Exception Handling

1
Exception Handling
Contents

1. Many classes in the Java standard library throw
   exceptions
2. throws clause (or try-catch block) is needed
   in the function header of any function that
   contains a message to a library method that
   throws an exception
3. Error message if throws clause or try-catch
   blocks are missing
4. Types of exceptions
5. Try-catch blocks
6. The Exception hierarchy
7. An example using try-catch blocks
8. The anatomy of an exception

2
Exception Handling
Most of Javas I/O classes (and many others)
throw exceptions.
For example Consider a function with a message
readLine( ) directed to a BufferedInputReader
object
import java.io.
//needed for streams and IOException
public class ExceptionalExample public
static void main(String args)
throws IOException
InputStreamReader isr new InputStreamReader(Syst
em.in) BufferedReader br new
BufferedReader(isr) System.out.println(Enter a
number)
String str br.readLine( )
The statement throws IOException is required
when using readLine( ). It must be included in
the header of any function in which readLine( )
appears.
Provide stream readers to convert characters
(read as integers) into a string, then prompt the
user for a number.
Read the supplied number (as a String) from the
keyboard.
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Exception Handling
If we fail to include the throws IOException
clause in the main function header we will see
the following error reported when we compile the
program ExceptionalExample
c\javacodegtjavac ExceptionalExample.java
Method readLine( ) throws an IOException. Any
function that uses it must either throw the same
exception (and pass the buck to the operating
system to handle the exception) or catch the
exception and provide its own error handling
methodology. In the previous example we chose
the first strategy.
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Exception Handling
Types of exceptions
Checked exceptions inability to acquire system
resources (such as insufficient memory, file does
not exist)
Java checks at compile time that some mechanism
is explicitly in place to receive and process an
exception object that may be created during
runtime due to one of these exceptions occurring.
Unchecked exceptions exceptions that occur
because of the user entering bad data, or failing
to enter data at all.
Unchecked exceptions can be avoided by writing
more robust code that protects against bad input
values. Java does not check at compile time to
ensure that there is a mechanism in place to
handle such errors.
It is often preferred to use Javas exception
handling capabilities to handle bad user input
rather than trying to avoid such circumstances by
providing user-input validation in the code.
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Exception Handling
The try / catch blocks
try //statements one of which is capable of
throwing an exception
catch (ExceptionTypeName objName) //one or
more statements to execute if this exception
occurs
finally //statements to be executed whether or
not exception occurs
Encapsulate statement or statements that can
throw an exception in a try block.
One or more catch blocks must immediately follow
a try block to provide error handling routines
for any exceptions that occur while executing the
statements in the try block.
Each catch block specifies the type of exception
that it handles in a parameter list.
An optional finally block can be added at the end
of the catch blocks to provide a set of
statements that are always executed whether or
not an exception occurs.
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Exception Handling
The exception hierarchy (partial)
All exceptions inherit from a base class Exception
Common Exception sub classes include
IOException can be decomposed into specific
classes of I/O errors
NumberFormatException is a subclass of
DataFormatException
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Exception Handling
Example
Consider implementing the BufferReader readLine(
) example with try-catch clocks.
import java.io. public class ExceptionalExample
public static void main(String
args) //no throws clause used here
InputStreamReader isr new InputStreamReader(S
ystem.in) BufferedReader br new
BufferedReader(isr) System.out.println(Enter a
number) try String str br.readLine(
) double num Double.parseDouble(str)
try block encapsulates the readLine( ) method and
the conversion from String to double.
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Exception Handling
Example (continued)
//add the catch blocks catch (IOException ioe)
//print a message to the screen
System.out.println(ioe.toString( )) catch
(Exception e) //catch any other exception
and print a message to the screen
System.out.println(e.toString( )) finally
System.exit(0)
Note! ioe is a handle (reference) to an
IOException object thrown by readLine( )
Note! toString( ) is a method that all Classes
inherit (implicitly).
In the finally clause we ensure that the program
terminates properly exit(0) signifies normal
termination.
Since both catch blocks have the same
implementation in this example, there is no need
to single out IOException in a separate block.
9
Exception Handling
The anatomy of an exception
In the main( ) function of ExceptionalExample a
BufferedReader object called br is created and
attatcher to isr.
br.readLine( ) is called inside of a try block in
function main( )
An error occurs while attempting to read from the
keyboard. An IOException object is created.
The IOException is passed to the catch block in
function main( )
Message is sent to object referenced by ioe to
execute its toString( ) method (inherited from
implicit base class Object)
try String str br.readLine( )
catch (IOException ioe) System.out.println(
ioe.toString())
new
readLine( ) ..
toString( )