CST242 Exception CST242 Handling Page 1

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1 CST242 Exception CST242 Handling Page Exceptions Exceptional events that occur during run-time that disrupt the program s normal flow Including but not limited to: Handling An array Exceptions index out-of-bounds CST242 Arithmetic overflow Division by zero for integers Exceptions Trying to parse a string with a invalid numeric format to a numeric value Exceptional events that occur during run-time that disrupt the program s normal flow Exceptions Including but not limited to: Exceptions An array may index come out-of-bounds from abnormal events generated during run-time, or They Arithmetic also may overflow be generated manually by programmers to handle events that could result in an invalid operation taking place Division by zero for integers Exceptions1.java Trying to parse a string with a invalid numeric format to a numeric value Exceptions Using if Statements to Catch Potential Run-time Exceptions Exceptions In previous may example, come a from run-time abnormal exception events may generated be generated during if run-time, a user enters: or They also value may zero be (0) generated for denominator manually(computers by programmers cannot to divide handle integers events by that zero) could result Non-integer an invalid values operation for either taking of the place ints in the showinputdialog Using Disadvantage if Statements of if processing: to Catch Potential Run-time Exceptions In Validation previous example, checking a must run-time take exception place for both may valid be generated and invalid if a values user enters: The Easy value to miss zero some (0) for errors denominator (computers cannot divide integers by zero) Exceptions1a.java Non-integer values for either of the ints in the showinputdialog Disadvantage of if processing: Exceptions1a.java Validation checking must take place for both valid and invalid values Exceptions1a.java Easy to miss some errors 3) Exception Processing Deal with abnormal events occurring as a result of some process during program execution Makes sense to use exception processing when the alternative is that program will: Crash, or Place the application into an inconsistent state Used in large systems to handle abnormal events in a standardized manner Exception Processing Generation of an non-normal event is called throwing an exception Occurs when a method detects an event during run-time with which it cannot deal Checks the type of exception to see if its parameters match one of a set of exception handling procedures Keywords try and catch A block of code in which statements with the potential to throw an exception are placed within a try block In the same method, the try block is followed immediately by one or more catch blocks: Each catch block is an exception handling routine (procedure) Specifies the type of exception that it can handle each exception type is a Class name Keywords try and catch try code that may throw an exception catch (ExceptionType parametervariable)

2 try number1 = Integer.parseInt(string; number2 = Integer.parseInt(string; quotient = number1 / number2; catch code (NumberFormatException that may throw an exception ex) catch System.out.print("Invalid (ExceptionType parametervariable) input"); catch exception (ArithmeticException handling codeex) catch System.out.print("Divide (ExceptionType parametervariable) by zero"); another exception handling code Keywords try and catch 4) If an exception occurs, the program: parametervariable is object that holds exception Abandons the try block Keywords Attempts try to and find catch a catch block 4) that matches the exception type If an exception occurs, type matches the program: the exception, its catch block is executed Abandons the try block Keywords try and catch 5) Attempts to find a catch block that matches the exception type If the exception fails to match the type of any catch block, none of the catch blocks If an exception type matches the exception, its catch block is executed are executed: Keywords Application try and may catch terminate ( crash 5) or hang ) If the Or the exception application fails could to match be placed the type into of any inconsistent catch block, state none (i.e. of the if an catch arithmetic blocks are overflow executed: occurred, an invalid result may be stored) Keywords Application try and may catch terminate ( crash 6) or hang ) Or the application could be placed into an inconsistent state (i.e. if an arithmetic If no exceptions are thrown during execution of the try block, the try block completes overflow occurred, an invalid result may be stored) and the catch blocks are ignored Keywords Program execution try and catch continues with 6) any statements that follow the last catch block If no True exceptions either way are whether thrown any during exception execution was of thrown the try or block, not the try block completes and the catch blocks are ignored The tostring() Method Program execution continues with any statements that follow the last catch block Method of the exception object that returns a String representation of the error True either way whether any exception was thrown or not message The tostring() Method Method exceptionobject.tostring() of the object that returns a String representation of the error Examples: message JOptionPane.showMessageDialog(null, ex.tostring() ); exceptionobject.tostring() JOptionPane.showMessageDialog(null, ex); Examples: Exceptions2.java JOptionPane.showMessageDialog(null, ex.tostring() ); Exceptions2.java JOptionPane.showMessageDialog(null, ex); CST242 Exception Handling Page RuntimeExceptions Exceptions generated by the occurrence of a run-time exception which are built into the Java VM (Virtual Machine) The super class for all run-time exceptions The RuntimeException class automatically throws an exception so programmer does not have to write if logic to check for it RuntimeExceptions For example, NumberFormatException is a RuntimeException thrown whenever Integer.parseInt fails to return an int This is true for all parse method types of all the wrapper classes Effectively a RuntimeException is thrown is a RuntimeException every a NumberFormatException thrown whenever is Integer.parseInt thrown (as is any fails exception to return class) an int

