©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Chapter 8  Errors and Exceptions Throwable class Handling Exceptions Throwing Exceptions Exception propagation

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Exceptions  An exception represents an error condition that can occur during the normal course of program execution.  When an exception occurs, or is thrown, the normal sequence of flow is terminated. The exception-handling routine is then executed; we say the thrown exception is caught. see Ch8Sample1

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Throwable Class  In Java an exception is represented as an instance of the Throwable class or its subclasses.  The Throwable class has two subclasses: Error - the Error class represents serious problems that should not be caught by ordinary applications. Exception - the Exception class represents error conditions that can be caught and handled.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Handling Exceptions  Until now, we've allowed Exceptions to terminate our programs.  Programs are much more reliable and robust if we put in code that allows the program to recover from Exceptions.  One way to do this is to wrap the statements that may throw an exception with the try-catch control statement.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 The try-catch statement  Try-catch is a kind of control statement that allows us to control what happens when an Exception occurs. try catch (Exception e)  We must specify which exception we are catching in the catch block’s parameter list.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Example inputStr = JOptionPane.showInputDialog(null, prompt); try { age = Integer.parseInt(inputStr); } catch (NumberFormatException e){ JOptionPane.showMessageDialog(null, "’" + inputStr +‘ is invalid\n" +"Please enter digits only");

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Execution Sequence  Statements in the try block are executed in sequence.  When one of the statements throws an exception, control is passed to the matching catch block and statements inside the catch block are executed.  The execution then continues to the statement following the try-block statement, ignoring any remaining statements in the try block.  If no statements in the try block throw an exception, the catch block is ignored. Execution continues with the statement following the try-catch statement.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 try-catch Control Flow  try-catch statement with one catch block.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Throwable Methods  There are two methods of the Throwable class we can call to get information about the thrown exception: getMessage - returns a description of the problem printStackTrace - prints the sequence of methods called to get to the line where the error occurred

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Run-Time Stack  The run-time stack is what is used to keep track of the methods that are currently being executed.  When you call a method, the system puts the parameter data and the location the method was called from on the top of a data structure called a stack.  When the method finishes, this data is removed from the stack and the calling method continues at the line the method was called from.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Throwing Exceptions  An exception can be thrown in a program using the throw statement. throw where is an instance of the Throwable class or its subclasses.  This allows you to alter the flow of your program by creating and throwing Exceptions

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Catching Exceptions  A try statement can have multiple catch blocks.  When there are multiple catch blocks in a try- catch statement, they are checked in sequence. It is important to check more specialized exception classes before the more general exception classes.  When an exception is thrown, its matching catch block is executed and the other catch blocks are ignored.  Alternately, you can have a single catch block and use the instanceof operator to determine what type of Exception was thrown.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 try-catch with multiple catch blocks

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Which catch Block?  If a catch block matches the Exception, it will be executed.  If none of the catch blocks matches the thrown exception, the system will search down the stack trace for a method with a matching catch block.  If no matching catch block is found, the system will handle the thrown exception.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 The finally Clause  If there is a block of code that must be executed regardless of whether an exception is thrown, we use the reserved word finally.  The finally block is executed even if there is a return statement inside the try block. When the return statement is encountered in the try block, statements in the finally block are executed before actually returning from the method.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 try-catch with finally

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Propagating Exceptions  It doesn't always make sense to handle an exception in the method where it occurs. Different users of a class may want to handle the same Exception differently. Do not catch an exception that is thrown as a result of violating a condition set by the client programmer.  Instead, propagate the exception back to the client programmer’s code and let him or her handle it.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Propagating Exceptions  When a method may throw an exception, either directly or indirectly, we call the method an exception thrower.  Every exception thrower must be one of two types: An exception catcher is an exception thrower that includes a matching catch block for the thrown exception. An exception propagator does not contain a matching catch block.  A method may be a catcher of one exception and a propagator of another.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Exception Propagation public A() { try { B() } catch (Exception e) { … } } public B() { try { C() } catch (Exception e) { … } } public C() { D();} public D() { if (cond)throw new Exception(); }

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Types of Exceptions  There are two types of exceptions: Checked. Unchecked.  A checked exception is an exception that is checked at compile time.  All other exceptions are unchecked exceptions, Also known as runtime exceptions.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 throws Clause  For a propagator of checked exceptions, we need to modify its header to declare the type of exceptions the method propagates. We use the reserved word throws for this declaration. void C( ) throws Exception {... } Without the required throws Exception clause, the program will not compile.  For runtime exceptions, the throws clause is optional.

©The McGraw-Hill Companies,Inc. Permission required for reproduction or display. COMPSCI 125 Spring 2005 Programmer-Defined Exceptions  Instead of using generic exception classes, we can define our own exception classes and attach useful information to the exception objects.  When creating a new customized exception class, define it as a checked exception to ensure client programmers handle thrown exceptions of this class in their code.