Handling Exceptions

1
Handling Exceptions
2
Exception

• Definition an occurrence of an undesirable
  situation that can be detected during program
  execution
• Examples
• division by zero
• trying to open an input file that does not exist
• an array index that goes out of bounds

3
Java Exception Hierarchy
4
Constructors and Methods of the class Throwable
5
The class Exception and its Subclasses from
java.lang
6
The class Exception and its Subclasses from
java.io
7
Javas Exception Class

• class Exception
• Subclass of class Throwable
• Superclass of classes designed to handle
  exceptions
• Various types of exceptions
• I/O exceptions
• Number format exceptions
• File not found exceptions
• Array index out of bounds exceptions
• Various exceptions categorized into separate
  classes and contained in various packages

8
The class Exception and its Constructors
9
Java Exception Classes
10
Exceptions Thrown by Methods
11
Checked Exceptions

• Definition any exception that can be analyzed by
  the compiler
• Examples
• IOExceptions

12
Unchecked Exceptions

• Definition exceptions that cannot be analyzed
  when the program compiles (must be checked for by
  programmer)
• Examples
• Division by zero
• Array index out of bounds
• Syntax
• throws ExceptionType1, ExceptionType2,
• ExceptionType1, ExceptionType2, etc are names of
  exception classes

13
Exceptions Example Code

• public static void exceptionMethod()
• throws
  NumberFormatException, IOException
• //statements
• The method exceptionMethod throws exceptions of
  the type NumberFormatException and IOException.

14
Handling Exceptions within a Program

• try/catch/finally block used to handle exceptions
  within a program
• try block
• Includes statements that may generate an
  exception
• Includes statements that should not be executed
  if an exception occurs
• Followed by zero or more catch blocks
• May or may not be followed by finally block

15
Handling Exceptions within a Program

• catch block
• Heading specifies type of exception it can catch
• Contains exception handler completely handles
  exception
• Can catch all exceptions of a specific type or
  all types of exceptions
• May or may not be followed by a finally block
• finally block
• Code contained in this block always executes
• try block with no catch block has finally block

16
Handling Exceptions within a Program

• try
• //statements
• catch(ExceptionClassName1 objRef1)
• //exception handler code
• catch(ExceptionClassName2 objRef2)
• //exception handler code
• ...
• catch(ExceptionClassNameN objRefN)
• //exception handler code
• finally

17
Order of catch Blocks

• If an exception in a try block is caught by the
  first catch block, the remaining catch blocks are
  ignored
• Must be careful about order catch blocks are
  listed

18
Combining catch Blocks With the Operator
instanceof
19
The Method printStackTrace

• Used to determine the order in which the methods
  were called and where the exception was handled