Exception Handling

1
Chapter 14

• E xception Handling

2
Chapter Goals

• To learn how to throw exceptions 
• To be able to design your own exception classes
• To understand the difference between checked and
  unchecked exceptions 
• To learn how to catch exceptions 
• To know when and where to catch an exception

3
Error Codes

• Traditional approach to error handling method
  returns error code
• Example JOptionPane.showInputDialog returns null
  if user hits Cancel
• Problem Calling method may forget to check for
  error code
• Problem Calling method may not know how to fix
  error--then it needs to return an error code
• Symptom Programming for successx.doSomething()i
  s replaced by programming for failureif
  (!x.doSomething()) return false

4
Exceptions

• Can't be overlooked
• Can be handled by a competent handler, not
  necessarily the calling method
• Throw an exception object to indicate failureif
  (failure)   XxxException e new
  XxxException(. . .)   throw e
• More conciselythrow new XxxException(. . .)

5
Exceptions
public class BankAccount public void
withdraw(double amount) if (amount
gt balance) throw new
IllegalArgumentException( "Amount exceeds
balance") balance balance - amount
...
6
Hierarchy of Exception Classes
7
Syntax 14.1 Throwing an Exception

•  throw exceptionObject
• Example
•  throw new IllegalArgumentException()
• Purpose
• To throw an exception and transfer control to a
  handler for this exception type

8
Checked Exceptions

• Compiler checks that you are aware of the
  exception
• Generally used for errors that can happen even
  in correct programs
• IOException and its sublcasses are checked
  exceptions
• NullPointerException, ArrayIndexOutOfBoundsExcep
  tion , ...  are unchecked --they are your fault
  -)
• Virtual machine errors (e.g. OutOfMemoryError)
  are unchecked
• Classification not perfect. For example,
  Integer.parseInt throws unchecked
  NumberFormatException
• Checked exceptions are subclasses of Exception
  that are not subclasses of RuntimeException

9
Checked and Unchecked Exceptions
10
Exception Specifications

• BufferedReader.readLine may throw IOException
• Tag calling method with throws IOException
• public class Coin
• public void read(BufferedReader in) throws
  IOException
• value Double.parseDouble(in.readLine())
  name in.readLine()
• ...

11
Exception Specifications

• Need to tag caller of Coin.read as well
• Stop at main or with handler (see below)
• Can have multiple exception typespublic void
  read()    throws IOException, ClassNotFoundExcept
  ion
• throws specifier not a sign of irresponsible
  programming
• Better to declare exception than to handle it
  incompetently

12
Syntax 14.2 Exception Specification

• accessSpecifier returnType methodName
• (parameterType parameterName, . . .)throws
  ExceptionClass, ExceptionClass . .
• Example
•   public void read(BufferedReader in) throws
  IOException
• Purpose
• To indicate the checked exceptions that a
  method can throw

13
Designing Your Own Exception Types

• if (amount gt balance)    throw new
  InsufficientFundsException(. . .)
• Make it an unchecked exception--programmer could
  have avoided it by calling getBalance first
• Extend RuntimeException
• Supply two constructors

14
Designing Your Own Exception Types
previous start next
File Purse.java

public class InsufficientFundsException extends
RuntimeException public InsufficientFundsEx
ception()   public
InsufficientFundsException(String reason)
super(reason)

previous start next
15
Catching Exceptions
try BufferedReader in new
BufferedReader( new
InputStreamReader(System.in))
System.out.println("How old are you?")
String inputLine in.readLine() int age
Integer.parseInt(inputLine) age
System.out.println("Next year,you'll be "
age) catch (IOException exception)
System.out.println("Input/output error
exception) catch (NumberFormatException
exception) System.out.println("Input
was not a number")
16
Catching Exceptions

• Statements in try block are executed
• If no exceptions occur, catch clauses are skipped
  
• If exception of matching type occurs, execution
  jumps to catch clause
• If exception of another type occurs, it is thrown
  to the calling method
• If main doesn't catch an exception, the program
  terminates with a stack trace

17
Syntax 14.3 General Try Block

• try statement statement ...
  catch (ExceptionClass exceptionObject)
  statement statement ... catch
  (ExceptionClass exceptionObject) statement
  statement ......