3 CST242 Exception For Handling example, NumberFormatException is a RuntimeException thrown whenever Page RuntimeExceptions Integer.parseInt fails to return an int This is true for all parse method types of all the wrapper classes Effectively a RuntimeException is thrown is a RuntimeException every a NumberFormatException thrown whenever is Integer.parseInt thrown (as is any fails exception to return class) an int For example, NumberFormatException is a RuntimeException thrown whenever RuntimeExceptions This is true for all parse method types of all the wrapper classes Integer.parseInt fails to return an int Effectively a RuntimeException is thrown every time a NumberFormatException is RuntimeExceptions This is true for all parse method types of all the wrapper classes thrown (as is any exception class) 3) Effectively a RuntimeException is thrown every time a NumberFormatException is RuntimeExceptions Wouldn t it be simpler just to specify the RuntimeException? thrown (as is any exception class) Implementing every potential run-time exception lets the programmer provide specific RuntimeExceptions feedback to the user 3) Wouldn t Exceptions2a.java it be simpler just to specify the RuntimeException? Implementing every potential run-time exception lets the programmer provide specific Exceptions2a.java feedback to the user Exceptions2a.java Sequencing of catch Blocks Exceptions2a.java Every catch block must be reachable Superclass exception catch blocks must follow their respective subclass exception catch Sequencing blocks of catch Blocks Every Failure catch to adhere block to must this be principle reachable will result in compile errors Superclass Sequencing exception of catch catch Blocks blocks must follow their respective subclass exception catch blocks Example of invalid sequencing: Failure to adhere to this principle will result in compile errors catch (RuntimeException ex) Sequencing of catch Blocks Example System.out.print("Invalid of invalid sequencing: input"); catch (RuntimeException ex) catch (NumberFormatException ex) System.out.print("Invalid input"); System.out.print("Invalid input"); catch (NumberFormatException ex) catch (ArithmeticException ex) System.out.print("Invalid input"); System.out.print("Divide by zero"); catch (ArithmeticException ex) RuntimeException Classes System.out.print("Divide by zero"); RuntimeException Generated when any run-time exception occurs RuntimeException The superclass of Classes all run-time exception classes RuntimeException NullPointerException Generated Attempting when to reference any run-time an object exception that does occurs not exist (declared but has not been The instantiated) superclass of all run-time exception classes NullPointerException RuntimeException Classes Attempting to reference an object that does not exist (declared but has not been ArithmeticException instantiated) Division by zero (0) for integers only and some other arithmetic exceptions RuntimeException NumberFormatException Classes ArithmeticException Attempt to parse a non-numeric string to a numeric value Division Or attempt by zero to parse (0) for a string integers with only digits and and some a decimal other arithmetic to an integer exceptions type NumberFormatException RuntimeException Classes 3) Attempt to parse a non-numeric string to a numeric value ArrayIndexOutOfBoundsException Or attempt to parse a string with digits and a decimal to an integer type Array index is outside the allowable range RuntimeException NegativeArraySizeException Classes 3) ArrayIndexOutOfBoundsException Declaring an array with a negative integer size Array index is outside the allowable range Mini-Quiz NegativeArraySizeException Consider the application Exceptions3.java, rewrite the program to catch array indexes

4 Include window a final RuntimeException to handle any other unforseen unchecked exceptions CST242 Exception The Handling array index out of range exception class is ArrayIndexOutOfBoundsException Page Exceptions3.java Also catch a NumberFormatException if the user does not enter an integer Include Exceptions3.java a final RuntimeException (solution) to handle any other unforseen unchecked exceptions Exceptions3.java (solution) Array index is outside the allowable range Exceptions3.java The Keyword finally (solution) NegativeArraySizeException Exceptions3.java Specifies a block that Declaring an array (solution) will be executed with a negative integer after the try catch blocks have been size evaluated The Guaranteed Keyword to finally be executed: Specifies Whether a or block not that an exception will be executed is thrown after the try catch blocks have been evaluated No matter which catch block is executed Guaranteed Whether or to not be executed: one of the catch blocks executes after an exception is thrown Whether or not an exception is thrown The Keyword finally No matter which catch block is executed Usually designed to release resources that may have been assigned (but not released Whether or not one of the catch blocks executes after an exception is thrown if an exception occurred) during the try block The E.g. Keyword files, memory, finally etc. Usually If application designed does to not release catch resources the exception, that may the have finally been block assigned still will execute (but not before released the if program an exception crashes occurred) during the try block The E.g. Keyword files, memory, finally etc. 3) If application does not catch the exception, the finally block still will execute before the program crashes try The Keyword statements finally 3) catch (OneException objectname) try statements catch statements (TwoException objectname) catch statements (OneException objectname) finally statements catch (TwoException objectname) statements; finally Exceptions4.java statements; Exceptions4.java Exceptions4.java Programmer-Defined Exceptions Exceptions4.java Exceptions that are thrown manually by the programmer These exceptions may be specified in the throws clause of the method s header (not Programmer-Defined required) Exceptions Exceptions Programmer-Defined that are thrown Exceptions manually are by handled the programmer by unique classes coded by the These programmer exceptions may be specified in the throws clause of the method s header (not Programmer-Defined required) Exceptions Programmer-Defined Exceptions are handled by unique classes coded by the programmer public type methodname( [parameterlist] ) [throws ExceptionType1,ExceptionType2, Programmer-Defined ] Exceptions ExceptionTypes are programmer-defined classes public type methodname( [parameterlist] ) [throws ExceptionType1,ExceptionType2, public ] void setquotient() throws DivideByZeroException ExceptionTypes In this example DivideByZeroException are programmer-defined is classes the name of the programmer-defined exception class The public Keyword void setquotient() throw throws DivideByZeroException In this example DivideByZeroException is the name of the programmer-defined Manually throws an exception from the called method back to the location of the exception class method call in the try block The Used Keyword in methods throw of programmer-defined exception classes that throw exceptions Required Manually throws if an exception an exception will be from thrown the called in a called method method back to but the the location try catch of the logic is

