CS1101: Programming Methodology Recitation 7

1
CS1101 Programming MethodologyRecitation 7
Exceptions
2
Exception Handling

• Program correctness
• Guarantees correct results for all valid input
• Program robustness/reliability
• Ability to run under various conditions
• Does not crash when wrong type of argument is
  passed to method or invalid input valid is
  entered
• Use mechanism called exception handling

3
Catching Exceptions

• An error condition occurs during the normal
  course of program execution
• Exception is thrown
• Exception handling routine is executed
• Exception is caught
• Case Study Age Input

4
Version 1 - No exception handling

• import java.io.
• import java.lang.
• import java.util.
• class AgeInputVer1
• private static final String
  DEFAULT_MESSAGE "Your age"
• public AgeInputVer1()
• public int getAge() throws IOException
• return getAge (DEFAULT_MESSAGE)
• public int getAge(String prompt) throws
  IOException
• BufferedReader stdin new
  BufferedReader(
• new
  InputStreamReader(System.in))
• System.out.println (prompt)

5
Program gets a persons age and outputs the year
in which the person was born.

• public class AgeInputMain
• public static void main(String args)
  throws IOException
• Calendar today
  Calendar.getInstance()
• int age, thisYear, bornYear,
  answer
• AgeInputVer1 input new
  AgeInputVer1()
• age input.getAge ("How old are
  you ?")
• thisYear today.get
  (Calendar.YEAR)
• bornYear thisYear - age
• System.out.println ("You are born
  in " bornYear)

What happens if user spells out age, e.g. nine,
instead of 9 ?
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AgeInputVer1

• What happens if user spells out age, e.g. nine,
  instead of 9 ?
• A number format exception is thrown
• Input value nine cannot be converted to an
  integer by using parseInt method.
• System will handle the thrown exception by
  displaying the error message
• java AgeInputMain
• How old are you ?
• nine
• Exception in thread "main" java.lang.NumberFormatE
  xception For input string "nine"
• gt at java.lang.NumberFormatException.forInput
  String(NumberFormatException.java48)
• at java.lang.Integer.parseInt(Integer.java468)
  
• at java.lang.Integer.parseInt(Integer.java518)
  
• at AgeInputVer1.getAge(AgeInputMain.java24)
• at AgeInputMain.main(AgeInputMain.java35)

7
Version 2 Loop until valid input

• public class AgeInputVer2
• private static final String DEFAULT_MESSAGE
  Your age
• public AgeInputVer2()
• public int getAge() throws IOException
• return getAge (DEFAULT_MESSAGE)
• public int getAge(String prompt) throws
  IOException
• BufferedReader stdin new BufferedReader(new
  InputStreamReader(System.in))
• int age
• while (true)
• System.out.println (prompt)
• String inputStr stdin.readLine()
• try
• age Integer.parseInt (inputStr)
• return age // input OK, return value and
  exit
• catch (NumberFormatException e)

Need to specify which exception we are catching
8
AgeInputVer2

• Loop until valid input

• java AgeInputMain
• How old are you ?
• nine
• Input is invalid. Please enter digits only
• How old are you ?
• ten
• Input is invalid. Please enter digits only
• How old are you ?
• 10
• You are born in 1994

9

• public class AgeInputVer3
• private static final String DEFAULT_MESSAGE
  Your age
• public AgeInputVer3()
• public int getAge() throws IOException
• return getAge (DEFAULT_MESSAGE)
• public int getAge(String prompt) throws
  IOException
• BufferedReader stdin new BufferedReader(new
  InputStreamReader(System.in))
• int age
• while (true)
• System.out.println (prompt)
• String inputStr stdin.readLine()
• try
• age Integer.parseInt (inputStr)
• if (age lt 0)
• throw new Exception(Negative age is
  invalid)
• return age

Version 3 Catch invalid negative integer
Multiple catch blocks checked in sequence List
more specialized exceptions first
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AgeInputVer3

• Catch invalid negative integer

• java AgeInputMain
• How old are you ?
• twelve
• Input is invalid. Please enter digits only
• How old are you ?
• -12
• Error Negative age is invalid
• How old are you ?
• 12
• You are born in 1992

11
AgeInputVer3

• What happens if we reverse the order of the catch
  blocks ?
• try
• catch (Exception e)
• catch (NumberFormatException e)

Second catch block will never be executed
because any exception object will match the
first catch block.
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AgeInputVer4

• Restrict valid input by specifying lower and
  upper bounds
• Client specify lower and upper bound as the time
  of object creation
• AgeInputVer4 input new AgeInputVer4 (10, 20)
• How should getAge respond when input is outside
  the range of client designated lower and upper
  bounds ?
• Instead of catching it, getAge should propagate
  the thrown exception back to caller (client) to
  handle since the condition is set by the client.
• Dont catch an exception that is thrown as a
  result of violating the condition set by client
  programmer.
• Propagate the exception back to client
  programmers code to handle

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Version 4 Restrict valid input by specifying
lower and upper bounds

• public class AgeInputVer4
• private static final String DEFAULT_MESSAGE
  Your age
• private static final int DEFAULT_LOWER_BOUND
  0
• private static final int DEFAULT_UPPER_BOUND
  99
• private int lowerBound
• private int upperBound
• public AgeInputVer4()
• setBounds (DEFAULT_LOWER_BOUND,
  DEFAULT_UPPER_BOUND)
• public AgeInputVer4(int low, int high) throws
  IllegalArgumentException
• if (low gt high)
• throw new IllegalArgumentException (lower
  bound is higher than upper bound)
• else
• setBounds (low, high)

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Version 4 Restrict valid input by specifying
lower and upper bounds

• public int getAge() throws Exception
• return getAge (DEFAULT_MESSAGE)
• public int getAge(String prompt) throws
  Exception
• BufferedReader stdin new BufferedReader(new
  InputStreamReader(System.in))
• int age
• while (true)
• System.out.println (prompt)
• String inputStr stdin.readLine()
• try
• age Integer.parseInt (inputStr)
• if (age lt lowerBound age gt upperBound)
• throw new Exception(Input out of bound)
• return age
• catch (NumberFormatException e)
• System.out.println (Input is invalid.
  Please enter digits only)

Propagate exception
No catch block for Exception
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AgeInputVer4

• Dont catch an exception that is thrown as a
  result of
• violating the condition set by client
  programmer.
• Propagate the exception back to client
  programmers code to handle

• java AgeInputMain
• How old are you ?
• twelve
• Input is invalid. Please enter digits only
• How old are you ?
• 5
• Exception in thread "main" java.lang.Exception
  Input out of bound
• at AgeInputVer4.getAge(AgeInputMain.java1
  22)
• at AgeInputMain.main(AgeInputMain.java139
  )

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AgeInputVer4
If client specify lower and upper bound at the
time of object creation as AgeInputVer4 input
new AgeInputVer4 (20, 10)

• java AgeInputMain
• Exception in thread "main" java.lang.IllegalArgume
  ntException lower bound is higher than upper
  bound
• at AgeInputVer4.ltinitgt(AgeInputMain.java9
  6)
• at AgeInputMain.main(AgeInputMain.java138
  )