EXCEPTION HANDLING. Summer 2018

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1 EXCEPTION HANDLING Summer 2018

2 EXCEPTIONS An exception is an object that represents an error or exceptional event that has occurred. These events are usually errors that occur because the run-time environment has detected an operation that is impossible to carry out. Exception objects are all children of the Throwable class. Exceptions represent normal error events that can occur in your program. Examples: array index out of bounds - IndexOutOfBoundsException trying to open a file that does not exist - FileNotFoundException trying to call a method that does not exist NoSuchMethodException Exceptions usually come in two types: Normal exceptions (checked exceptions) Good programs watch for these, you have to handle them (either catch them or declare that your method can throw them) Runtime Exceptions (unchecked exceptions) You do not need to handle these There is a class of exceptions called errors that are not usually recoverable (example VirtualMachineError) these do not need to be handled

3 COMMON BUILT-IN EXCEPTION TYPES ClassNotFoundException If you attempt to use a nonexistent class CloneNotSupportedException Raised on an attempt to call clone() for an object that doesn t implement the Cloneable interface RunTimeException Various types of programming errors that can cause the program to abort ArithmeticException NullPointerException IndexOutOfBoundsException IOException Input/Output problems. Several subtypes AWTException Graphics programming Errors You can also build your own exception types, which should be derived from class Exception or one of its subclasses.

4 WHY HAVE EXCEPTIONS? Exceptions allow more robust programs because they allow the programmer to recover from an exceptional event Java was originally designed for embedded systems (TVs, phones, watches, etc.) which should never stop working. Exceptions are needed in those systems. If you run this example, it can crash with an exception because of a division by zero However, this revised version of the example handles the exception instead Usually, if an exception is not handled, it can cause the program to exit unnaturally and prematurely

5 HOW TO HANDLE EXCEPTIONS The process involves: Claiming exceptions specifying which exceptions might occur in each method Throwing an exception When an error occurs that fits an exception situation, an exception object is created and thrown Catching an exception Exception handlers are blocks of code created to handle the different exception types. The appropriate handler catches a thrown exception and performs the code in the handler code block

6 CLAIMING EXCEPTIONS In a method, to claim an exception, use the keyword throws and list the exceptions that may occur in the method This tells the compiler that this method might throw this exception without catching it (there is no need to claim an exception if you catch every possible instance of it) Examples: public void mymethod() throws IOException public void yourmethod() throws IOException, AWTException, whateverexception

7 THROWING EXCEPTIONS Use the keyword throw, along with the exception being thrown An exception is an object, so it must be created with the new operator Be careful not to confuse with the keyword throws used in claiming exceptions Examples: throw new BadHairDayException(); MyException m = new MyException(); throw m if (persononphone!= friend) throw new Exception( Stranger on the phone!! );

8 CATCHING EXCEPTIONS Any group of statements that can throw a checked exception or a group of statements that you want to watch for Runtime or Error exceptions must be within a try block. At the end of the try block there must usually be either a catch or finally block A catch block has a parameter that is the type of exception this catch block will handle. There can be several catch blocks for a single try block If an exception is thrown, the first catch block that matches the exception s type will be the one that catches the exceptions A finally block is ALWAYS executed, no matter how control leaves a try block. This happens even if a return statement is executed in the try block and even if control passes to a catch block

9 CATCHING EXCEPTIONS, EXAMPLE try { //lots of code reading to/from files } catch (FileNotFoundexception) { //tell the user and probably repeat try block } catch (EOFException) { //hit end of file early } catch (IOException) { //handle all other IO problems } finally { //Close any open files }

10 TRY WITH RESOURCES Beginning in Java 7, Java allows try blocks that do not end in a catch or finally in certain situations This is allowed when a try statement declares one or more resources A resource is an object that must be closed after the program is finished with it (such as a FileReader) Anything that implements java.lang.autocloseable or java.io.closeable may be used as a resource After the try block ends (either because of an exception or because execution reaches the end of the block), all resources that were created inside of the try block run their close methods in the opposite order of creation

11 UNCAUGHT EXCEPTIONS If your method does not catch a checked exception and does not declare that your method can throw it, then the compiler will complain If your method throws an exception, then the method that called your method must handle the exception or declare that it can throw that exception If no method handles the exception, then the program crashes and a message is printed out describing the exception The same happens if an unchecked exception should occur The only difference between the two is that checked exceptions must be handled

12 RETHROWING EXCEPTIONS Writing code to handle exceptions is tedious and you often have no idea what to do for error recovery. Sometimes, it s easier to just rethrow the checked exception as an unchecked exception Simple example code that rethrows exceptions: catch (Exception e) { throw new RuntimeException(e); }

13 WHEN TO USE EXCEPTIONS Exceptions are not always appropriate Exceptions are good for situations in which the error doesn t need to be handled in the same block where it occurred Conventional error-checking is better for simple tests Validating user input often falls into this category, it is best to test user input with simple if-statements and loops Note that this is good for validating user input values (e.g. ensuring positive numbers), but exceptions are better for validating user input types (e.g. user types in c when expecting a double) Exceptions are good for handling errors that would otherwise result in the termination of the program

14 INSTANCE METHODS IN EXCEPTION OBJECTS Since exception objects are created from classes, they can have instance methods Here are some instance methods that all exception objects have (they are inherited from Throwable) public String getmessage() returns a detailed message about the exception public String tostring() returns a short message describing the exception public String getlocalizedmessage() often overridden to give a localized version of getmessage (in a different language, for example) public void printstacktrace() prints stack trace of the exception

15 CONDITIONS When debugging, it is often helpful to state certain conditions that should be met at specific points in the code Preconditions are commonly used to indicate what should be true when a method is called Postconditions are commonly used to indicate what should be true when a method returns

16 ASSERT The assert statement can be used to easily test whether these conditions are true while debugging. The two formats for the assert statement are: assert expression; assert expression : message; If the expression is false, an AssertionError is thrown. With the second format, message is the error message attached to the exception Because assert is meant for debugging, you have to explicitly enable them when executing the program if you wish to turn them on (this prevents them from affecting performance for normal users) using the ea command-line option java ea Class

17 CODE EXAMPLES Deitel code examples (Chapter 11) Some other code examples (From Bob myers)

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