Exceptions

1
Exceptions

• CSC 171 FALL 2004
• LECTURE 24

2
READING

• Read Horstmann Chapter 14
• This course covered Horstmann Chapters 1 - 15

3
EXAM

• Thursday 12/9 in class
• Chapters 11, 13-15

4
Make up exam

• Friday 12/3 1215PM-125PM
• CSB 703

5
GRADING

• OLD
• Midterm 10
• Projects (4) 40
• Final 15
• Quizes (10) 10
• Labs (15) 15
• Workshops 10

• NEW
• Exam 1 13
• Projects (4) 40
• Exam 2 12
• Quizes (6) 5
• Labs (21) 20
• Workshops 10

6
Errors in Programming
7
Errors in Programming

• Sometimes cause by our code
• Sometimes caused by external code
• Reasonable to take precautions

8
Two aspects of errors

• Recognizing when an error occurs
• Dealing with the errors

9

• There are two aspects of handling failure
  ________________ and ________________.

10

• There are two aspects of handling failure
  ___detection____ and ____recovery____________.

11
Old School

• have the method return an indication of
  success/failure (an error code)
• x.doSomething()
• // becomes
• if (x.doSomething() -1) return false

12
New School

• Java has an exception handling mechanism which
  can require potential errors to be recognized.
• This mechanism is flexible and efficient.

13

• In Java, ___________________________ provides a
  flexible mechanism for passing control from the
  point of error detection to a recovery handler.

14

• In Java, __exception handling_______ provides a
  flexible mechanism for passing control from the
  point of error detection to a recovery handler.

15
Exception Objects
16
THROWING EXCEPTIONS

• If I am a method
• AND If a problem occurs
• then I can deal with it by
• Constructing a new exception object describing
  the problem
• Throw the object to the method that invoked me
  (I have to terminate to do this)

17
Example

• public class BankAccount public void
  withdraw(double amount) if (amount gt
  balance) throw new IllegalArgumentException(
  "Amount exceeds balance") balance
  balance - amount ...

18

• To signal an exceptional condition, use the
  _____________ statement to throw an
  _____________________ object.

19

• To signal an exceptional condition, use the
  ____throw____ statement to throw an
  __________exception____ object.

20

• When you throw an exception, the current method
  _____________________.

21

• When you throw an exception, the current method
  __terminates______.

22
CHECKED/UNCHECKED

• Its hard to anticipate all possible exceptions
  at compile time.
• The compiler checks to see if we deal with
  exceptions (checked exeptions)
• Unchecked exceptions are not enforced by the
  compiler.

23

• import java.io.BufferedReader
• import java.io.InputStreamReader
• import java.io.IOException
• public class Console
• public static void main(String args)
• InputStreamReader isreader new
  InputStreamReader(System.in)
• BufferedReader console new BufferedReader(isrea
  der)
• String input1 console.readLine()
• System.out.println(input1)

24

• import java.io.BufferedReader
• import java.io.InputStreamReader
• import java.io.IOException
• public class Console
• public static void main(String args) throws
  IOException
• InputStreamReader isreader new
  InputStreamReader(System.in)
• BufferedReader console new BufferedReader(isrea
  der)
• String input1 console.readLine()
• System.out.println(input1)

25

• import java.io.BufferedReader
• import java.io.InputStreamReader
• import java.io.IOException
• public class Console
• public static void main(String args)
• InputStreamReader isreader new
  InputStreamReader(System.in)
• BufferedReader console new BufferedReader(isrea
  der)
• try
• String input1 console.readLine()
• System.out.println(input1)
• catch (IOException e)
• System.out.println(Problem)

26
Unchecked Exceptions

• int k Integer.parseInt(Hello World)
• int a 2,3,4,5,6,7,8,9
• a20 5

27

• There are two kind of exceptions
  ______________________ and _____________________
  exceptions.

28

• There are two kind of exceptions
  ___checked_______ and ___unchecked________
  exceptions.

29

• Unchecked exceptions extend the class
  _________________________ or _________________.

30

• Unchecked exceptions extend the class
  _RuntimeException______ or _____Error______.

31

• Checked exceptions are due to ____________________
  ______________. The compiler checks that your
  program handles these exceptions.

32

• Checked exceptions are due to _external
  circumstances___. The compiler checks that your
  program handles these exceptions.

33

• Add a _____________________ specifier to a method
  that can throw a checked exception.

34

• Add a __throws_______ specifier to a method that
  can throw a checked exception.

35
(No Transcript)
36

• You can design your own exception types
  subclasses of _______________________ or
  ______________________.

37

• You can design your own exception types
  subclasses of __Exception____ or
  ___RuntimeException_.

38

• public class InsufficientFundsException
• extends RuntimeException
• public InsufficientFundsException()
   public InsufficientFundsException(String
  reason) super(reason)

39
TRY/CATCH

• 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

40

• 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")

41

• In a method that is ready to handle a particular
  exception type, place the statements that can
  cause the exception inside a ____________________,
  place the handler inside a ______________________
  ________.

42

• In a method that is ready to handle a particular
  exception type, place the statements that can
  cause the exception inside a ________try
  block________, place the handler inside a __catch
  clause_________.

43

• It is better to _________________________________
  than to _______________________.

44

• It is better to ________give_______________ than
  to ________ receive _________.

45
finally

• 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"

46

• Once a try block is entered, the statements in a
  ___________________ clause are guaranteed to be
  executed, whether or not an exception is thrown.

47

• Once a try block is entered, the statements in a
  ______ finally _____ clause are guaranteed to be
  executed, whether or not an exception is thrown.

48
Example

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