Exceptions

1
Exceptions

• Protecting methods from errors
• CATCHING an exception protects your program from
  bad users
• THROWING an exception protects others who use
  your methods

2
catching an error

• try
• x Integer.parseInt( userString )
• catch (Exception e)
• System.out.println(Not an integer!)

3
trycatch

• Tries the try-bracketed statement.
• try
• // whatever is here
• Stops at an error. Does not complete.
• Error info is placed into the object e, of the
  Exception class - can decipher ALL exceptions
• catch (Exception e)
• // do something here

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Exception class

• catch (Exception e)
• declares an object e of the Exception class
• there are methods to be used
• catch (Exception e)
• e.printStackTrace( ) // where the error was
• System.out.println(user did e.getMessage(
  ) )
• // what input
  caused error
• System.out.println(computer thought
  e.toString( ) )
• // the
  computers description of
• // the
  message

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Summary - Exception methods( )

• catch (Exception e)
• e.getMessage( ) - what the user typed that
  generated the error
• e.toString( ) - the computers description of the
  message
• e.printStackTrace( ) - list all the methods from
  main, to the one that generated the error

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catch different types

• try
• myClass myObject new myClass()
• myObject.myMethod( )
• // method 2( )
• // method 3( )
• catch (NullPointerException e)
• catch (DivideByZeroException e)
• System.out.println(Cant divide by zero)

7
different types

• NullPointerException - method called in try, was
  supposed to return an object (like Color), but
  didnt (user hit cancel in JColorChooser)
• ArrayIndexOutOfBoundsException - method called
  in try, accessed index greater than the array
  definition.

8
different types

• NumberFormatException - method called in try,
  like parseInt, tried to convert a string to a
  number, and the string wasnt a digit.
• ArithmeticException - method called in try, did
  something arithmetically wrong - like mixing
  variable types.

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different types

• AWTException - method called in try, attached a
  handler object that was inappropriate for the
  component (a MouseListener to a JButton, or an
  object not of an ActionListener class).
• DivideByZeroException - method called in try, did
  a divide by zero.

10
throwing an exception

• public myClass
• public double myMethod( int q) throw
  DivideByZeroException
• if (q 0)
• throw new DivideByZeroException( )
• else
• return(10/q)

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User Defined Exceptions
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1. extending Exception

• class HighTempException extends Exception
• public HighTempException()
• super(Temperature Exceeds Safe Level")

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super gives us all the necessary methods

• getMessage( ) - what the user typed that
  generated the error
• toString( ) - the computers description of the
  message
• printStackTrace( ) - list all the methods from
  main, to the one that generated the error

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2. throwing

• public void increasePowerLevel() throws
  highTempException
• powerLevel powerLevel 10
• temperature powerLevel 100
• if (temperature gt 10000)
• throw new highTempException()
• // end increasePower

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3. using (i.e. catching)

• try
• chernobyl.increasePowerLevel()
• catch (highTempException hte)
• hte.printStackTrace()
• System.exit(0)