Exceptions

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• Exceptions

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Exceptions

• Exceptions are objects that store information
  about the occurrence of an unusual or error
  condition. 
• They are thrown when that error or unusual
  condition occurs. 
• You have seen these before
• OutOfBoundsException, NullPointerException,
  ArithmeticException, etc.
• Until now, any exception that occurred always
  resulted in a program crash. 
• This doesnt have to happen!

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Exceptions

• Java can handle exceptions keep your program from
  crashing. 
• This is known as catching an exception.
• If any exception occurs and is not caught, the
  program will crash. 
• You can do 2 things with Exceptions
• catch Do something about the problem
  (hopefully fix it)
• throw pass the buck (hope some one else fixes
  it)

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Exceptions

• There are two types of exceptions in Java,
  unchecked exceptions and checked exceptions

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Unchecked Exceptions

• Unchecked exceptions are any class of exception
  that extends the RuntimeException class at some
  point in its inheritance hierarchy
• This includes the most common exceptions. 
• ArithmeticException, NullPointerException,
  OutOfBoundsException
• Unchecked exceptions occur all over a program and
  can be very numerous

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Unchecked Exceptions

• The cost of checking them can be greater than the
  benefit of handling them
• Thus, Java compilers do not require that you
  declare or catch unchecked exceptions in your
  program
• They can be handled if the programmer deems it
  necessary

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Checked Exceptions

• Checked exceptions are exceptions that do not
  extend the RuntimeException class
• Checked exceptions must be handled to avoid a
  compile-time error
• An example of a checked exception is the
  IOException that may occur when the readLine
  method is called on a BufferedReader object

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Checked Exceptions

• There are two ways to handle checked exceptions
• You may declare the exception using a throws
  clause or you may catch the exception
• To declare an exception, you add the keyword
  throws followed by the class name of the checked
  exception to the method header
• public void method1() throws IOException
• // stuff here

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Checked Exceptions

• Any method that calls a method that throws a
  checked exception must also handle the checked
  exception by either catching to throwing it
• If an exception is always handled using the
  throws clause, it will eventually propagate all
  the way back to the main method
• If the main method throws the exception (and the
  exception occurs while the program is running)
  then the program will crash.

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Checked Exceptions

• public static void main(String args) throws
  IOException
• method1()
• public static void method1() throws IOException
• method2()
• public static void method2() throws IOException
• stdin.readLine()

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Checked Exceptions

• public static void main(String args) throws
  IOException
• method1()
• public static void method1() throws IOException
• method2()
• public static void method2() throws IOException
• stdin.readLine()

main calls method1
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Checked Exceptions

• public static void main(String args) throws
  IOException
• method1()
• public static void method1() throws IOException
• method2()
• public static void method2() throws IOException
• stdin.readLine()

main calls method1
method1 calls method2
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Checked Exceptions

• public static void main(String args) throws
  IOException
• method1()
• public static void method1() throws IOException
• method2()
• public static void method2() throws IOException
• stdin.readLine()

main calls method1
method1 calls method2
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Checked Exceptions

• public static void main(String args) throws
  IOException
• method1()
• public static void method1() throws IOException
• method2()
• public static void method2() throws IOException
• stdin.readLine()

main calls method1
method1 calls method2
method2 can cause an IOException
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Checked Exceptions

• public static void main(String args) throws
  IOException
• method1()
• public static void method1() throws IOException
• method2()
• public static void method2() throws IOException
• stdin.readLine()

main calls method1
method1 calls method2
If it does, it will throw it to whoever called it
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Checked Exceptions

• public static void main(String args) throws
  IOException
• method1()
• public static void method1() throws IOException
• method2()
• public static void method2() throws IOException
• stdin.readLine()

main calls method1
method1 calls method2
If it does, it will throw it to whoever called it
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Checked Exceptions

• public static void main(String args) throws
  IOException
• method1()
• public static void method1() throws IOException
• method2()
• public static void method2() throws IOException
• stdin.readLine()

main calls method1
If method1 causes and exception, it will throw it
as well
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Checked Exceptions

