Java Exception Handling

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JavaException Handling
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Exception

• An exception is an exceptional condition that
  arises in a code sequence at run time.
• A Java exception is an object that describes an
  exceptional condition that happens.
• When an exception condition arises, an object
  representing that exception is created and thrown
  in the method that caused the error.

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Exception Types

• All exception types are subclasses of the
  built-in class Throwable.
• There are two subclasses of the Throwable
  Exception and Error.
• Exception is the class that you will create
  subclass for your own expception handling. The
  Error class handles system-oriented, catastrophic
  failure at run-time.

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Run-time Default Exception Handler

• When there is no code for exception handling and
  an error is detected, Java rum-time will
  construct a new exception object and then throws
  this exception.
• The default exception handler displays a string
  describing the exception, prints a stack trace
  from the point at which the exception occurred,
  and terminates the program. (ExceptionNo1.java)

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Exception Handling

• Java exception can be handled manually by your
  own code via five keywords.
• Program statements that you want to monitor for
  exceptions are contained within a try block.
• If an exception occurs within the try block, it
  is thrown and caught by the catch keyword.

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Exception Handling

• To manually throw an exception, use the keyword
  throw.
• Any code that absolutely must be executed before
  a method returns is put in a finally block.
• Manual exception handling allows you to fix the
  error and prevents the program from automatically
  terminating.

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Try, catch, finally

• The general form
• try
• block of code for monitoring errors
• catch (ExceptionType1 exceptionObj)
• block of code for exception handling
• catch (ExceptionType2 exceptionObj)
• finally
• block of code to executed before try block
  ends
• (Exception1.java)

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Try, catch, finally

• A try and its catchs and finally statement form a
  unit.
• The scope of the catch clause is restricted to
  those statements specified by the immediately
  preceding try statement.
• A catch statement cannot catch an exception
  thrown by another try statement.

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Try, catch, finally

• The goal of most well-constructed catch clauses
  should be to resolve the exception condition and
  then continue on as if the error had never
  happened.
• (Exception2.java)

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Multiple catch Clauses and Nested try
Statements
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Java Built-in Exceptions

• Inside the standard package java.lang, Java
  defines several exception classes.
• The most general of these exceptions are
  subclasses of the standard type RuntimeException.
• Since java.lang is automatically imported,
  exceptions derived from RuntimeException are also
  automatically available.

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Creating Your Own Exception Subclasses

• You can define your own exception handling
  subclasses under Exception.
• Because Exception is subclass of Throwable, it
  has all the methods defined by Throwable.
• (Exception3.java)