Exception

1
Exception

• Error handling

2
Exception 4

• An unusual occurrence during program execution
  that requires immediate handling
• Errors are the most common type of exception
• Java enables centralization of exception handling
  code in a location that is most logical
• When an exception is reported (or thrown)
  execution is automatically transferred the
  closest exception handler, no matter what method
  was executing when the exception was thrown.

3
Try/catch 4

• Try establishes a block of code that is to have
  its exceptions and abnormal exits (through break,
  continue,return, or exception propagation)
  handled
• Catcha try block may be followed by zero or more
  catch clauses that specify code to handle various
  types of exceptions
• Catch clauses have unusual syntax each is
  declared an argument, much like a method
  argument. This argument must be of type throwable
  or a subclass
• Do not need a catch clause for every possible
  exception

4
Trycatch 4

• try
• statement1
• catch(ExceptionClass1 e1)
• statement2
• catch(ExceptionClass2 e2)
• statement3
• finally
• statementn
• The statement following try is executed
  unconditionally as part of the normal flow of
  execution

5
Trycatch 3

• During execution of this statement (or block), an
  exception may be raised (even indirectly, i.e.,
  statement may call a method that may raise an
  exception)
• If the exception is raised and not handled by a
  more tightly nested block or method, statement1
  terminates execution and the interpreter checks
  if one of the catch blbocks can handle the
  exception
• Every exception class implements the throwable
  interface

6
Exception 3

• Statements inside the catch block are executed
  when the respective exception type is matched
• Inside the catch block, the code may decline to
  handle the exception by executing a throw
  statement. Here, the exception is passed along
  and the interpreter must search for another
  handler
• Exceptions not handled by any code eventually
  reach the Java default exception handler, which
  prints a message and terminates execution

7
Finally 3

• Code in the finally block is executed
  unconditionally after execution leaves the try
  block
• The finally block is guaranteed to be executed,
  whether the try block terminated normally or
  exited early because of a n exception or break
  statement
• The finally block is a logical place to put code
  that cleans up

8
Exception No Exception

• try
• ?
• exception raised?
• ?
• ? catch()
• ?
• ?

• try
• ?
• ?
• ? catch()
• ?
• ?

9
Console.java

• public static int readInt()
• String tmpX readString().trim()
• try
• return Integer.parseInt(tmpX)
• catch(NumberFormatException ne)
• System.err.println("Not an integer.
  Quitting ...")
• System.exit(-1)
• return -1