Exceptions Jan 14 2019 Errors and Exceptions An error is a bug in your program dividing by zero going outside the bounds of an array trying to use a null reference An exception is a problem whose cause is outside your program trying to open a file that isn t there running out of memory What to do about errors and exceptions An error is a bug in your program An exception is a problem that your program may encounter It should be fixed The source of the problem is outside your program An exception is not the normal case but your program must be prepared to deal with it This is not a formal distinction it isn t always clear whether something should be an error or an exception Dealing with exceptions Most exceptions arise when you are handling files A needed file may be missing You may not have permission to write a file A file may be the wrong type Exceptions may also arise when you use someone else s classes or they use yours You might use a class incorrectly Incorrect use should result in an exception The problem with exceptions Here s what you might like to do But here s what you might have to do open a file read a line from the file open a file if the file doesn t exist inform the user if you don t have permission to use the file inform the user if the file isn t a text file inform the user read a line from the file if you couldn t read a line inform the user etc etc All this error checking really gets in the way of understanding the code Three approaches to error checking 1 Ignore all but the most important errors 2 Do something appropriate for every error 3 The code is cleaner but the program will misbehave when it encounters an unusual error The code is cluttered but the program works better You might still forget some error conditions Do the normal processing in one place handle the errors in another this is the Java way The code is at least reasonably uncluttered Java tries to ensure that you handle every error The try statement Java provides a new control structure the try statement also called the try catch statement to separate normal code from error handling try do the normal code ignoring possible exceptions catch some exception handle the exception catch some other exception handle the exception Exception handling is not optional As in other languages errors usually just cause your program to crash Other languages leave it up to you whether you want to handle exceptions There are a lot of sloppy programs in the world It s normal for human beings to be lazy Java tries to force you to handle exceptions This is sometimes a pain in the neck but the result is almost always a better program Error and Exception are Objects In Java an error doesn t necessarily cause your program to crash When an error occurs Java throws an Error object for you to use You can catch this object to try to recover You can ignore the error the program will crash When an exception occurs Java throws an Exception object for you to use You cannot ignore an Exception you must catch it You get a syntax error if you forget to take care of any possible Exception The exception hierarchy Throwable the superclass of throwable objects Error Usually should not be caught instead the bug that caused it should be fixed Exception A problem that must be caught RuntimeException A special subclass of Exception that does not need to be caught Hence it is the Exceptions that are most important to us since we have to do something about them The Exception hierarchy II Throwable Error Need not be caught Exception Must be caught RuntimeException A few kinds of Exceptions IOException a problem doing input output FileNotFoundException no such file EOFException tried to read past the End Of File NullPointerException tried to use a object that was actually null this is a RuntimeException NumberFormatException tried to convert a nonnumeric String to a number this is a RuntimeException OutOfMemoryError the program has used all available memory this is an Error There are about 200 predefined Exception types What to do about Exceptions You have two choices You can catch the exception and deal with it You can pass the buck and let some other part of the program deal with it For Java s exceptions this is usually the better choice This is often better for exceptions that you create and throw Exceptions should be handled by the part of the program that is best equipped to do the right thing about them What to do about Exceptions II You can catch exceptions with a try statement When you catch an exception you can try to repair the problem or you can just print out information about what happened You can pass the buck by stating that the method in which the exception occurs throws the exception Example void openFile String fileName throws IOException Which of these you do depends on whose responsibility it is to do something about the exception If the method knows what to do it should do it If it should really be up to the user the method caller to decide what to do then pass the buck How to use the try statement Put try around any code that might throw an exception This is a syntax requirement you cannot ignore For each Exception object that might be thrown you must provide a catch phrase catch exception type name You can have as many catch phrases as you need name is a formal parameter that holds the exception object You can send messages to this object and access its fields finally After all the catch phrases you can have an optional finally phrase try catch AnExceptionType e catch AnotherExceptionType e finally Whatever happens in try and catch even if it does a return statement the finally code will be executed If no exception occurs the finally will be executed after the try code In an exception does occur the finally will be executed after the appropriate catch code How the try statement works The code in the try part is executed If there are no problems the catch phrases are skipped If an exception occurs the program jumps immediately to the first catch clause that can handle that exception Whether or not an exception occurred the finally code is executed Ordering the catch phrases A try can be followed by many catches The first one that can catch the exception is the one that will catch the exception Bad catch Exception e catch IOException e This is bad because IOException is a subclass of Exception so any IOException will be handled by the first catch The second catch phrase can never be used Using the exception When you say
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