LECTURE 8: EXCEPTIONS CSC 212 – Data Structures

Error Handling Goals  What should we do when an error occurs?  Should alert system to the error  May want to execute code to handle to fix error  Minimize amount of rewritten code  May want to change in subclass  Future inputs & uses of code should be considered  Changing printing of error message is not possible

Exceptional Circumstances  ex-cep-tion : n.  Instance or case not conforming to the general rule  Proper way to signal error in object-oriented code  Seen Java exceptions in CSC111 already  Most are unchecked exceptions (no real solution)  ArrayIndexOutOfBoundsException -- accessing a nonexistent array entry  NullPointerException -- used reference that is equal to null

Exception Classes Java  Exception is a class defined by Java  Java defines many subclasses of Exception  New Exception subclasses can be written, also  Error handling uses instances of these classes  Classes are no different than others  Will include fields, methods, & constructors  Exception instances are normal objects  Use anywhere, not strictly for error handling  Must instantiate before use

Throwing an Exception public class BankAccount { private float balance; // Lots of code here… float withdraw(float amt) throws ReqException{ if (amt balance) { ReqException re = new ReqException(); re.setBadRequest(amt, balance); throw re; } balance -= amt; return balance; } }

Error Handling Codes  throw exception upon detecting problem  Handle problem by catch ing exception  Do not need to catch an exceptions  If it is never caught, program will crash  Not a bad thing – had an unfixable error!

Handling Exceptions  Once an exception is thrown, methods can:  Include code to catch exception and then ignore it

Handling Exceptions  Once an exception is thrown, methods can:  Include code to catch exception and then ignore it  Catch & handle exception so error is fixed

Handling Exceptions  Once an exception is thrown, methods can:  Include code to catch exception and then ignore it  Catch & handle exception so error is fixed  Catch the exception and throw a new one

Handling Exceptions  Once an exception is thrown, methods can:  Include code to catch exception and then ignore it  Catch & handle exception so error is fixed  Catch the exception and throw a new one  Ignore exception so passed on to calling method

Handling Exceptions  Exception can be thrown anywhere & anytime  throws lists method’s fixable uncaught exceptions void controller() throws LostPlane {…} int scheduler() throws NoFreeTime {…}  Calling methods now aware of possible errors  If possible, could catch and correct errors  List exception in own throws clause otherwise void plan(int i) throws NoFreeTime { // Lots of interesting code here… scheduler(); }

try {…} Blocks  Cannot catch exceptions thrown outside try block void cantCatch() throws Oops, MyBadExcept { try { // There is code here that does something interesting… System.err.println(“This is not my exception”); } catch (Oops oop) { // Here be code… } methodThatMightThrowOopsExcept(); throw new MyBadExcept(); }

try {…} Blocks  Catch some (or all) exceptions thrown in try  Each try needs at least 1 catch void catchSome() throws MyBadException { try { methodThatMightThrowOops(); throw new MyBadExcept(); } catch (Oops oop) { oop.printStackTrace(); System.err.println(“Oops was caught.”); System.err.println(“Method ends normally.”); } }

try {…} Blocks  try can have multiple catch s void catchAll() { try { methodThatMightThrowOops(); methodThatShouldThrowOops(); throw new MyBadExcept(); } catch (Oops oop) { oop.printStackTrace(); System.err.println(“Oops was caught.”); System.err.println(“Method ends normally.”); } catch (MyBadExcept mbe) { mbe.printStackTrace(); System.err.println(“MyBad was caught.”); System.err.println(“Method ends normally.”); } }

Handling Exceptions public void forcedWithdrawal(float amount) throws BadRequestException { callPolice(); addDyePacks(); withdrawal(amount); } public void defeatOfJesseJames(float amt) { try { forcedWithdrawal(amt); } catch (BadRequestException bre) { formPosse(); killSomeGangMembers(); } finally { giveLollipop(); } }

2 Types of Exceptions  Subclass of Exception  Must list uncaught exceptions in throws  Use for fixable errors  Java forces methods to consider them  Only useful if fixable  Subclass of RuntimeException  Can list uncaught ones in throws  “You are hosed”  Usually can’t be fixed  Can ignore in method  Still crashes program unless it is caught Checked Exception Unchecked Exception

Tracing Example public static int generate() throws TraceException { TraceException te = new TraceException(); System.out.println(“Starting gE”); throw te; System.out.println(“Ending gE”); return 0; } public static void handler(boolean callIt) { try { System.out.println(“Starting cE”); if (callIt) { generate(); } System.out.println(“Ending cE”); } catch (TraceException te) { System.out.println(“Caught te”); } } public static void main(String[] args) { handler(false); handler(true); }

Before Next Lecture…  Continue week #3 weekly activity  Should now be able to do problems #1 & 2  Continue programming assignment #1  Assignment does not require any new material  Read about interfaces & abstract classes  (Will form the basis of the mysterious ADT)  Very important when working with GUIs  Useful way of beating inheritence