18

• Example
•  try System.out.println("What is your
  name?") String name console.readLine()
  System.out.println("Hello,"name "!")
  catch (IOException exception)
  exception.printStackTrace() System.exit(1)
  
• Purpose
• To execute one or more statements that may
  generate exceptions. If an exception of a
  particular type occurs, then stop executing those
  statements and instead go to the matching catch
  clause. If no exception occurs, then skip the
  catch clauses.

19
The finally Clause

• Exception terminates current method
• Danger Can skip over essential code
• Example BufferedReader in in new
  BufferedReader(new FileReader(filename))
  purse.read(in) in.close() 
• Must execute in.close() even if exception happens
  
• Use finally clause for code that must be executed
  "no matter what"

20
The finally Clause

• BufferedReader in null try in new
  BufferedReader( new FileReader(filename))
  purse.read(in) finally if (in !null)
  in.close()

21
The finally Clause

• Executed when try block comes to normal end
• Executed if a statement in try block throws an
  exception, before exception is thrown out of try
  block
• Can also be combined with catch clauses

22
Syntax 14.4 The finally Clause

•  try    statement    statement    ...
  finally
•     statement    statement    ...

23
previous start next

• Example
•  BufferedReader in null
• try
• in new BufferedReader( new
  FileReader(filename)) purse.read(in)
• Finally
• if (in !null) in.close()
• Purpose
• To execute one or more statements that may
  generate exceptions, and to execute the
  statements in the finally clause whether or not
  an exception occured. 

24
A Complete Example

• Program 
• reads coin descriptions from file
•  adds coins to purse
• prints total
• What can go wrong?
• File might not exist
• File might have data in wrong format
• Who can detect the faults?
• main method of PurseTest interacts with user
• main method can report errors
• Other methods pass exceptions to caller

25
The read method of the Coin class

• Distinguishes between expected and unexpected end
  of file
• public boolean read(BufferedReader in) throws
  IOException
• String input in.readLine() if (input
  null) // normal end of file return false
  value Double.parseDouble(input) // may
  throw unchecked NumberFormatException name
  in.readLine() if (name null) // unexpected
  end of file throw new EOFException("Coin
  name expected") return true

26
The read method of the Purse class

• Unconcerned with exceptions
• Just passes them to caller
• public void read(BufferedReader in) throws
  IOException
• boolean done false while
  (!done) Coin c new Coin() if
  (c.read(in)) add(c) else done true

27
The readFile method of the Purse class

• finally clause closes files if exception happens
  
• public void readFile(String filename) throws
  IOException
• BufferedReader in null try in
  new BufferedReader( new FileReader(filename))
  read(in) finally if
  (in ! null) in.close()

28
User interaction in main

• If an exception occurs, user can specify
  another file name
• boolean done false
• String filename JOptionPane.showInputDialog(
  "Enter file name") while (!done)
  try Purse myPurse new Purse()
  myPurse.readFile(filename) System.out.print
  ln("total" myPurse.getTotal()) done
  true

29
catch (IOException exception)
System.out.println("Input/output error "
exception) catch (NumberFormatException
exception) exception.printStackTrace()
// error in file format if (!done)
Filename JOptionPane.showInputDialog("Try
another file") if (filename null)
done true
30
Scenario

• PurseTest.main calls Purse.readFile
• Purse.readFile calls Purse.read
• Purse.read calls Coin.read
• Coin.read throws an EOFException
• Coin.read has no handler for the exception and
  terminates immediately. 
• Purse.read has no handler for the exception and
  terminates immediately
• Purse.readFile has no handler for the exception
  and terminates immediately after executing the
  finally clause and closing the file. 
• PurseTest.main has a handler for an IOException ,
  a superclass of EOFException. That handler prints
  a message to the user. Afterwards, the user is
  given another chance to enter a file name. Note
  that the statement printing the purse total has
  been skipped.

