JAVA EXCEPTION HANDLING N.V.Raja Sekhar Reddy www.technolamp.co.in Want more interesting... Watch and Like us @ https://www.facebook.com/Technolamp.co.in subscribe videos @ http://www.youtube.com/user/nvrajasekhar

1. Exception Handling

2. Throwing Exceptions
 What is a good program? A program that is
reliable?
 Not just giving correct answer on correct input
 Should protect against possible errors (invalid
password, not a picture file, etc)

3. Good programs
 Need a robust program, reliable in the eyes of a
novice user
 It should not (“cannot”) crash easily - what if
Word always crashed when you hit the control
key?

4. Good Programs
 How do we ensure a robust program?
 Option 1 – return a special value to determine if
the method succeeded or failed
 Ex. Make withdraw return true or false
 What’s wrong with this?
 Calling method may not check answer
 Calling method may not know what to do

5. Good Programs
 Option 2 - Use exceptions
 Exception – an error condition that can occur
during the normal course of a program
execution
 Exception handling is another form of control
structure (like ifs and switch statements)
 When an error is encountered, the normal flow of
the program is stopped and the exception is handled

6. Exceptions
 We say an exception is thrown when it occurs
 When the exception-handling code is executed,
the error is caught
 Examples:
 Dividing by zero
 Accessing a null object

7. Exceptions
 What happens when exceptions occur?
 An Exception object is thrown
 What happens when an Exception is thrown?
 normal execution stops and exception handling begins
 What does the Exception object know?
 the name of the problem
 the location where it occurred
 and more…

8. Exceptions
 Why use exceptions?
 consistency (everyone else does it)
 Java API classes use exceptions.
 Other programming languages do too!
 flexibility
 programmer can decide how to fix problems.
 simplicity
 Easy to pinpoint problems.

9. Handling Exceptions
 Do nothing
 program crashes if the exception occurs!
 Propagate (throw) it
 tell the method’s caller about it and let them decide what
to do
 Resolve (try-catch) it in our method
 fix it or tell the user about it and let them decide what to
do

10. Handling Exceptions
 So far, we have let the system handle exceptions
int score = in.nextInt();
If the user enters “abc123”
Exception in thread "main"
java.util.InputMismatchException
at java.util.Scanner.throwFor(Scanner.java:819)
at java.util.Scanner.next(Scanner.java:1431)
at java.util.Scanner.nextInt(Scanner.java:2040)
at java.util.Scanner.nextInt(Scanner.java:2000)
at Test.main(Test.java:5)

11. Handling Exceptions
 Says in English:
 System has caught an error described as a
InputMismatchException
 Thrown because a String cannot be converted to an
integer
 When system handles, we often get a program crash
 Instead of the system, we can handle to improve
robustness

12. Handling Exceptions
 Better solution: Instead of letting the program
crash, use Exception Handling to improve
program’s robustness
 Exceptions cannot be ignored
 Exceptions are handled by Exception Handlers
directly

13. Throwing Exceptions
 If there is an error in the value of a parameter,
we can throw exceptions to make the user
accountable
1. Decide what type of exception to throw
2. Test for condition, and throw the exception if
condition is violated

14. Example
 Throw an exception object to signal an
exceptional condition
 Example: What do we do if the amount to
withdraw is greater than the balance?
 IllegalArgumentException: illegal parameter value

15. Problem
public class BankAccount
{
public void withdraw(double amount)
{
if (amount > balance)
{
?????????
}
balance = balance - amount;
}
. . .
}

16. Solution #1
public class BankAccount
{
public void withdraw(double amount)
{
if (amount > balance)
{
IllegalArgumentException exception
= new IllegalArgumentException("Amount
exceeds balance");
throw exception;
}
balance = balance - amount;
}
. . .
}

17. Solution #2
public class BankAccount
{
public void withdraw(double amount)
{
if (amount > balance)
{
throw new IllegalArgumentException("Amount
exceeds balance");
}
balance = balance - amount;
}
. . .
}