• public static void main(String args) throws
  IOException
• method1()
• public static void method1() throws IOException
• method2()
• public static void method2() throws IOException
• stdin.readLine()

Now main gets the Exception
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Checked Exceptions

• public static void main(String args) throws
  IOException
• method1()
• public static void method1() throws IOException
• method2()
• public static void method2() throws IOException
• stdin.readLine()

Since it throws it, the program will crash
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Checked Exceptions

• A method that calls a method that throws a
  checked exception must also handle the exception
  by catching or throwing it
• If an exception is always handled using the
  throws clause, it will eventually propagate all
  the way back to the main method
• If the main method throws the exception (and the
  exception occurs while the program is running)
  then the program will crash.

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Checked Exceptions

• An alternative to simply passing the buck, is to
  catch the exception
• A method that catches and Exceptoin will do
  something to correct the error or recover and
  continue the program's execution, if the
  exception occurs
• To catch an exception, you must first indicate
  what code may cause the exception. 
• This is called a try block and is a part of a
  try-catch statement.

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Checked Exceptions

• The try block surrounds the statement and any
  related statements that may cause the exception
  to occurs
• The catch block follows the try block and defines
  the code that should be executed if the exception
  occurs
• If more than one type of checked exception needs
  to be caught, write a separate catch clause for
  each type.

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Checked Exceptions

• public void method2()
• try
• String s stdin.readLine( )
• System.out.println(s)
• catch (IOException ex)
• System.out.println(IOException ex
  occurred)

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Checked Exceptions

• All Exceptions that DO NOT extend the
  RuntimeException class are checked exceptions. 
• The compiler requires a throws clause or a
  try-catch statement for any method that may cause
  a checked exception
• If a checked exception occurs and is not caught
  before it reaches the main method of the
  application, the program will crash.

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Exceptions

• How do we know if it is checked or unchecked?
• Compiler will tell you
• Check the java API!!!

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Exceptions

• Questions??

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Other Exceptions

• int x // Get a valid integer from the user
• try
• System.out.print( "Enter an integer " )
• x Integer.parseInt( stdin.readLine() )
• catch ( NumberFormatException e )
• // A NumberFormatException occurred
• System.out.println( e.getMessage() " is not a
  valid format for an integer." )

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Other Exceptions

• boolean valid false
• int x // Get a valid integer from the user
• while ( ! valid )
• try
• System.out.print( "Enter an integer " )
• x Integer.parseInt( stdin.readLine() )
• valid true // only executes if the string
• // entered does not cause an Exception
  
• catch ( NumberFormatException e )
• // A NumberFormatException occurred
• System.out.println( e.getMessage() " is not a
  valid format for an integer." )
• // repeats until a valid integer has been
  entered

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Multiple Exceptions

• try
• FileReader fileReader new FileReader(
  file.txt )
• BufferedReader inFile new BufferedReader(
  fileReader )
• String temp inFile.readLine()
• while ( inFile.ready() )
• System.out.println(inFile.readLine()
• fileReader.close()
• catch ( FileNotFoundException e )
• System.out.println( e.getMessage() " FILE NOT
  FOUND" )
• catch ( IOException e )
• System.out.println( e " EXCEPTION" )

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Exceptions

• Questions??

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Exceptions

• After the try-catch statement, you may optionally
  include a finally clause
• A finally clause is used to perform operations
  that must happen whether or not an exception
  occurs in the try block
• If a finally clause is used, it must follow all
  catch clauses for a given try block
• There can only be one finally clause for a given
  try block.

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Exceptions

• try
• ...
• catch ( IOException e )
• ...
• catch ( Exception e )
• ...
• finally
• // any statements that you always want to occur

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Exceptions

• You may declare the exception using a throws
  clause or you may catch the exception
• Any method that calls a method that throws a
  checked exception must also handle the checked
  exception by either catching to throwing it
• Whatever is in a finally block is always executed
  at the end of a try-catch block

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Exceptions

• Exceptions are a good way to separate the code
  that handles errors or very special cases from
  the code that is most commonly executed. 
• This can help you focus on the intended purpose
  of the code without having to write many nested
  control statements to verify information at each
  step of execution.

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Exceptions

• Questions??