Handling errors

1
Handling errors

• Exception handling and throwing
• Simple file processing

2
Outline

• Error example
• Defensive Programming
• Throwing Exception
• Catching Exception
• Defining new Exception Types
• Checked vs unchecked

3
Error situations

• Programmer errors
• Incorrect implementation
• Inappropriate object request
• Errors often arise from the environment
• incorrect URL entered
• network interruption
• File processing is particularly error prone
• missing files
• lack of appropriate permissions

4
Example Runtime error

• Use
• AddressBook friends new AddressBook()
• friends.removeDetails("frederik")
• Result
• java.lang.NullPointerException
• at AddressBook.removeDetails(AddressBook.java119
  )
• ...
• In class AddressBook
• public void removeDetails(String key)
• ContactDetails details (ContactDetails)
  book.get(key)
• book.remove(details.getName())
• ...

1. returns null when key does not exist in book
2. trying to call method on null results in
exception
5
Outline

• Error example
• Defensive Programming
• Throwing Exception
• Catching Exception
• Defining new Exception Types
• Checked vs unchecked

6
Defensive programming

• Defensive programming in client-server
  interaction
• server assumes that clients are not well behaved
  (and potentially hostile)
• E.g. server checks arguments to constructors and
  methods
• Significant overhead in implementation

7
Defensive programming

• Modified method in class AddressBook
• public void removeDetails(String key)
• if(keyInUse(key))
• ContactDetails details
• (ContactDetails)
  book.get(key)
• book.remove(details.getName())
• book.remove(details.getPhone())
• numberOfEntries--

check argument
8
Server error reporting

• How to report illegal arguments?
• To the user?
• is there a human user
• can they solve the problem?
• To the client object
• return a diagnostic value
• throw an exception

9
Returning a diagnostic
did any errors arise?

• public boolean removeDetails(String key)
• if(keyInUse(key))
• ContactDetails details
• (ContactDetails)
  book.get(key)
• book.remove(details.getName())
• book.remove(details.getPhone())
• numberOfEntries--
• return true
• else
• return false

10
Client responses

• Test the return value
• Attempt recovery on error
• avoid program failure
• Ignore the return value
• cannot be prevented
• likely to lead to program failure
• Exceptions are preferable

11
Outline

• Error example
• Defensive Programming
• Throwing Exception
• Catching Exception
• Defining new Exception Types
• Checked vs unchecked

12
Exception-throwing principles

• A special language feature
• No special return value needed
• Errors cannot be ignored in the client
• The normal flow-of-control is interrupted
• Specific recovery actions are encouraged

13
Throwing an Exception

• public ContactDetails getDetails(String key)
• if (keynull)
• throw new IllegalArgumentException(
• "null key in getDetails")
• return (ContactDetails) book.get(key)
• An exception object is constructed
• new ExceptionType("...")
• The exception object is thrown
• throw ...

14
The effect of an exception

• The throwing method finishes prematurely
• No return value is returned
• Control does not return to the client's point of
  call
• so the client cannot carry on regardless
• A client may 'catch' an exception

15
Outline

• Error example
• Defensive Programming
• Throwing Exception
• Catching Exception
• Defining new Exception Types
• Checked vs unchecked

16
The try block

• Clients catching an exception must protect the
  call with a try block
• try
• Protect one or more statements here.
• catch(Exception e)
• Report and recover from the exception here

17
The try block

• try
• addressbook.saveToFile(filename)
• tryAgain false
• catch(IOException e)
• System.out.println("Unable to save to "
  filename)
• tryAgain true

exception thrown from here
control transfers to here
18
Catching multiple exceptions

• try
• ...
• ref.process()
• ...
• catch(EOFException e)
• // Take action on an end-of-file exception.
• ...
• catch(FileNotFoundException e)
• // Take action on a file-not-found exception.
• ...

19
The finally clause

• try
• Protect one or more statements here.
• catch(Exception e)
• Report and recover from the exception here.
• finally
• Perform any actions here common to whether or
  not
• an exception is thrown.

