Exceptions

1
Exceptions

• Three categories of errors
• Syntax errors
• Runtime errors
• Logic errors
• Syntax errors rules of the language have not
  been followed.
• Runtime error program is running and environment
  detects an operation that is impossible to carry
  out.
• Logic errors the program doesnt perform the way
  it intend to

2
Runtime error

• Runtime errors occur for various reason
• Invalid input
• Open a file which doesnt exist
• Access out-of-bounds array
• Runtime errors cause exceptions
• Exception are handled differently from the events
  of GUI programming
• A event may be ignored in GUI programming, but an
  exception cannot be ignored.
• Java provides programmers with the capability to
  elegantly handle runtime errors.

3
An example

• import javax.swing.JOptionPAne
• public class test
• public static void main(String args)
• String input JOptionPane.showInputDialog(null
  , Please enter an integer)
• int number Integer.parseInt(input)
• JOptionPane.shoeMessageDialog(null, the number
  entered is number)
• // if you entered a floating-point value, the
  program terminates

4
Catch this error trycatch block

• import javax.swing.JOptionPAne
• public class test
• public static void main(String args)
• try
• String input JOptionPane.showInputDialog
  (null, Please enter an integer)
• int number Integer.parseInt(input)
• JOptionPane.shoeMessageDialog(null, the
  number entered is number)
• catch Exception ex)
• JOptionPane.showMessageDialog(null,
  Incorrect input an integer is required)

5
Exception classes

• A Java exception is an instance of a class
  derived from Throwable.
• The Throwable class is contained in the java.lang
  package
• Subclasses of Throwable are contained in various
  packages
• Errors related to GUI components are included in
  the java.awt package
• Numeric exceptions are included in the java.lang
  package
• You can create your own exception classes by
  extending Throwable or a subclass of Throwable.

6
Exception classes

• The exception classes can be classified into
  three types
• System errors are thrown by the JVM and
  represented in the Error class. The Error class
  describe internal system errors
• Exceptions are represented in the Exception
  class, which describe errors caused by your
  program and by external circumstance
• Runtime exceptions are represented in the
  RuntimeException class, which describes
  programming errors, such as bad casting,
  accessing an out-of bounds array, and numeric
  errors.

7
Exception classes and subclasses

• Object
• Throwable
• Exception
• ClassNotFoundException
• IOException
• AWTException
• RuntimeException
• ArithmeticException
• NullPointerException
• IndexOutOfBoundsException
• Error
• LinkageError
• VirtualMachineError
• AWTError

8
Unchecked Exceptions

• RuntimeException, Error and their subclasses
• Reflect programming logic errors
• Not recoverable
• Should be corrected in the program
• Can occur anywhere
• Java does not mandate that you write code to
  catch or declare
• For example, a NullPointerException is thrown if
  you access an object through a reference variable
  before an object is assigned to it

9
Checked exception

• All other exception are known as checked
  exception
• The compiler forced the programmer to check and
  deal with them

10
Exception handling

• Javas exception-handling model is based on three
  operations
• Declaring an exception
• Throwing an exception
• Catching an exception
• Every method must state the types of checked
  exceptions it might throw
• Java doesnot require that you declare Error and
  RuntimeException.

11
Declaring exception

• Other exception thrown by the method must be
  explicitly declared
• public void myMethod() throws IOException
• If the method throws multiple exceptions
• public void myMehtod() throws Exception1,
  Exception2, , ExceptionN

12
Throwing Exceptions

• Suppose the argument must be non-negative, but a
  negative argument is passed, the program can
  created an instance of IllegalArgumentException
  and throw it
• IllegalArgumentException ex new
  IllegalArgumentException(Wrong Argument)
• throw ex
• Or
• throw new IllegalArgumentException(Wrong
  Argument)

13
Catching Exceptions

• try
• statements
• catch (Exception1 e1)
• catch (Exception2 e2)
• finally