Java Exception Handling

1
Java Exception Handling

• Exception an event that occurs during the
  execution of a program that disrupts the normal
  flow of instructions
• Examples Hard disk crash Out of bounds array
  access Divide by zero, etc
• When an exception occurs, the executing method
  creates an Exception object and hands it to the
  runtime system--throwing an exception''
• The runtime system searches the runtime call
  stack for a method with an appropriate handler,
  which catches the exception''

2
Java's Exception Handling Policy

• Java is fussy about (some) exceptions, requiring
  that the programmer either
• deal with the exceptions when they occur, using
  try and catch, or
• explicitly hand off the exception to the method
  that calls the method in which the exception
  occurs, effectively passing the buck'' to the
  calling method.
• The exceptions Java is fussy about are called
  checked'' exceptions, because the compiler will
  check that one of the above options is satisfied

3
Advantages of Java Exception Handling

1. Separating Error Handling Code from Regular''
   Code
2. Propagating Errors Up the Call Stack
3. Grouping Error Types and Error Differentiation

4
Advantage 1Separating Error Handling Code from
Regular'' Code
Example Here is pseudocode for reading a file
into memory
readFile open the file determine its
size allocate that much memory read the
file into memory close the file
5
Problems with readFile

• What happens if the file can't be opened?
• What happens if the length of the file can't be
  determined?
• What happens if enough memory can't be allocated?
• What happens if the read fails?
• What happens if the file can't be closed?

6
Error Handling With Traditional Programming
errorCodeType readFile initialize errorCode
0 open the file if (theFileIsOpen)
determine the length of the file
if (gotTheFileLength) allocate that
much memory if (gotEnoughMemory)
read the file into memory
if (readFailed)
errorCode -1
else errorCode -2
else errorCode -3
close the file if
(theFileDidntClose errorCode 0)
errorCode -4 else
errorCode errorCode and -4
else errorCode -5 return
errorCode
7
Error Handling With Java
readFile try open the file
determine its size allocate that much
memory read the file into memory
close the file catch (fileOpenFailed)
doSomething catch (sizeDeterminationF
ailed) doSomething catch
(memoryAllocationFailed) doSomething
catch (readFailed) doSomething
catch (fileCloseFailed) doSomething