20
The finally clause

• A finally clause is executed even if a return
  statement is executed in the try or catch clauses
• An uncaught or propagated exception still exits
  via the finally clause

21
Outline

• Error example
• Defensive Programming
• Throwing Exception
• Catching Exception
• Defining new Exception Types
• Checked vs unchecked

22
Defining new exceptions

• Extend Exception or RuntimeException
• Define new types to give better diagnostic
  information
• include reporting and/recovery information

23
The exception class hierarchy
24
Exception categories

• Checked exception
• subclass of Exception
• use for anticipated failures
• where recovery may be possible
• Unchecked exception
• subclass of RuntimeException
• use for unanticipated failures
• where recovery is unlikely

25
Unchecked exceptions

• Use of these is 'unchecked' by the compiler
• Cause program termination if not caught
• this is normal practice
• NullPointerException is a typical example

26
Checked exceptions

• Checked exceptions are meant to be caught
• The compiler ensures that their use is tightly
  controlled
• in both server and client
• Used properly, failures may be recoverable

27
The throws clause

• Methods throwing a checked exception must include
  a throws clause
• public void saveToFile(String destinationFile)
  throws IOException ...

28
Example new checked exception

• public class NoMatchingDetailsException extends
  Exception
• private String key
•  
• public NoMatchingDetailsException(String key)
• this.key key
•  
• public String getKey()
• return key
• public String toString()
• return "No details matching '" key "'
  were found."

29
QUIZ
IOException
XXX
NullPointerException
30
QUIZ
IOException
XXX
NullPointerException
31
Outline

• Input/output
• Text files
• Terminal / keyboard
• Binary Files

32
Input-output

• Input-output is particularly error-prone
• it involves interaction with the external
  environment
• java.io.IOException is a checked exception

33
Two ways to store data

• Text format
• .txt, .java, .xml
• human-readable form
• Sequence of characters
• integer 12345 stored as 49 50 51 52 53 (ascii
  for "1" "2" "3" "4" "5)
• use Reader and Writer (and their subclasses)
• (based on char type)
• Binary format
• .doc, .class, .pdf
• more compact and efficient
• integer 12345 stored as 00 00 48 57 (12345
  4825657)
• Use InputStream and OutputStream (and their
  subclasses)
• (based on byte type)

34
Outline

• Input/output
• Text files
• Terminal / keyboard
• Binary Files

35
Text output

• Use the FileWriter class
• open a file
• write to the file
• close the file
• Failure at any point results in an IOException.

36
Text output

• try
• FileWriter writer new FileWriter("name of
  file")
• while(there is more text to write)
• ...
• writer.write(next piece of text)
• ...
• writer.close()
• catch(IOException e)
• something went wrong with accessing the file

37
Text input

• Use the FileReader class
• Augment with Scanner for line-based input
• try
• Scanner in new Scanner(
• new FileReader("name of
  file"))
• while(in.hasNextLine())
• String line in.nextLine()
• do something with line
• in.close()
• catch(FileNotFoundException e)
• the specified file could not be found
• IOExceptions from FileReader swallowed by
  Scanner

38
Outline

• Input/output
• Text files
• Terminal / keyboard
• Binary Files

39
User input from terminal window

• Similar to reading from a text file
• construct a Scanner from System.in
• no closing needed
• Scanner in new Scanner(System.in)
• System.out.println("type a line of input")
• String input in.nextLine()
• do something with input

40
Outline

• Input/output
• Text files
• Terminal / keyboard
• Binary Files

41
Object streams

• ObjectOutputStream class can save entire objects
  to disk
• ObjectInputStream class can read objects back in
  from disk
• Objects are saved in binary format (and streams
  are used)
• similar to text files
• open, read/write, close

42

• Writing object to file
• BankAccount b1 ...
• ObjectOutputStream out new ObjectOutputStream(ne
  w
• FileOutputStream("accounts.dat"))
• out.writeObject(b1)
• Reading object from file
• ObjectInputStream in new ObjectInputStream(new
• FileInputStream("accounts.dat")
• BankAccount b2 (BankAccount) in.readObject()
• Objects that are written to a file must be
  instances of a class implementing the
  Serializable interface
• public class BankAccount implements Serializable
  ...

readObject may throw a (checked)
ClassNotFoundException
Serializable interface has no methods
43
Use container and write a single object

• Objects of any size written/read in a single
  statement
• ArrayList a new ArrayList()
• ... // add many objects to array list
• out.writeObject(a)
• java.util.ArrayList implements Serializable
• Objects stored in the array list must also be
  instances of a class implementing Serializable