SE1020

1
More Exception Handling

• and Throwing Exceptions

2
A few of 60 exception classes
3
Catch or let go?

• The Error class represents serious problems that
  should not be caught by ordinary applications
• (e.g. thread death, out-of-memory).
• The Exception class represents error conditions
  that should be caught
• (e.g. divide-by-0, null reference, conversion
  errors).

4
Catching different types of Exceptions

• When there are multiple catch blocks in a
  try-catch statement, they are checked in
  sequence.
• It is important to check more specialized
  exception classes before the more general
  exception classes.
• When an exception is thrown, its matching catch
  block is executed and the other catch blocks are
  ignored.

5
Fumbling to the VM

• If none of the catch blocks matches the thrown
  exception, the system will search down the stack
  trace for a method with a matching catch block.
• If none is found, the system (VM) will handle the
  thrown exception
• And terminate your program after calling
  printStackTrace()

6
Getting information from exceptions

• There are two methods of the Throwable class that
  provide information about the thrown exception
• getMessage a String description
• printStackTrace call stack dump

7
Catching Exceptions

• If there is a block of code that must be executed
  regardless of whether an exception is thrown, we
  use the reserved word finally
• inputStr JOptionPane.showInputDialog(null, )
• try
• number Integer.parseInt(inputStr)
• return// will not return until finally is
  processed
• catch (NumberFormatException e)
• System.out.println(Cannot convert to int)
• return // will not return until finally is
  processed
• finally
• System.out.println(DONE)

8
You can also throw exceptions

• An exception is thrown using the throw statement.
• throw lta throwable objectgt
• where lta throwable objectgt is an instance of the
  Throwable class or its subclasses.
• Exception is a subclass of Throwable
• By convention, we create Exceptions rather than
  Throwable objects

9
A few of 60 exception classes
Throwable is the root of all Exceptions and Errors
Your program cannot catch or throw Errors
Your program can catch or throw Exceptions
10
Throwing your own Exceptions

• inputStr JOptionPane.showInputDialog(null,
  Enter a value less than 100)
• number Integer.parseInt(inputStr)
• if (numgt100)
• throw new Exception(Out of bounds)

11
Passing the buck

• When a method may throw an exception, either
  directly or indirectly, the method is called an
  exception thrower.
• An exception thrower method can be a
• Catcher if it catches it
• Propagator if it lets it go
• Can be both if it catches some types and lets
  others go

12
Catcher/Propagator Example

• inputStr JOptionPane.showInputDialog(null,
• Enter a value less than 100)
• try
• number Integer.parseInt(inputStr)
• if (numgt100)
• throw new Exception(Out of bounds)
• catch (NumberFormatException e)
• System.out.println(Cannot convert to int)

Here, NumberFormatExceptions are caught but
Exceptions are propagated.The Out of bounds
exception will not be caught here!
13
RuntimeException a special class of Exception

• Some exceptions have to be caught by your code
  and not allowed to propagate to the VM
• A checked exception is an exception that is
  checked at compile time
• RuntimeException and classes that derive from it
  are unchecked.
• They are detected only at runtime

14
Only RuntimeExceptions can be left unchecked
RuntimeExceptions and descendants are unchecked
exceptions
Exception and IOException are checked exceptions
15
Propagating Exceptions

• If a method is a checked exception propagator,
  its header must be modified to declare the type
  of exceptions the method propagates.
• Without the required throws clause, the program
  will not compile.
• The reserved word throws is used for this
  declaration.
• void SomeMethod( ) throws Exception // some
  code that may cause an Exception
• // that is not caught here in the method. . .

16
Callers of a method that can throw a checked
exception must include the try-catch statement in
the method body or the throws clause in the
header.
17
Throws is optional for RuntimeExceptions

• For the exceptions of the type (or derived from
  the type) called RuntimeException, the throws
  clause is optional.
• If a method is a propagator of runtime exceptions
  or errors, the throws clause is optional.

18

• It is optional for callers of a method that can
  throw runtime exceptions to include the try-catch
  statement in the method body or the throws clause
  in the header.