EXCEPTIONS

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EXCEPTIONS
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Whats an exception??

• Change the flow of control when something
  important happens
• ideally - we catch errors at compile time
• doesnt happen
• data error
• someone pulls a plug
• hardware error
• software error (blame it on the machine)

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Exceptions Errors

• Exceptions are actual objects
• They are instances of classes that inherit from
  the class Throwable
• an instance of a Throwable class is created when
  an exception is thrown
• Throwable has two sub classes
• Error - internal runtime error (no hope)
• Exception - we can do something here

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Error Class

• These Exceptions are internal errors in the JAVA
  runtime environment (such as)
• LinkageError
• NoClassDefFoundError
• NoSuchMethodError
• OutOfMemoryError
• VirtualMachineError

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Exception class

• is divided into three major categories
• gtRuntime exception classes
• gtI/O exception classes
• gtother classes that are directly derived
  from the Exception class

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RunTimeException

• We know these guys (they are all classes)
• NullPointerException
• IllegalArgumentException
• NumberFormatException
• IllegalThreadStateException
• IndexOutOfBoundsException
• StringIndexOutOfBoundsException
• Array IndexOutOfBoundsException

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I/O Exceptions

• General I/O failures
• EOFException
• FileNOtFoundException
• MalformedURLException
• SocketException

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other classes of Exceptions

• These exceptions are their own classes derived
  from the Exception class (I think)
• ClassNotFoundException
• AWTException
• Instantiationexception
• InterruptedException

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Some terms

• exception - an error condition at run time
• throwing - cause an exception to occur
• catching - capturing an exception and
  executing some statements to try and resolve the
  error
• catch clause - the block of code that does this
  (the catching)
• stack trace - sequence of method calls that
  brought control to this point

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What causes an exception?

• Two ways
• explicitly - the programmer throws an exception
  with the throw statement
• EX throw ExceptionObject
• implicitly - carry out some illegal action
• EX divide by zero
• array index out of bounds
• (we know that one)

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errors that might be thrown

• We are using a JAVA provided method and get the
  following compiler error
• blah blah Exception blah blah must be caught or
  it must be declared in the Throws clause of this
  method
• whats going on???

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try/catch blocks

• a JAVA method can indicate the kinds of errors it
  might throw
• EX - a method that reads a file might throw an
  I/OException error
• you have to protect your code against these
  errors.
• This is compiler enforced
• we use the try/catch blocks to do this

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The try part

• You put a try block around the code that might
  throw an exception
• the try statement says try these statements
  and see if you get an exception
• try
• statements

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the catch part

• Still have errors - missing the catch block
• the catch block says I will handle any exception
  that matches my argument
• a try block is followed by a catch block
• catch(Exception e)
• statements

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an example

• Thread.sleep might throw an
• InterruptedException
• try
• Thread.sleep (500)
• catch (InterruptedException e)
• // does nothing

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Added notes

• A try block may have many statements
• therefore, the code in the try block may throw
  more than one exception
• there may be more than one catch block
  following a try block
• try
• catch(IOException e1)
• catch(InterruptedException e2)
• catch(SocketException e3)

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the finally clause

• Assume there is some code you absolutely must do
• put the code in a finally block
• the finally block executes whether an exception
  was thrown or not, if an exception was caught or
  not, and will execute even if a return has been
  executed

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uses of the finally clause

• You open a file
• you read the file
• you must close the file
• (weather an exception was thrown or not)
• return resources
• dispose of a socket
• dispose of a file descriptor

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create your own exceptions

• The new class should inherit from some other
  exception in the JAVA hierarchy
• inherit from Exception, not Error
• you can inherit from Exception but try and find
  one close the one you are creating
• an Exception usually has two constructors
• first takes no parameters
• second takes a String as a parameter

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example - create an Exception

• public class Oops extends Exception
• public Oops()
• public Oops (String msg)
• super(msg)

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example - create an Exception

• Public class Oops extends IOException
• public Oops()
• public Oops (String msg)
• super(msg)

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new methods and exceptions

• You can create a new method that throws an
  exception by using the throws keyword
• modifiers return name() throws e1, e2, e3
• public void ReadF() throws Something
• Something s new Something(oops)
• if ((x /y) gt 4)
• throw s

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handling the Exception

• When an Exception occurs, execution stops
• the machine looks for a catch clause to handle
  the error
• if no catch clause is found in the offending
  method to handle the error, (final is run) and
  control is passed to the calling method
• this method is searched for a catch clause
• and so on up the call chain
• you bomb if no catch clause is found

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some fancy stuff

• Two methods are inherited from Throwable
• Did you wonder why there was that String in the
  second Exception constructor example??
• There is a method that returns that String
• can be used to pass information
• the method getMessage returns that String
• another method printStackTrace() will print the
  call chain from the point of the exception