Exceptions

1
Exceptions

• 11-07-05

2
Exception

• Conveys the idea that something has gone wrong
• Good-quality software should be able to cope with
  predictable problems
• Examples
• Loading a file that doesnt exist
• Trying to print on a printer with no paper

3
If-then construct

• if (something wrong)
• handle the problem
• else
• handle the normal situation
• DoA()
• DoB()
• DoC()

4
If-then

• DoA()
• If (DoA went wrong)
• handle the DoA problem
• Else
• DoB()
• if (DoB went wrong)
• handle the DoB problem
• else
• DoC()
• if (Doc went wrong)
• hande the DoC problem
• else
• Etc.

5
If-then

• The preceding code has further drawbacks
• Methods can have many input parameters but can
  return only a single result
• How to we signal a problem?
• Negative numbers?
• Zero-length string?
• Such an approach is not general

6
Exceptions and Objects

• Java is OO
• Exceptions are an object
• Use new to create an instance of the appropriate
  exception class
• Another region of the program can then check for
  its existence

7
When to use exceptions

• They provide a kind of control structure for
  unlikely events
• Not to be used for normal bad things
• User inputting a number of out range
• Use exception handling facilities for errors
  rather than predictable normal cases

8
Jargon of Exceptions

• Exceptions are indicated by being thrown
• Exceptions are detected by being caught
• Java uses these keywords
• Throws
• Throw
• Try
• Catch

9
Try-Catch example

• See ExceptionDemo1.Java

10
Try and Scope

• When a try block produces an exception, its
  execution is halted
• Any variables declared within the try block
  become inaccessible
• They cant be used in the catch block
• Any variables that are required inside the catch
  block must be declared outside the try block

11
Search for a catcher

• Method A invokes Method B which invokes Method C
• Exception occurs in C
• Looks in Method C
• If found handles the exception there
• Looks in Method B
• Looks in Method A

12
Throwing -- an Introduction

• Public static int parseInt(String s)
• throws NumberFormatException
• // code for method
• throw new NumberFormatException()
• // more code
• // end of parseInt

13
Exception Classes

• To catch we need a method to handle the
  appropriate exception
• See figure 1 for different kinds of exceptions
• Unchecked exceptions are ones you must be
  prepared for
• Compiler will look for the other kind of
  exceptions (checked)

14
Using the exception class structure

• try
• // some code
• catch (NumberFormatException e)
• // handle number format problem
• catch (IllegalArguementException e)
• // hande this
• catch (Exception e)
• // handle this type of exception here

15
Finally

• try
• while (line myFile.readlin() ) ! null)
• // process line
• catch (IOexception e)
• errorField.setText(Error in file input)
• finally
• myFile.close()