Exception-Handling in Java

• Exceptions are represented by obects of classes belonging to a particular subtree of the Object hierarchy
• The root is the Throwable class
• Any subclass of this class is eligible to be used for exception handling

Indicating the Possibility of an Exception

• throws clause in method header

  void open(String fileName) throws FileNotFoundException {...}
  		

  public static int parseInt(String s) throws NumberFormatException {...}
  		

  public static void main(String [] args) throws Exception {...}
  		

Throwing An Exception

	throw new Exception();

	throw new Exception("Some message");

	throw new UnsupportedOperationException();

• What Exception to Throw?
  • Use already-defined exception class
  • Create your own exception class
• Propogation of the Exception
  • Up the run-time call chain (stack)

Catching Exceptions

try {
	...		// code potentially throwing an exception (possibly via a method invocation)
} catch (exception-class-1 e1) {
	... // exception handling code
} catch (exception-class-1 e2) {
	... // exception handling code
} ...
...
}catch (exception-class-n en) {
	... // exception handling code
}

The Exception Class Hierarchy

• Root is actually Throwable
  • Only objects of this class (or one of its subclasses) may be thrown
  • Only objects of this class (or one of its subclasses) may be caught
• Two immediate subclasses
  • Exception
  • Error

• Superclass of conditions that a reasonable application might wish to catch.
• Examples
  • FileNotFoundException
  • MalformedURLException
• Subclass RuntimeException

Error class

• Superclass of conditions that indicate a serious problem, which probably should not be handled (abnormal condition)
  • No throws clause required
  • Examples
    • OutOfMemoryError
    • StackOverflowError
    • ClassFormatError
    • VirtualMachineError

RuntimeException class

• Superclass of exceptions thrown by the Java Virtual Machine during normal operation.
• No throws clause required
• Examples
  • NullPointerException
  • ArrayIndexOutOfBoundsException
  • ArithmeticException
  • NoSuchElementException
    • Thrown by Iterator.next()