Applets

1
Applets Applications

• CSC 171 FALL 2001
• LECTURE 15

2
History FORTRAN

• 1954 - John Backus proposed the development of a
  programming language that would allow uses to
  express their problems in commonly understood
  mathematical formulae -- later to be named FORTRAN

3
Change in Schedule

• 11/12 Chapter 14 Exception Handling
• 11/16 Midterm Hoyt 8AM
• 11/19 Chapters 11, 12, 13
• Graphics GUIs (optional)
• Enjoy the Turkey
• 11/26 Chapter 16 Files Streams
• 11/29 Project due
• 12/3 Chapter 19 Data Structures
• 12/11 last lecture
• 12/19 FINAL EXAM - 12/19 4PM

4
Exam Friday 11/16

• 8 AM - 9 AM Hoyt
• Chapters 1-10
• end of chapter questions make good study materiel
• Labs
• Projects
• Workshops
• Multiple choice (20 _at_ 2)
• Some Write a method/class that . . . (10 _at_ 6)

5
Throwing Exceptions

• What should a method do when a problem is
  detected?
• Traditionally, methods return some special code
  to indicate failure
• However,
• The caller may forget to check the return value
• The caller may not be able to fix it

6
Problems with return signals

• If the caller forgets to check
• Bad data is processed
• String input myTextBox.getText()
• Double d new Double(input) // what if
  I type hello
• i d.doubleValue()
• If the caller cant fix it
• We have to punt to the callers caller
• x.doStuff() // becomes
• If (!x.doStuff()) return false //man, thats a
  lot of code

7
Exception-handling mechanism to the rescue

• Exceptions cant be overlooked
• Excepts can be handled by a competent handler
• Not just the caller

8
Simple Try Block Syntax

• try
• statement
• statement
• catch (ExceptionClass ExceptionObject)
• statement
• statement

9
Example Console Input

• import java.io.
• public class ReadingDoubles
• public static void main (String args)
• double i, j
• try
• // open the console for
  bufferd I/O
• InputStreamReader reader
• new InputStreamReader(Sys
  tem.in)
• BufferedReader console
• new BufferedReader(reader
  )

10
Alternately

• // instead of
• InputStreamReader reader
• new InputStreamReader(Sys
  tem.in)
• BufferedReader console
• new BufferedReader(reader
  )
• // could use nested constructors
• BufferedReader console
• new BufferedReader(new
  InputStreamReader(System.in))

11
What happens?

• public static void main (String args)
• BufferedReader console
• new BufferedReader(InputStreamReader(Syste
  m.in))
• System.out.print("Please enter a floating
  point number ")
• String input console.readLine()
• Double d new Double(input)
• double d1 d.doubleValue()

12
This happens

• cd d/courses/CSC171/CSC171FALL2001/code/
• d/devenv/jdk1.3/bin/javac Console1Bad.java
• Console1Bad.java14 unreported exception
  java.io.IOException must be caught or declared
  to be thrown
• String input console.readLine() //read a
  String
• 1 error
• Compilation exited abnormally with code 1 at Mon
  Nov 12 211452

13
Deal with the potential exception

• try
• String input console.readLine()
• catch (IOException e)
• System.out.println(e bad read)
• System.exit(0)

14
Example

• //get the value
• System.out.print("Please enter a floating point
  number ")
• try
• input console.readLine() //read a String
• catch (IOException e)
• System.out.println(e " bad read ")
• System.exit(0)

15
Now check the number format

• try
• Double d new Double(input)
• double d1 d.doubleValue()
• // Calculate the sum printout
• System.out.println(d1 "2 " (d1d1))
• catch (NumberFormatException e)
• System.out.println(e
• " What you entered was not a double")

16
Checked Unchecked Exceptions

• Java Exceptions fall into two categories
• Checked
• You MUST tell the compiler what you are going to
  do about the exception
• Unchecked (sub-classes of RuntimeException)
• NumberFormatException
• IllegalArgumentException
• NullPointerException

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Exception Class Hierarchy
18
Why Checked Unchecked?

• Checked Exceptions are not your fault
• So, you have to deal with them
• Unchecked Exceptions are your fault
• So, we trust you to deal with them
• You have to deal with things you cannot prevent!

19
Cheap hacks

• Lazy empty clauses
• try System.in.read()
• catch (IOException e)
• Punt the exception to the caller
• public static void main(String args) throws
  IOException

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Alternate version I

• import java.io.
• public class ReadingDoubles3
• public static void main (String args)
  throws IOException
• InputStreamReader reader new
  InputStreamReader(System.in)
• BufferedReader console new BufferedReader(reade
  r)
• //get the first value
• System.out.print("Please enter a floating point
  number ")
• String input console.readLine() //read a
  String
• Double d new Double(input)
• double d1 d.doubleValue()

21
Alternate Version II

• // Calculate the sum printout
• System.out.println(d1 "2 " (d1d1))
• // wait for user to end
• System.out.println(" Hit return to exit")
• System.in.read()

22
Throwing your own exeptions

• public static double myfun(double
  ValueBetweenZeroAndOne)
• if ((ValueBetweenZeroAndOne lt 0)
• ( ValueBetweenZeroAndOne gt 1))
• throw new IllegalArgumentException("RTFM -
  you dolt!")
• return ValueBetweenZeroAndOne
  ValueBetweenZeroAndOne

23
Making them deal with it

• public static double myfun(double
  ValBetZeroAndOne) throws Exception // now, they
  have to write a handler
• if ((ValBetZeroAndOne lt 0) ( ValBetZeroAndOne
  gt 1))
• throw new IllegalArgumentException("RTFM you
  dolt!")
• return ValBetZeroAndOne ValBetZeroAndOne

24
Defining your own exceptions

• public class DivideByZeroException extends
  RuntimeException
• public DivideByZeroException()
• super(Attempt to divide by Zero)
• public DivideByZeroException( String msg)
  
• super(msg)

25
Defining your own checked exceptions

• public class myCheckException extends Exception
• public myCheckException()
• super(Attempt to divide by Zero)
• public myCheckException( String msg)
• super(msg)

26
Finally

• try
• statements
• catch (ExceptionClass ExceptionObject)
• statements //exception specific
• finally
• statements //clean up

27
Finally

• The Finally block executes regardless of what
  happens in the try/catch
• Good for cleaning up resources
• Good for conditions where the catch blocks may
  throw exceptions.