18. Throwing Exceptions: Syntax
throw exceptionObject;

19. Checked/Unchecked Exceptions
 Checked Exception – checked at compile time
 Complier ensures that you are handling a possible
problem
 These errors are due to external circumstances that
the programmer cannot prevent
 Majority occur when dealing with input and output
 For example, IOException

20. Checked/Unchecked Exceptions
 Unchecked Exception = Runtime Exceptions
 Extend the class RuntimeException or Error
 They are the programmer's fault
 Examples of runtime exceptions:
 NumberFormatException
 IllegalArgumentException
 NullPointerException
 Optional to deal with these
 Example of error: OutOfMemoryError
 Can’t do anything about these catastrophic
problems, so don’t deal with it

22. Checked/Unchecked Exceptions
 Unchecked Exceptions result from deficiencies
in your code, so should check on own
 Null object reference
 Sending a negative value to Math.sqrt()
 Checked Exceptions are not the fault of the
coder
 Problems with the file format, user input, etc.

23. Checked/Unchecked Exceptions
 Categories aren't perfect:
 Scanner.nextInt throws unchecked
InputMismatchException
 Programmer cannot prevent users from entering
incorrect input
 This choice makes the class easy to use for beginning
programmers

24. Checked/Unchecked Exceptions
 Deal with checked exceptions principally when
programming with files and streams
 For example, use a Scanner to read a file
String filename = . . .;
FileReader reader = new FileReader(filename);
Scanner in = new Scanner(reader);
 But, FileReader constructor can throw a
FileNotFoundException

25. Handling Checked Exceptions
1. Handle the exception
2. Tell compiler that you want method to be
terminated when the exception occurs
 Use throws specifier so method can throw a
checked exception
public void read(String filename) throws
FileNotFoundException
{
FileReader reader = new
FileReader(filename);
Scanner in = new Scanner(reader);
. . .
}

26. Handling Checked Exceptions
 throws tells the compiler to “pass the buck” to the
method that called this method
 Can propagate multiple exceptions:
public void read(String filename) throws IOException,
ClassNotFoundException
 Can also group using hierarchy
 If method can throw an IOException and
FileNotFoundException, only use IOException

27. Handling Checked Exceptions
 Why propagate when we could handle the error
ourselves?
 We may not know how to
 Let user of my code decide
 Better to declare exception than to handle it
incompetently

28. Catching Exceptions
 At some point, an exception should be dealt
with
 If not, program terminates with error message
 Professional code requires more sophistication –
cannot just allow errors to kill program
 What would happen if all of my.wisc.edu turned off
if you entered wrong password?

29. Catching Exceptions
 To deal with this problem, install exception
handlers in your code to deal with possible
exceptions
 Handlers are try/catch statements

30. Try-Catch
 Put statement(s) that could cause an error in the
try block
 Error handling code goes in catch block
 Only is executed if there was an error
 Can have multiple catch blocks, one for each
possible type of exception

31. try-catch Syntax
try
{
<try block>
}
catch ( <ExceptionClass> <name> )
{
<catch block>
}
catch ( <ExceptionClass> <name> )
{
<catch block>
}…

32. try
{
String filename = . . .;
FileReader reader = new FileReader(filename);
Scanner in = new Scanner(reader);
String input = in.next();
int value = Integer.parseInt(input);
. . .
}
catch (IOException exception)
{
exception.printStackTrace();
}
catch (NumberFormatException exception)
{
System.out.println("Input was not a number");
}

33. Example
 3 types of error can be thrown
 FileNotFoundException is thrown by
FileReader constructor  caught by
IOException clause
 NoSuchElementException is thrown by
Scanner.next  not caught, thrown to caller
 NumberFormatException is thrown by
Integer.parseInt()  caught by second clause

34. Catching Exceptions
 If there are no errors, the catch block is skipped
 If an exception is thrown, the try block stops
executing immediately and jumps to the catch
block

35. Catching Exceptions

36. Exception Information
 Why do we have Exception objects?
 We can get information on where the error happened
and what exactly happened
 Exception objects have 2 methods defined:
 getMessage() (what happened?)
 printStackTrace() (where did it happen?)

