Question of the Day

A landscaper plants 5 rows of 4 trees each, but only uses 10 trees. How is this possible?

Question of the Day

A landscaper plants 5 rows of 4 trees each, but only uses 10 trees. How is this possible?

Question of the Day

A landscaper plants 5 rows of 4 trees each, but only uses 10 trees. How is this possible?

LECTURE 14:EXCEPTIONS

Error Handling Goals

Error Handling Goals

What should we do when an error occurs? Should alert system to the error Stop immediately and execute code

handling error Minimize amount of rewritten code

How error handled & fixed may change in subclass

Unknown future uses of code demands flexibility

Changing printing of error message is not possible

Exceptional Circumstances

ex-cep-tion: n. Situation or case not conforming to the

general rule Proper way to signal error in object-

oriented code

Seen Java exceptions in CSC111 already Most of these were unchecked (not

solvable) ArrayIndexOutOfBoundsException --

accessing a nonexistent array entry NullPointerException -- used null

reference

Exception Classes Java

Exception is class defined by Java Java already defines many subclasses of Exception

New Exception subclasses can be written, also

Error handling uses instances of these classes Classes are not magic and work like all

others Classes can include fields, methods, &

constructors Exception instances not special, either

Must instantiate before use Use anywhere, not strictly for error

handling

Error Handling Codes

throw exception upon detecting problem

Handle problem by catching exception Do not need to catch an exceptions

If it is never caught, program will crash Not a bad thing if the error was unfixable,

however

Throwing an Exception

public class BankAccount { private float balance;

// Lots of code here…

float withdraw(float amt) throws ReqException{ if (amt < 0) { throw new ReqException(); } else if (amt > balance) { ReqException re = new ReqException(); re.setBadRequest(amt, balance); throw re; } balance -= amt; return balance; }}

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 it would like, could catch and correct errors List exception in own throws clause otherwisevoid plan(int i) throws NoFreeTime { // Lots of interesting code here… scheduler();}

try {…} Blocks

Can only catch exceptions thrown in try block

void cantCatch() throws MistakeExcept,MyBadExcept{try { // There is code here that does something interesting… System.err.println(“No exceptions”);} catch (Oops oop) { // Here be code to fix the exception}methodThatMightThrowMistakeExcept();throw new MyBadExcept();

}

try {…} Blocks

Can only catch exceptions thrown in try block

void cantCatch() throws MistakeExcept,MyBadExcept{try { // There is code here that does something interesting… System.err.println(“No exceptions”);} catch (Oops oop) { // Here be code to fix the exception}methodThatMightThrowMistakeExcept();throw new MyBadExcept();

}

try {…} Blocks

Can catch some (or all) exceptions from try Each try needs at least 1 catch

void catchSome() throws MyBadException {try { methodThatMightThrowExceptions(); throw new MyBadExcept();} catch (Oops oop) { oop.printStackTrace(); System.err.println(“Method ends normally.”);} catch (MistakeException me) { System.err.println(“Boy was I dumb!”);}

}

try {…} Blocks

If exception thrown, stop try & go to catchvoid catchAll() {try { methodThatMightThrowOops(); methodThatShouldThrowOops(); throw new MyBadExcept();} catch (Oops oop) { oop.printStackTrace(); System.err.println(“Oops was handled.”);} catch (MyBadExcept mbe) { mbe.printStackTrace(); System.err.println(“MyBad fixed.”);}

}

Not Handling Exceptions

float withdraw(float amt) throws BadReqEx {if (amt > balance) { BadReqEx re = new BadReqEx(amt,balance); throw re;}balance -= amt;return balance;

}

void forcedWithdrawal(float amount) {callPolice();addDyePacks();withdrawal(amount);

}

public void robbedByJesseJames(float amt) {forcedWithdrawal(amt);

}

Not Handling Exceptions

float withdraw(float amt) throws BadReqEx {if (amt > balance) { BadReqEx re = new BadReqEx(amt,balance); throw re;}balance -= amt;return balance;

}

void forcedWithdrawal(float amount) {callPolice();addDyePacks();withdrawal(amount);

}

public void robbedByJesseJames(float amt) {forcedWithdrawal(amt);

}

Unfair - by not stating exception,caller hasn't chance to handle

Handling Exceptions

void forcedWithdrawal(float amount) throws BadReqEx{callPolice();addDyePacks();withdrawal(amount);

}

public void northfieldMN(float amt) {try { forcedWithdrawal(amt);} catch (BadReqEx bre) { formPosse(); killSomeGangMembers();} finally { giveLollipop();}

}

Handling Exceptions

void forcedWithdrawal(float amount) throws BadReqEx{callPolice();addDyePacks();withdrawal(amount);

}

public void northfieldMN(float amt) {try { forcedWithdrawal(amt);} catch (BadReqEx bre) { formPosse(); killSomeGangMembers();} finally { giveLollipop();}

}

2 Types of Exceptions

Checked Exception Unchecked Exception

2 Types of Exceptions

Subclass of Exception throws must list

uncaught

Use for fixable errors Java forces methods to

consider them Only useful if fixable

Subclass of RuntimeException Can be listed in throws

“You are hosed” Usually can’t be fixed Can ignore in method Unless it is caught,

these will still crash program

Checked Exception Unchecked Exception

Tracing Example

public static void generate() throws TraceException {TraceException te = new TraceException();throw te;System.out.println(“Ending gE”);

}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);

}

Tracing Example

public static void generate() throws TraceException {TraceException te = new TraceException();throw te;System.out.println(“Ending gE”);

}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);

}

Tracing Example

public static void generate() throws TraceException {TraceException te = new TraceException();throw te;

}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);

}

Your Turn

Get into your groups and complete activity

For Midterm

You can use on this midterm: You can always use your textbook & notes Printout of slides IF has notes on that day's

slides At the same time, you may NOT use:

Computer, calculator, cell phone, or similar Copies of daily activities and/or solutions Friends, Romans, countrymen or their ears

To be certain rules are followed, when test ends Hand in all printed material you had with

you

How to Prepare for Midterm

DO DON'T

Make cheat sheets for the test

Review how parts of Java work

Add post-its to important pages

Memorize Drink case of 40s before

test Use post-its as clothing