1 Why do we need exceptions? In C, return variables must be used to indicate errors: if((fd = fopen(path,...)) == -1){ if(errno==a){...} else if(errno==b){...} // other error code } // do something

2 Why do we need exceptions? What are the problems with this? –This complicates the code by adding extra clutter to check the value of the return arguments. –If you have a function f() that calls g() that calls h(), and h() needs to return an error, both g() and f() have to contain special code to handle the exception

3 Why do we need exceptions? How does the code change when we add exceptions? try{ fd = new FileReader(path,...); } catch(IOException e){ // error code } catch(FileNotFoundException e){ // error code } // do something

4 When should exceptions be used? Exceptions provide a way to signal errors without having to use return variables They should generally be used when an unexpected error occurs For example, Java uses an exception to signal when a file you are trying to open could not be found. However, when searching for an item in an array, Java returns a -1 if it cannot find the item.

5 Checked Exceptions You can either use one of the existing exceptions, or create your own Most exceptions extend Exception, but a function can throw any class that extends Throwable Such exceptions are known as checked exceptions –Functions that throw a checked exception must advertise this by using the throws keyword –Every function that calls a function that can throw an exception must either use a try…catch block or advertise that it throws the Exception with a throws keyword

6 Unchecked Exceptions Unchecked exceptions do not need to be caught by your program. There are two types: –Exceptions that extend Error are serious virtual machine errors, like OutOfMemoryError –Exceptions that extend RuntimeException can be caught, but do not need to be. These are problems like NullPointerException and IndexOutOfBoundsException

7 Using Exceptions In order to signal an error has occurred, your code must throw an exception: throw new MyException(“Error Message”) Also, all functions that throw exceptions must specifically state that they throw exceptions. A function that throws MyException and MyOtherException would be defined as follows: int myFunction() throws MyException, MyOtherException

8 Using Exceptions A function that calls a function that can throw an exception must deal with it in one of two ways: –Advertise that it throws the exception –Use a try…catch block

9 Using Exceptions try{ fd = new FileReader(path,...); } catch(IOException e){ // error code } catch(Exception e){ // error code }

10 Exceptions and Inheritance When creating a subclass, do not change the exceptions thrown by a method you override, unless you are changing them to a subclass of the previous exception. However, you can remove one or more of the exceptions from the throws keyword. –If a method throws MyException and MyOtherException, a method that overrides it can throw only MyException

11 Finally… When working with resources such as files or network connections, special handling is required for exceptions. Resources often need to be properly closed to ensure that normal behavior continues, regardless of whether an exception will be thrown or not.

12 Why Finally? import java.io.*; class ProcessFile{ public static void main(String[] args){ if(args.length > 0){ FileReader f; try{ // Open a file: f = new FileReader(args[0]); SomeOtherClass.process(f); f.close(); } catch (IOException x){ System.out.println(x); if (f != null) f.close(); }}}}

13 Why Finally? import java.io.*; class ProcessFile{ public static void main(String[] args){ if(args.length > 0){ FileReader f; try{ // Open a file: f = new FileReader(args[0]); SomeOtherClass.process(f); } catch(IOException x){ System.out.println(x); } finally{ if(f != null) f.close(); }}}

14 SaM and Exceptions Why do you need to know this? In Part 2, your compiler will not have to throw exceptions. However, you will be required to use SaMTokenizer SamTokenizer can throw an exception when you request a token

15 SamTokenizer and Exceptions public int getInt() throws TokenizerException { if(next() == StreamTokenizer.TT_NUMBER) return (int)tokens.nval; else throw new TokenizerException("Attempt to read non-numerical value as an integer", lineNo()); }

16 Exceptions in your Project All methods that use SamTokenizer, and any methods that call those methods, must advertise that they throw TokenizerException. This will allow you to avoid handling the exceptions. Under normal conditions, you should never get have a TokenizerException thrown, since you can check the type of the next token using peekAtKind()