Note that the error handling code and regular''
code are separate
8
Advantage 2Propagating Errors Up the Call Stack
readFile
method1 call method2 method2 call
method3 method3 call readFile
call
method3
call
method2
call
method1
Call Stack
Suppose also that method1 is the only method
interested in the errors that occur within
readFile. How does the error code get propogated
up the stack?
9
Propagating Errors Using Traditional Programming
method1 errorCodeType error error
call method2 if (error)
doErrorProcessing else
proceed errorCodeType method2
errorCodeType error error call method3
if (error) return error else
proceed errorCodeType method3
errorCodeType error error call readFile
if (error) return error else
proceed
readFile
return
call
method3
return
call
method2
return
call
method1
Call Stack
10
Propagating Errors Using Java
method1 try call method2
catch (exception) doErrorProcessing
method2 throws exception call
method3 method3 throws exception call
readFile
readFile
call
method3
throw
call
method2
call
method1
Call Stack
11
Advantage 3Grouping Error Types and Error
Differentiation
Java exceptions are first class Java objects, and
so they are grouped into a class hierarchy. For
example
Exception
RunTimeException
IOException
NullPointerException
FileNotFoundException
ArithmeticException
MalformedURLException
12
Example
Suppose (remember, this is pseudocode)
method1 try call method2
catch(Exception) doErrorProcessing
method2 throws ArithmeticException
... call method3 ... method3 throws
FileNotFoundException ... call method4
... method4 throws NullPointerException
...
13
Throwing Structure
method4
call
method3
call
throw
method2
call
method1
Call Stack
In this case, the exception handler in method1
must handle all exception types
14
Modified Example
method1 try call method2
catch(RunTimeException)
doErrorProcessing method2 throws
ArithmeticException ... call method3
... method3 throws FileNotFoundException
... call method4 ... method4 throws
NullPointerException ...
15
New Throwing Structure
method4
call
method3
throw
call
method2
call
method1
Call Stack
In this case, method1 will handle only
runtime exceptions. Any FileNotFoundException
thrown by method3 will have to be handled
further down the stack.
16
Another Modified Example
method1 try call method2
catch(RunTimeException)
doErrorProcessing method2 throws
ArithmeticException try ... call
method3 ... catch(IOException)
doMoreErrorProcessing method3 throws
FileNotFoundException ... call method4
... method4 throws NullPointerException
...
17
New Throwing Structure
method4
call
method3
throw
call
method2
call
method1
Call Stack
18
FileCopy Example Revisited
import java.io. public class FileCopy
public static void main(String args) throws
IOException BufferedReader br
new BufferedReader(new InputStreamReader(Syst
em.in)) FileReader r FileWriter
w System.out.print("Source file name
") String inFileName br.readLine()
r new FileReader(inFileName)
System.out.print("Destination file name ")
String outFileName br.readLine() w
new FileWriter(outFileName) int c
while ((c r.read()) ! -1)
w.write(c) w.flush()
19
If The Input File Does Not Exist
In the example, the main method chooses to
pass the buck,'' but there is nowhere to pass
it to. Thus, the program will crash
4 java FileCopy Source file name foo Exception
in thread "main" java.io.FileNotFoundException
foo (No such file or directory) at
java.io.FileInputStream.open(Native Method) at
java.io.FileInputStream.ltinitgt(FileInputStream.jav
a103) at java.io.FileInputStream.ltinitgt(FileInpu
tStream.java66) at java.io.FileReader.ltinitgt(Fil
eReader.java39) at FileCopy.main(FileCopy.java1
2) 5
20
Catching the Exception
public class FileCopy public static void
main(String args) throws IOException
BufferedReader br new
BufferedReader(new InputStreamReader(System.in))
FileReader r FileWriter w
System.out.print("Source file name ")
String inFileName br.readLine() try
r new FileReader(inFileName)
catch(FileNotFoundException ex)
System.out.println(ex.getMessage())
System.out.print("Source file name ")
inFileName br.readLine()
r new FileReader(inFileName)
...
21
Catching the Exception (cont'd)
This approach will catch the first instance of a
FileNotFoundException, but only that instance
5 java FileCopy Source file name foo foo (No
such file or directory) Source file name
bar Exception in thread "main" java.io.FileNotFoun
dException bar (No such file or directory) at
java.io.FileInputStream.open(Native Method) at
java.io.FileInputStream.ltinitgt(FileInputStream.jav
a103) at java.io.FileInputStream.ltinitgt(FileInpu
tStream.java66) at java.io.FileReader.ltinitgt(Fil
eReader.java39) at FileCopy.main(FileCopy.java1
9) 6
Note that you can use the getMessage()
method (inherited by the Exception class) in your
handler.
22
Generating True Looping Behavior
public class FileCopy public static void
main(String args) throws IOException
BufferedReader br new
BufferedReader(new InputStreamReader(System.in))
FileReader r null FileWriter w
null boolean fileFound do
fileFound true
System.out.print("Source file name ")
String inFileName br.readLine()
try r new FileReader(inFileNam
e) catch(FileNotFoundEx
ception ex) fileFound false
System.out.println(ex.getMessage())
while ( !fileFound )
...
23
New Output
226 java FileCopy Source file name foo foo (No
such file or directory) Source file name bar bar
(No such file or directory) Source file name
foo foo (No such file or directory) Source file
name bar bar (No such file or directory) Source
file name X.java Destination file name
Y.java 227
24
Another Way
public static void main(String args) throws
IOException BufferedReader br
new BufferedReader(new InputStreamReader(Syst
em.in)) FileReader r
getFileReader(br) FileWriter w
getFileWriter(br) int c while
((c r.read()) ! -1) w.write(c)
w.flush()
25
Generating Looping Behavior Through Recursion
private static FileReader getFileReader(Buffer
edReader br) System.out.print("Source
file name ") try String
inFileName br.readLine() return
new FileReader(inFileName)
catch(IOException ex)
System.out.println(ex.getMessage())
return getFileReader(br)

Note No looping code needed. Since
the exception is caught, no throws clause
is necessary.
26
Generating Looping Behavior Through Recursion
(cont'd)
private static FileWriter getFileWriter(BufferedRe
ader br) System.out.print("Destination
file name ") try String
outFileName br.readLine() return
new FileWriter(outFileName)
catch(IOException ex)
System.out.println(ex.getMessage())
return getFileWriter(br)
Now the only calls in main that can throw an
exception are read, write, and flush
27
A Version of main that Does Not throw
public static void main(String args)
BufferedReader br new
BufferedReader(new InputStreamReader(System.in))
FileReader r getFileReader(br)
FileWriter w getFileWriter(br) int
c try while ((c
r.read()) ! -1) w.write(c)
w.flush()
catch(IOException ex)
System.out.println(ex.getMessage())

Now the program will not crash if/when an I/O
error occurs.
28
Checked vs. Unchecked Exceptions

• A program that catches all I/O exceptions may
  still experience a different type of exception
  (say, a runtime exception)
• I/O exceptions are checked'' by the compiler
• That is, you are required to either catch them or
  mention them in a throws clause
• Unlike I/O exceptions, runtime exceptions are
  unchecked''
• That is, you can choose to ignore them at your
  peril

29
Some Checked Exceptions

• IOException and any of its subclasses
• InterruptedException (thrown when a thread is
  interrupted)
• Any exception you invent by subclassing Exception

30
Some Unchecked Exceptions

• Subclasses of RuntimeException
• ArithmeticException
• IllegalArgumentException
• NumberFormatException
• IndexOutOfBoundsException
• ArrayIndexOutOfBoundsException
• StringIndexOutOfBoundsException
• NullPointerException
• Those that you invent by subclassing
  RuntimeException

31
Exceptions vs. Errors

• Exceptions are processing conditions that a
  program ought to be able to recover from if it is
  robust enough
• Errors are conditions that are ordinarily not
  recoverable
• Death of a thread
• Linking error
• Virtual machine error

32
The Throwable Class

• Exceptions and Errors are both subclasses of the
  Throwable class

Object
Throwable
Error
Exception
LinkageError
ThreadDeath
VirtualMachineError