Exceptions Don’t Frustrate Your User – Handle Errors

Exceptions Don’t Frustrate Your User – Handle Errors KR – CS 1401 Spring 2005 Picture – sysprog.net

Exceptions Writing Reliable Code • We want to write applications that are reliable. • Reliability depends upon correctness and robustness: • Correctness - the program produces correct results for valid input. • Robustness – the program can handle error conditions without crashing too easily. • Robustness can be increased by planning for and handling unusual circumstances, called exceptions. KR – CS 1401 Spring 2005

Exceptions Possible sources of Error • An exception is an unusual error condition that occurs while the program is running. • What are some sources of these errors ? • System or network-related I/O problems: - A file may become corrupted or removed. - Network congestion or an overloaded server may prevent access to a file. - The server may be down. • User input error, such as entering a string that cannot be converted into a number as expected by application. KR – CS 1401 Spring 2005

Exceptions • When an exception occurs, the system is alerted and an exception is said to be thrown. • When an exception is thrown, the normal sequence of code execution is halted, and the exception is caught by appropriate exception-handling routines. KR – CS 1401 Spring 2005

ExceptionsClasses of Exception Objects • Exceptions in Java are defined as objects of various exception classes. • Exception classes form an inheritance hierarchy. • All exceptions are subclasses of the class Throwable. • You may throw any of the predefined exceptions to handle an error condition, or you may define and throw your own exception subclass to handle errors in a specific way appropriate for your application. KR – CS 1401 Spring 2005

ExceptionsInheritance Diagram of Exception Classes KR – CS 1401 Spring 2005 Diagram – Sun Microsystems

ExceptionsHow is an exception handled? • If an exception is thrown within a method, the JVM looks for code within the local method to catch (handle) it. • If there is none, the method returns and the JVM looks in the calling method for code to catch the exception. • This can cause a chain of returns all the way back to the JVM. At that point, the program terminates and a stacktrace is displayed. • Up to now we have thrown exceptions to the JVM to catch: public static void main (String [] args) throws IOException {…} KR – CS 1401 Spring 2005

ExceptionsException handling KR – CS 1401 Spring 2005 Diagrams – Sun Microsystems

Exceptions Writing our own code • We can write our own code to throw and catch exceptions. • The structure to use is a try-catch-finally statement: • Block of code that may result in an error and thrown exception (try). • One or more blocks of code to handle exception(s) thrown (catch). • Optional block of code performing “clean-up” or other statements to be executed regardless of whether there is an error or not (finally). Ex: closing a file. KR – CS 1401 Spring 2005

ExceptionsSyntax of try-catch-finally statement try { statement statement … } catch ( ExceptionClassexceptionObject ) { statement statement … } finally { statement statement … } KR – CS 1401 Spring 2005

ExceptionsExample: One Catch Block int number; try { String userInput = inFile.readLine( ); number = Integer.parseInt ( userInput ); } catch ( NumberFormatException excep ) { System.out.println ( “Cannot convert input. Please enter integer.” ); } finally { System.out.println ( “End of try-catch-finally statement.” ); } KR – CS 1401 Spring 2005

ExceptionsMultiple Catch Blocks May Be Defined try{ statement statement … } catch ( ExceptionClassexceptionObject ) { statement statement … } catch ( ExceptionClassexceptionObject ) { statement statement … } finally { … } KR – CS 1401 Spring 2005

Exceptions Example: Multiple Catch Blocks int number; try{ String userInput = inFile.readLine( ); number = Integer.parseInt ( userInput ); } catch ( IOException excep ) { System.out.println ( “Problem reading file.” ); } catch ( NumberFormatException excep ) { System.out.println ( “Cannot convert input. Please enter integer.” ); } finally { … } KR – CS 1401 Spring 2005

Exceptions Throwing Our Own Exceptions int number; // Ask user to enter a number > = 100 try{ String userInput = inFile.readLine( ); number = Integer.parseInt ( userInput ); if ( number < 100 ) { throw new Exception (userInput + “must be > = 100” ); } } catch ( NumberFormatException excep ) { System.out.println ( “Cannot convert input. Please enter integer.” ); } catch ( Exception excep ) { System.out.println ( “Error: ” + excep.getMessage ( ) ); } finally { … } KR – CS 1401 Spring 2005

ExceptionsOrder of Multiple Catch Blocks • List multiple catch blocks in the order of more specialized exception classes to more general exceptions: catch ( NumberFormatException excep ) { System.out.println ( “Cannot convert input. Please enter integer.” ); } catch ( Exception excep ) { System.out.println ( “Error: ” + excep.getMessage ( ) ); } • Only one catch block will be executed; the rest will be skipped. KR – CS 1401 Spring 2005

Exceptions Defining Our Own Exception Subclass int number; // Ask user to enter a number > = 100 try{ String userInput = inFile.readLine( ); number = Integer.parseInt ( userInput ); if ( number < 100 ) { throw new ourInputException(userInput + “must be > = 100” ); } } catch ( NumberFormatException excep ) { System.out.println ( “Cannot convert input. Please enter integer.” ); } catch ( ourInputException excep ) { System.out.println ( “Error: ” + excep.getMessage ( ) ); } finally { … } KR – CS 1401 Spring 2005

Exceptions What is ourInputException ? • It is the subclass of Exception that we defined: public class ourInputException extends Exception { public ourInputException ( ) { // Call constructor of Exception Class with no argument super ( ) ; } public ourInputException ( String message ) { // Call constructor of Exception Class and pass message that catch // clause can use for error handling. super ( message ); } KR – CS 1401 Spring 2005

Exceptions Don’t Frustrate Your User – Handle Errors