31
File PurseTest.java

• 1 import javax.swing.JOptionPane
• 2 import java.io.IOException
• 3
• 4 /
• 5 This program prompts the user to enter a
  file name
• 6 with coin values. A purse object is filled
  with
• 7 the coins specified in the file. In case of
  an exception,
• 8 the user can choose another file.
• 9 /
• 10 public class PurseTest
• 11
• 12 public static void main(String args)
• 13
• 14 boolean done false
• 15 String filename
• 16 JOptionPane.showInputDialog("Enter
  file name")
• 17

32

• 18 while (!done)
• 19
• 20 try
• 21
• 22 Purse myPurse new Purse()
• 23 myPurse.readFile(filename)
• 24 System.out.println("total"
  myPurse.getTotal())
• 25 done true
• 26
• 27 catch (IOException exception)
  
• 28
• 29 System.out.println("Input/output
  error " exception)
• 30
• 31 catch (NumberFormatException
  exception)
• 32
• 33 exception.printStackTrace()
• 34
• 35
• 36 if (!done)

33

• 38 filename JOptionPane.showInputDia
  log(
• 39 "Try another file")
• 40 if (filename null) done true
  
• 41
• 42
• 43 System.exit(0)
• 44
• 45

34
File Purse.java

• 1 import java.io.BufferedReader
• 2 import java.io.FileReader
• 3 import java.io.IOException
• 4
• 5 /
• 6 A purse computes the total of a collection
  of coins.
• 7 /
• 8 public class Purse
• 9
• 10 /
• 11 Constructs an empty purse.
• 12 /
• 13 public Purse()
• 14
• 15 total 0
• 16
• 17

35

• 18 /
• 19 Read a file with coin descriptions and
  adds the coins
• 20 to the purse.
• 21 _at_param filename the name of the file
• 22 /
• 23 public void readFile(String filename)
• 24 throws IOException
• 25
• 26 BufferedReader in null
• 27 try
• 28
• 29 in new BufferedReader(new
  FileReader(filename))
• 30 read(in)
• 31
• 32 finally
• 33
• 34 if (in ! null) in.close()
• 35
• 36

36

• 38 /
• 39 Read a file with coin descriptions and
  adds the coins
• 40 to the purse.
• 41 _at_param in the buffered reader for
  reading the input
• 42 /
• 43 public void read(BufferedReader in)
• 44 throws IOException
• 45
• 46 boolean done false
• 47 while (!done)
• 48
• 49 Coin c new Coin()
• 50 if (c.read(in))
• 51 add(c)
• 52 else
• 53 done true
• 54
• 55
• 56

37

• 58 Add a coin to the purse.
• 59 _at_param aCoin the coin to add
• 60 /
• 61 public void add(Coin aCoin)
• 62
• 63 total total aCoin.getValue()
• 64
• 65
• 66 /
• 67 Get the total value of the coins in the
  purse.
• 68 _at_return the sum of all coin values
• 69 /
• 70 public double getTotal()
• 71
• 72 return total
• 73
• 74
• 75 private double total
• 76

38
File Coin.java

• 1 import java.io.BufferedReader
• 2 import java.io.EOFException
• 3 import java.io.IOException
• 4
• 5 /
• 6 A coin with a monetary value.
• 7 /
• 8 public class Coin
• 9
• 10 /
• 11 Constructs a default coin.
• 12 Use the read method to fill in the value
  and name.
• 13 /
• 14 public Coin()
• 15
• 16 value 0
• 17 name ""

39

• 18
• 19
• 20 /
• 21 Constructs a coin.
• 22 _at_param aValue the monetary value of the
  coin.
• 23 _at_param aName the name of the coin
• 24 /
• 25 public Coin(double aValue, String aName)
• 26
• 27 value aValue
• 28 name aName
• 29
• 30
• 31 /
• 32 Reads a coin value and name.
• 33 _at_param in the reader
• 34 _at_return true if the data was read,
• 35 false if the end of the stream was
  reached
• 36 /

40

• 38 throws IOException
• 39
• 40 String input in.readLine()
• 41 if (input null) return false
• 42 value Double.parseDouble(input)
• 43 name in.readLine()
• 44 if (name null)
• 45 throw new EOFException("Coin name
  expected")
• 46 return true
• 47
• 48
• 49 /
• 50 Gets the coin value.
• 51 _at_return the value
• 52 /
• 53 public double getValue()
• 54
• 55 return value
• 56

41

• 58 /
• 59 Gets the coin name.
• 60 _at_return the name
• 61 /
• 62 public String getName()
• 63
• 64 return name
• 65
• 66
• 67 private double value
• 68 private String name
• 69
• 70
• 71
• 72
• 73
• 74
• 75
• 76