Introduction to Exceptions in Java CS201, SW Development Methods

Failures at run-time Divide by zero? Try to access an object reference that’s null? –Your program halts –It prints the location of where it failed You might think this is “built-in” as part of how Java runs a program But Java gives you the same ability to recognize error or situations and respond however you want to

When Things Go Wrong Java uses exceptions. When something “goes wrong” at a line of code: –An exception object gets created –Flow of control changes to some place in your code that can handle the exception Note: usually changes in flow-of-control are clearly marked by Java keywords –return, if/else, while, for, switch, call to a method, break, continue –Exceptions and a “jump” to somewhere else can occur where it’s not so obvious

What We Need to Know We need to understand –What an exception object is and what we might do with it. –How to manage the flow-of-control when an exception occurs Can we ignore exceptions? Some of them: –Unchecked exceptions: serious system problems, the program fails, you normally don’t try to handle these –Checked exceptions: Java requires you to do something with these. Either: Handle them when they occur Declare that you are passing responsibility back up to the method that called your code

Exception class hierarchy Java developers create a new exception class for each “type” of exception that can occur. A few examples: –NullPointerException: attempt to access an object reference that is null –IOException: some sort of I/O problem has occurred –FileNotFoundException: failed to open a file Each one its own class, all derived from the Exception superclass –You can easily create your own exceptions by extending Exception (but we won’t today)

Serious problems you shouldn’t try to worry about! Superclass of all Errors and Exceptions Problems we must catch We could catch this or any of its subclasses

How to Use an Exception Object Something bad has happened, and you have access to the exception object. What methods can you call on it? –String getMessage() Each exception encapsulates a text-message that says what the problem is –void printStackTrace() You’ve see this! It’s a list of the methods called that led to this exception, with the class and line numbers.

Control Flow and Exceptions Java lets you surround code that might generate an exception… –Called a “try block” And attach handlers that will take care of problems if they occur in this block –Called “catch blocks” Java also lets you do something else: –Declare that your method will pass any exception back up to it’s caller –We’ll explain later

try/catch/finally try { // lines of code that // might throw exceptions } catch ( ExceptionType1 e) { // handle 1 st type of exceptions } catch ( ExceptionType2 e) { // handle 2 nd type of exceptions } finally { // code executed in all cases } If some code in here throws an exception, then an exception object is created and we jump to the first catch block. Can have 1 or more catch- blocks. The first one where the thrown exception object matches the “parameter type” is executed. If we need to make sure something is always done after try-block and/or catch- block, it goes here

Comments on try/catch/finally try block –Any variable declared here is local to the try-block –Often, declare and initialize outside the try-block, then change it inside the block. catch blocks –Often there’s just one kind of exception that can happen. –If more than one possible Often we test for specific types of exceptions first Then have catch (Exception e) which will capture any other kind of exception since they all match the super-class type

Comments on try/catch/finally finally block –Always executed! whether or not an exception is thrown even if the try or catch blocks execute a return –Useful for closing files, freeing resources like database connections, etc.

What to Do in a Catch Block? You can do anything to try to recover from the problem –Ask for new input, use a different value, whatever makes sense –Print more useful information Remember getMessage() and printStackTrackTrace() can be called on the exception object –(To be explained later) You can throw the same exception which passes it back to the caller

Advice on Writing Exception Handling Code Empty catch blocks are used to ignore exceptions –Don’t do this unless it makes sense for your code! Keep it simple: wrap as few lines as possible in a try-block Recognize you can’t always safely recover –Sometimes best to “die gracefully”