAVA method can indicate the kinds of errors it might throw EX - a method that reads a file might throw an I/OException error you have to protect your code against these errors. This is compiler enforced we use the try/catch blocks to do this

13. The try part You put a try block around the code that might throw an exception the try statement says “try these statements and see if you get an exception try { ‘statements’ }

14. the catch part Still have errors - missing the catch block the catch block says “I will handle any exception that matches my argument” a try block is followed by a catch block catch(Exception e) { statements }

15. an example Thread.sleep might throw an InterruptedException try { Thread.sleep (500); } catch (InterruptedException e) { // does nothing }

16. Added notes A try block may have many statements therefore, the code in the try block may throw more than one exception there may be more than one catch block following a try block try{ } catch(IOException e1) { } catch(InterruptedException e2) { } catch(SocketException e3) { }

17. the finally clause Assume there is some code you absolutely must do put the code in a finally block the finally block executes whether an exception was thrown or not, if an exception was caught or not, and will execute even if a return has been executed

18. uses of the finally clause You open a file you read the file you must close the file (weather an exception was thrown or not) return resources dispose of a socket dispose of a file descriptor

19. create your own exceptions The new class should inherit from some other exception in the JAVA hierarchy inherit from Exception, not Error you can inherit from Exception but try and find one ‘close’ the one you are creating an Exception usually has two constructors first takes no parameters second takes a String as a parameter

20. example - create an Exception public class Oops extends Exception { public Oops() {}; public Oops (String msg) { super(msg); } }

21. example - create an Exception Public class Oops extends IOException { public Oops() {}; public Oops (String msg) { super(msg); } }

22. new methods and exceptions You can create a new method that throws an exception by using the throws keyword modifiers return name() throws e1, e2, e3 public void ReadF() throws Something { Something s = new Something(“oops”); if ((x /y) > 4) throw s;

23. handling the Exception When an Exception occurs, execution stops the machine looks for a catch clause to handle the error if no catch clause is found in the offending method to handle the error, (final is run) and control is passed to the ‘calling’ method this method is searched for a catch clause and so on up the ‘call chain you bomb if no catch clause is found

24. some fancy stuff Two methods are inherited from Throwable Did you wonder why there was that String in the second Exception constructor example?? There is a method that returns that String can be used to pass information the method getMessage returns that String another method printStackTrace() will print the call chain from the point of the exception