Exception Handling

• Exception


  An object that describes an unusual or erroneous situation
• Exception handling


  1. Ignore it (possible only with "unchecked" exception – abnormal termination)

  2. Handle it where it occurs

  3. Handle it in another place in the program
• Java has a predefined set of exceptions and errors that can occur during execution
• Error


  Represents an unrecoverable situation and should not be caught
• try statement


  • Line that throws the exception is executed within a try block
  • A try block is followed by one or more catch clauses
  • Each catch clause has an associated exception type and is called an exception handler
  • When an exception occurs, processing continues at the first catch clause that matches the exception type
• finally clause


  • Statements in the finally clause always are executed
  • If no exception is generated, the statements in the finally clause are executed after the statements in the try block complete
  • If an exception is generated, the statements in the finally clause are executed after the statements in the appropriate catch clause complete
• Exception class hierarchy


  • All error and exception classes are descendants of the Throwable class
  • A programmer can define an exception by extending the Exception class or one of its descendants
  • The parent class used depends on how the new exception will be used

  
• Checked exceptions


  • Must be explicitly handled by the programmer
  • Compile-time exceptions that extend the Exception class (except RuntimeException and its subclasses)
  • Forced handling: The compiler ensures that you either handle them using try-catch blocks or declare them in the method signature using the throws keyword

  
• Unchecked exceptions


  • Do not need explicit handling
  • Run-time exceptions that extend the RuntimeException class
  • Examples: ArithmeticException, NullPointerException
• Errors


  • Indicate a serious problem that usually occurs due to system resources or other external factors
  • Cannot be caught or handled by the program
  • Occur at run time and are always unchecked
  • Examples: OutOfMemoryError, StackOverflowError, LinkageError
• A checked exception must either be caught by a method, or be listed in the throws clause of any method that may throw or propagate it
• A throws clause is appended to the method header
• The compiler will issue an error if a checked exception is not caught or asserted in a throws clause
• Unchecked exceptions do not require explicit handling
• Errors are similar to RuntimeException and its descendants in that errors should not be caught and errors do not require a throws clause
• throw keyword


  • Used to throw an exception explicitly
  • Specify the exception object which is to be thrown
  • Can throw either checked or unchecked exceptions
• throws keyword


  • Used to declare an exception
  • Gives information to the programmer that there may occur an exception
  • Used to handle checked exceptions
• Unchecked exceptions and errors do not need to be declared using throws keyword
• User-defined exception


  • If it extends Exception class, it is a checked exception
  • If it extends RuntimeException class, it is an unchecked exception