5 46 The Keyword throw Exceptions3.java Functionality is similar to a return statement (terminates processing of the method Exception method and passes call Classes in new the try exception block object back to calling method) Used in methods of programmer-defined exception classes that throw exceptions Exceptions5.java Written by a programmer to extend some Java API exception type Required They typically if an have exception two constructors will be thrown (which in a called is similar method to Java but API the exception try catch classes): logic is Exceptions5.java located Manually One that in the throws takes calling an no exception arguments method from and the specifies called a method hard-coded back to default the location exception of the message The Exceptions3.java method One Keyword that call takes in throw the a try string block argument usually a more specific exception message Used in methods of programmer-defined exception classes that throw exceptions Exception DivideByZeroException.java Classes Required throw new if an ExceptionType([argumentList]); exception will be thrown in a called method but the try catch logic is Written DivideByZeroException.java located in by the a programmer calling method to extend some Java API exception type They The if (denominator typically have Keyword throw == two 0) constructors (which is similar to Java API exception classes): Exceptions6.java One that takes no arguments and specifies a hard-coded default exception message Exceptions6.java One throw throw that new new takes ExceptionType([argumentList]); DivideByZeroException(); a string argument usually a more specific exception message DivideByZeroException.java Quotient.java if Functionality (denominator is == similar 0) to a return statement (terminates processing of the method DivideByZeroException.java Quotient.java and passes new exception object back to calling method) Exceptions5.java Exceptions6.java Quotient.java throw new DivideByZeroException(); 3) Exceptions5.java Exceptions6.java Quotient.java 4) Functionality is similar to a return statement (terminates processing of the method Exceptions3.java Quotient.java and passes new exception object back 5) to calling method) Exception Quotient.java Exceptions Classes and GUI Components Written Quotient.java Exception by handling a programmer is especially to extend useful some 3) when Java there API will exception be user type input They In the typically case of have Java GUI two constructors interfaces, the (which window is similar still may to be Java executing API exception ( hanging ) classes): even Quotient.java after an unhandled exception 4) One that takes no arguments and specifies a hard-coded default exception message Quotient.java The program does not just simply One that takes a string argument usually crash 5) a more specific exception message DivideByZeroException.java Exceptions and GUI Components DivideByZeroException.java Exception handling is especially useful when there will be user input In the case of Java GUI interfaces, the 3) window still may be executing ( hanging ) even Exceptions6.java after an unhandled exception Exceptions6.java The program does not just simply crash 4) The getmessage() Method Quotient.java Method of the exception object that returns a descriptive String message stored in an Quotient.java exception object reference 3) Quotient.java Either a default or programmer custom 3) message 4) Quotient.java exceptionobject.getmessage() 4) The getmessage() Method Quotient.java 5) Method JOptionPane.showMessageDialog(null, of the exception object that returns ex.getmessage() a descriptive ); String message stored in an Exceptions exception object and GUI reference Components The printstacktrace() Method Exception Either a handling default or is programmer especially useful custom when message there will be user input In Displays the case the of following: Java GUI interfaces, the window still may be executing ( hanging ) even after exceptionobject.getmessage() The an exception unhandled type The program exact statement does not in just the simply execution crash of the Java class that threw the exception JOptionPane.showMessageDialog(null, If more than one method was involved, ex.getmessage() the sequence of ); method calls leading to the exception (in reverse order of the calls) The Outputs printstacktrace() to the standard Method error stream Displays Usually the the following: command line or console window The exception type 3) The printstacktrace() Method The exact statement in the execution 4) of the Java class that threw the exception If more than one method was involved, the sequence of method calls leading to the The exceptionobjectname.printstacktrace(); exception getmessage() (in reverse Method order of the calls) Method Outputs of to the exception standard error object stream that returns a descriptive String message stored in an exception ex.printstacktrace(); Usually the object command reference line or console window Either Not a returned a default or String programmer that can be custom displayed message (the statement stands alone) The printstacktrace() Method Exceptions8.java CST242 Exception Manually Handling throws an exception from the called method back to the location of the Page

6 CST242 Exception Handling Page 6 exceptionobjectname.printstacktrace(); ex.printstacktrace(); Not a returned String that can be displayed (the statement stands alone)