CSCI 143

1
CSCI 143

• Chapter 15
• Exception Handling

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Exception

• A problem that occurs during program execution
• An exception to the rule
• An object, with data and methods
• Is thrown by a method or the interpreter
• Can be handled or controlled
• Creates robust, fault-tolerant, programs
• If uncaught, will crash the program

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Exception Handling

• try block
• enables exception handling
• contains statements that might cause exceptions
• catch block (exception handler)
• specifies an exception type
• contains code to deal with handle an exception
• need one catch for each exception type

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Try-Catch Syntax

• try
• //code that might throw an exception
• catch (ExceptionType name)
• //do something about it
• //processing resumes here

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Exception Handling

• Throw point where the exception occurs
• Try block terminates at throw point
• Program looks for a matching catch clause
• identical types
• exception is a subtype of the declared type
• If match is found, catch handler executes
• If no match is found, exceptions propagate up the
  call stack (stack unwinding)

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Example

• String s keyboard.readString()
• try
• double d Double.parseDouble(s)
• catch (NumberFormatException e)
• System.out.println (Invalid number)

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Exception Hierarchy

• Throwable
• Subclass of object
• Superclass for all exceptions
• Exception
• Can be caught and handled
• Example IOException
• Error
• Can not typically be caught by the program
• Example OutOfMemoryError

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Exception Hierarchy
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Checked Exceptions

• The Java compiler requires Exceptions which are
  not descended from RuntimeExceptions to be handled

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Checked Exceptions

• public void readData() throws FileNotFoundExceptio
  n
• FileReader frd new FileReader(test.txt)
• FileReader constructor might throw an exception
• readData() throws the exception to the method
  that called it
• Method that called readData() will either
• catch the exception or
• throw it further up the call sequence

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Unchecked Exceptions

• All direct and indirect subclasses of
  RuntimeException
• Package java.lang
• Can typically be prevented by proper coding
• Are not required to be listed in a methods
  throws clause

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Practice

• Are the following exceptions checked or
  unchecked?
• ArithmeticException
• IOException
• ClassCastException
• InterruptedException
• UnsupportedFlavorException
• FileNotFoundException

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Catching Multiple Exceptions
throws FileNotFoundException

• try
• BufferedReader rdr
• new BufferedReader (new FileReader
  (Text.txt))
• String s rdr.readLine()
• double d Double.parseDouble (s)
• catch (FileNotFoundException e)
• System.out.println (Cannot find file)
• catch (IOException e)
• System.out.println (Cannot read file
  e.getMessage())
• catch (NumberFormatException e)
• System.out.println (Invalid input)

throws IOException
throws NumberFormatException
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Catching Multiple Exceptions

• All exceptions can be caught by catching a common
  superclass
• try
• BufferedReader rdr
• new BufferedReader (new FileReader
  (Text.txt))
• String s rdr.readLine()
• double d Double.parseDouble (s)
• catch (Exception e)
• System.out.println (Something is wrong!)
• May result in a loss of error handling precision
• May be used to handle errors in the same
  inheritance hierarchy polymorphically!

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finally Clause

• Always executes
• Used for cleanup (releases resources)
• database and network connections, files
• Executes before catch clause returns or exits
• try
• //code that might generate an exception
• catch
• //code to handle the exception
• finally
• //cleanup here

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In Conclusion

• Good programs catch and handle errors
• Exception handling is designed to handle runtime
  errors
• Class Exception and its subclasses can be caught
  and handled
• Descendents of RuntimeException are unchecked
• Methods can handle their own exceptions or throw
  them up the call sequence