C Sc 335 4Sep03 More Java from Ch1 Horstmann

1
C Sc 335 4-Sep-03More Java from Ch1 Horstmann

• When programs run, exception events occur (a 3rd
  thing that is sure in life).
• Examples of "exceptional events" include
• Division by 0
• Attempt to open and it fails
• Array subscript out of bounds
• Trying to convert a string that is not a number
  into a number
• Can deal with these via exception handling.
• General form on next slide

2
Handling exceptional events

• Put code that "throws" exceptional event into a
  try block and add a catch block.
• try
• code that cause an exceptional event
  (throws an exception)
• catch (Exception anException)
• code that executes when the code in try
  above throws an exception
• If the code in the try block causes an
  except-ional event, control transfer to the catch
  block.

3
Example

• ParseDouble may be passed a string that does not
  represent a valid number.
• String numberAsString depositField.getText()
• double amount 0.0
• try
• // the next message may "throw an exception"
• amount Double.parseDouble(numberAsString)
• System.out.println("this may not be sent to
  System.out")
• catch (NumberFormatException nfe)
• JOptionPane.showMessageDialog(null,
• "Not a valid deposit amount"
  numberAsString)

4
parseDouble documentation

• public static double parseDouble(String s)
• throws NumberFormatException
• Returns a number new represented by s
• Parameters s - the string to be parsed.
• Returns the double value represented by the
  string argument.
• Throws NumberFormatException - if the string
  does not represent a valid number, 1o0.0 for
  example.
• Many methods throw an exception
• Your methods could also

5
A small piece of Java's large Exception hierarchy

• Code that throws RuntimeException need not be in
  a try block, All others must be "tried" (there is
  an alternative).

6
Examples of Exceptions

• parseDouble and parseInt when the String argument
  does not represent a valid number.
• Integer expressions that result in division by 0.
• Sending a message to an object when the reference
  variable has the value of null.
• Indexing exceptions
• attempting to access an ArrayList element with an
  index that is out of range or a character in a
  string outside the range of 0 through length()-1

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Two Examples of Exceptions

• String str null
• String strAsUpperCase str.toUpperCase()
• Output
• Exception in thread "main" java.lang.NullPointerEx
  ception at main(OtherRunTimeExceptions.java6)
• ArrayList stringList new ArrayList()
• stringList.add("first")
• String third (String) stringList.get(1)
• Output
• IndexOutOfBoundsException Index 1, Size 1
• java.util.ArrayList.RangeCheck(ArrayList.java491)
  
• at java.util.ArrayList.get(ArrayList.java307)
• at main(OtherRunTimeExceptions.java10)

8
Throwing your own Exceptions

• throws and throw are keywords.
• The object after throw must be an instance of a
  class that extends the Throwable class.
• public void deposit(double depositAmount)
• throws IllegalArgumentException
• // Another way to handle preconditions
• if (depositAmount lt 0.0)
• throw new IllegalArgumentException()
• my_balance my_balance depositAmount

9
Input/Output Streams

• Input is read through input stream objects.
• Output is written through output stream objects.
• A stream is a sequence of items read from some
  source or written to some destination.
• Let's replace TextReader with standard Java
  classes.

10
Standard input/output

• Java has two objects for standard I/O
• System.in The "standard" input stream object (an
  instance of InputStream)
• System.out The "standard" output stream object
  (an instance of PrintStream)
• InputStream has readLine
• PrintStream has all of the print and println
  methods that you have been using
• print(Object) print(double) print(int)
  
• println(Object) println(double) println(int)

11
Standard Input

• InputStreamReader bytesToChar
• new InputStreamReader(System.in)
• // Can only read chars now, so decorate
  bytesToChar
• // with and instance of BufferedReader so to
• // get the added funcrtionality of readLine
• BufferedReader keyboard
• new BufferedReader(bytesToChar)
• String line ""
• System.out.println("Enter a line of text")
• try
• // Java forces you to try to read from keyboard
• line keyboard.readLine()
• catch (IOException ioe)
• System.out.println("Couldn't read keyboard")

12
JOptionPane

• import javax.swing.JOptionPane
• public class JOption
• public static void main(String args)
• String prompt "Enter a number to square"
• String numberAsString
• JOptionPane.showInputDialog(null, prompt)
• double number Double.parseDouble(numberAsStrin
  g)
• // Note Java also has Integer.parseInt("123")
• // No user input this time (other than button
  clicks)
• JOptionPane.showMessageDialog(null,
• number "2 " Math.pow(number,
  2))