Exceptions Chapter 16 This chapter explains: What as exception is Why they are useful Java exception facilities

An exception = something has gone wrong – an error has occurred – not a good thing Examples: the use was asked to input a size (numeric) and put in a name (string) The program could: –Quit and return to the operating system –Ignore the input –Display a help message

Another example: an attempt to load a file It cannot be found (possible responses – same as last example) Attempt to print and out of paper –Some printer software examines the status and will indicate out of paper condition

Why do we need error notification? How is it provided? Software and hardware systems in Java are pre-packaged (classes and methods) we don’t care how they work internally – but – vital we are informed of error conditions Software may be set up to detect errors and take alternative action. Some errors handled locally – more serious are passed “upstairs.”

A printer runs out of paper… Normally a technician refills the printer but what if the organization is out – a manager may need to be informed. A technician trips on a cable and breaks a leg (possible legal action) Should be handled by a managing director. Analogy: a person doing a job and a method when a error occurs. Java exception facilities allow us to set up a plan of action. If something wrong – handle the problem else handle the normal situation.

Given: do A(); do B(); do C(); Becomes: – do A(); if ( do A() went wrong) handle the do A() problem – else do B(); – if ( do B() went wrong) handle the do B() problem – else do C(); if ( do C() went wrong) handle the do C() problem – else …

If an error condition occurs (we do not want it to occur) Java exception facilities allow us to code the normal and the exceptional use Java methods can only pass back a single result

Exceptions and objects Boolean variables are not always the answer as to when an error occurs: boolean errorhappened = false; … code whichaffects errorCase – if (errorHappened = = true ) { –… handle problem …

This is not how errors are indicated by Java We use the object approach. We use ‘new’ to create an instance of the appropriate exception class. Another region of the program may check for the errors existance An instance of the exception class is created to indicate “an error happened”

When to use exceptions Exceptions provide a control structure –When should we use them? Rather than an if statement or while loop. Example: we wish to add a series of positive numbers and end the series with a -1. That is not the exception. (it is normal input) We use exception handling for errors not normal input.

The jargon of exceptions – Java’s own terminology throws, throw, try, catch and finally are keywords that carry out the task or error detection. A try – catch example program: ExceptionDemo1 pages “a number doubling program” User inputs an integerr and program doubles it and displays the answer in another field. If the user inputs something ‘not a number’ the ‘parseInt method will not be able to process it. Screen shot page 302 shows ‘good’ integer and doubles it. ‘bad’ integer ‘xx’ blanks the output field and error message

Key part of program try {int number = Integer.parseInt (inputField.setText (Integer.toString( 2 * number)) } catch (Number FormatException errorObject) { JOptionPane.showMessageDialog (null, “Error in number retype” ); }

The ‘new’ statement: general form try { a series of statements } cach (some exception errorObject) { handle the exception;} A Java block { } all statement within curly braces Execute statements in the try block if no error catch block ignored. If error occurs catch block is executed.

catch (NumberFormatException errorObject) similar to a methos declaration. parseInt may throw a NumberFormatException if it does catch block will execute. –Catch (NumberFormatException errorObject) { OPtionPane.showMessageDialog (null, “Error “ + errorObject.toString ( ) );} –// returns the name of the exception

private void aMethod ( ) { – try { // some code …} – catch (Exception errorObject) {handle it} –return

try and scope When a try block produces an exception – execution terminates – variables declared within it become inaccessible (can not be used in catch block) They must be declared outside the try block

String s; try { code involving s…} catch (exception errorObject) { – JOptionPane.showMessageDialog ( null, “Error: s is “ + s);}