COP 2800 Lake Sumter State College Mark Wilson, Instructor

Exceptions

 Methods with unknown side effects Versioning Latent bug  Methods that may not work at runtime Unavailable system resources Abnormal or unexpected response

 An event that occurs during the execution of a program that disrupts the normal flow of instructions  Method Java uses to handle errors and other exceptional events  An exception object, contains information about the error, including its type and the state of the program when the error occurred  Creating an exception object and handing it to the runtime system is called throwing an exception.

 Traditional processing breaks well formed constructs, makes spaghetti code  The runtime system searches the call stack for a method that contains a block of code that can handle the exception (exception handler)  An exception handler is considered appropriate if the type of the exception object thrown matches the type that can be handled by the handler  Uncaught exceptions cause the runtime system (and, therefore, the program) to terminate

 Separates Error-Handling Code from "Regular" Code In traditional programming, error detection, reporting, and handling often lead to confusing spaghetti code  Propagating Errors Up the Call Stack Java runtime environment searches backward through the call stack to find any methods that are interested in handling a particular exception  Grouping and Differentiating Error Types grouping or categorizing of exceptions is a natural outcome of the class hierarchy

 Checked Exception – Exception class well-written application anticipates and recovers  Error – Error class external to the application application usually cannot anticipate or recover  Runtime Exception – RuntimeException class internal to the application application usually cannot anticipate or recover  Error and runtime exceptions are unchecked exceptions

 throws Method declares it may throw an exception  try Sets off the code block for risky behavior  catch Sets off code to handle an exception Follows try Multiple catches possible  finally Always executes after the try block May not execute if the JVM exits during a try or catch Good place to put cleanup code (e.g. close open files)

public void writeList() { PrintWriter out = null; try { System.out.println("Entering" + " try statement"); out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i < SIZE; i++) out.println("Value at: " + i + " = " + vector.elementAt(i)); } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Caught ArrayIndexOutOfBoundsException: " + e.getMessage()); } catch (IOException e) { System.err.println("Caught IOException: " + e.getMessage()); } finally { if (out != null) { System.out.println("Closing PrintWriter"); out.close(); } else { System.out.println("PrintWriter not open"); }

 Methods that don’t catch checked exceptions must specify that they can throw exceptions  Identifies exceptions that may need to be caught  Unchecked exceptions are optional public void writeList() throws IOException, ArrayIndexOutOfBoundsException { public void writeList() throws IOException {

 Any code can throw exceptions  Exceptions are thrown with throw  All exceptions are descendants of Throwable  Most programs throw and catch objects derived from the Exception class public Object pop() { Object obj; if (size == 0) { throw new EmptyStackException(); } obj = objectAt(size - 1); setObjectAt(size - 1, null); size--; return obj; }

 Exception type isn't represented by those in the Java platform?  Help users differentiate your exceptions from other vendors?  Does your code throw more than one related exception?  Will users have access to other’s exceptions?  Most applications will throw objects that are Exceptions. Errors are normally used for serious, hard errors in the system  For readable code, it's good practice to append the string Exception to the names of all classes that inherit (directly or indirectly) from the Exception class.

 A program can use exceptions to indicate that an error occurred.  To throw an exception, use the throw statement and provide it with an exception object — a descendant of Throwable — to provide information about the specific error that occurred.  A method that throws an uncaught, checked exception must include a throws clause in its declaration.  A program can catch exceptions by using a combination of the try, catch, and finally blocks.  The try block identifies a block of code in which an exception can occur.  The catch block identifies a block of code, known as an exception handler  The finally block identifies a block of code that is guaranteed to execute,  The try statement should contain at least one catch block or a finally block and may have multiple catch blocks.  The class of the exception object indicates the type of exception thrown.  The exception object can contain further information about the error  With exception chaining, an exception can point to the exception that caused it

Packages, Jars and Deployment. Oh, my!

Beginning GUI