Exception Handling

1
Exception Handling

• Exceptions are unexpected events
• i/o errors
• unintended inputs from users
• divide by zero errors
• Handling them appropriately helps us to achieve
  the robust quality.
• Not handling them appropriately can cause program
  crashes or propagation of errors.

2
Throwing Exceptions

• Here are some methods that we have used that can
  throw an exception.
• public static float parseInt(String s) in the
  Integer class can throw a NumberFormat Exception
• public static float parseFloat(String s) in the
  Float class can throw a NumberFormat Exception
• public char charAt(int index) in the String class
  can throw an IndexOutOfBoundsException

3
Throwing Your Own Exceptions

• try
• if (n lt 0)
• throw new Exception (Error Cant average 0
  items.)
• catch (Exception e)
• String str e.getMessage ()
• ...

4
try Blocks

• Convention says not to place every throw
  statement in its own try block. Include as many
  statements as possible.
• If an exception is thrown, the try block is
  immediately exited and control does not return to
  it.
• Do not put a non-void methods return statement
  inside a try block near the end. Why?
• They must be followed immediately by one or more
  catch blocks.

5
try Block Example

• try
• float x Float.parseFloat(str1)
• float y Float.parseFloat(str2)
• if ( y 0 )
• throw new ArithmeticException (y cannot be
  0)
• System.out.println(We made it this far.)

variable scope?
6
catch Block

• Catch blocks must be immediately preceded by a
  try block or another catch block.
• Catch blocks have the general form
• catch (ExceptionClassName myExc)
• If an exception is thrown in the try block, Java
  exits the try block and transfers control to the
  first catch block that matches the particular
  kind of exception thrown.

7
Checked Exceptions

• Checked Exceptions must either be caught within
  the method where it is thrown or declared within
  the method.
• IOException and ClassNotFoundException are
  checked exceptions.
• Any method that can throw an IOException that
  doesnt catch it must declare that it throws an
  IOException in its header.

8

• import java.io.
• public class Example
• BufferedReader input new BufferedReader (new
  InputStreamReader(System.in))
• public void doRead () throws IOException
• String inputString input.readLine ()
• public static void main (String argv throws
  IOException
• Example ex new Example()
• ex.doRead()

9
Unchecked Exceptions

• Belong to some subclass of RunTimeException
• Are not checked by the compiler.
• Do not have to be handled within the program.
• Do not have to be mentioned in the throws clause
  if they are not handled within a method.

10
Advantages to Javas Exception Handling System

• Separates error handling code from regular
  code. The main logic of the program isnt lost in
  error handling code.
• Propagates errors up the method call stack. Only
  the method that care about errors have to detect
  them.
• Groups error types. Methods can be written to
  handle 1 or 2 types.