37. getMessage
 Returns the data that cause the error
 Example:
…
}catch(NumberFormatException e){
System.out.println(e.getMessage());
}

38. printStackTrace
 Prints out a trace of methods that caused the
error starting at the root of the error
 Where did the exception occur? What method called
this code to execute..etc.
 This is what the system does when an exception is
thrown
 Example:
…
}catch(NumberFormatException e){
e.printStackTrace();
}

39. Catching ALL Exceptions
public double average( String data )
{
try
{
int sum = 0;
for ( int i=0; i < data.length(); i++ )
sum += Integer.parseInt(data.charAt(i));
return sum/data.length();
}
catch ( Exception e ) // catch ALL exceptions
{
System.out.println( e );
return 0;
}
}

40. Catching ALL Exceptions
 Why is this probably a bad idea?
 Probably want to handle each type of exception
differently
 Don't want to catch things like
NullPointerExeption

41. Catching Exceptions
 IMPORTANT! Order of catch blocks matters
} catch (Exception e){
System.out.println(e.getMessage());
} catch (NumberFormatException e){
System.out.println("'" + str + "'not valid
input, Please use digits only");
}
 You should go specific to generic
 NumberFormatException is a specific type of the class
Exception (inheritance)

42. Demo
p.s.v. main( String [] args )
{
try {
String filename = args[0], input;
scanner = new Scanner( new File(filename) );
input = scanner.nextLine();
int value = Integer.parseInt( input );
double reciprocal = 1.0 / value;
System.out.println( "rec=" + rec );
}
catch ( Exception e )
{
S.o.pln( "Found Exception: " + e );
}
S.o.pln( "Done" );
}

43. String filename=args[0], input;
try {
scanner = new Scanner(new File(filename));
input = scanner.nextLine();
int value = Integer.parseInt( input );
double reciprocal = 1.0 / value;
System.out.println( "rec=" + rec );
}
catch ( NumberFormatException e ) {
S.o.pln( input + " is not an integer");
}
catch ( ArithmeticException e ) {
S.o.pln( "Can't divide by zero" );
}
catch ( FileNotFoundException e ) {
S.o.pln( filename + " Not Found" );
}
finally { S.o.pln( "Done" ); }

44. Catching Exceptions
 What if an exception is thrown and there is no
catch block to match?
 The system handles it (terminates program and
prints the stack trace)

45. The finally clause
 Recall that whenever an exception occurs, we
immediately go to the catch block
 What if there is some code we want to execute
regardless of whether there was an exception?
 finally block is used

46. The finally clause
try{
distance = Double.parseDouble(str);
if (distance < 0){
throw new Exception("Negative distance is not
valid");
}
return distance;
} catch (NumberFormatException e){
System.out.println("'" + str + "'not valid
input, Please use digits only");
} catch (Exception e){
System.out.println(e.getMessage());
} finally {
System.out.println(“Done”);
}

47. The finally clause
 finally is executed NO MATTER WHAT
 Even if there is a break or return in the try block
 Good for “cleanup” of a method

49. Throwable Exception Inheritance Hierarchy
Exception
RuntimeException
NullPointerException
ArithmeticException
NegativeArrayIndexException
IndexOutOfBoundsException
ArrayIndexOutOfBoundsException
StringIndexOutOfBoundsException
IllegalArgumentException
NumberFormatException
IOException
FileNotFoundException
EOFException

50. try-catch Control Flow
code before try
try block
no exceptions occur
code after try

51. try-catch Control Flow
code before try
exception occurs
try block catch block
code after try

52. try-catch Control Flow
code before try
exception occurs
try block catch block
no exceptions occur
code after try

53. try-catch Control Flow
code before try
try block
no exceptions
occurred
finally block (if it exists)
code after try

54. try-catch Control Flow
code before try
exception occurs
try block catch block
finally block (if it exists)
code after try

55. try-catch Control Flow
exception occurs
code before try
true matches first
try block catch block?
no exceptions
occurred 1st catch block false
true matches next
2nd catch block catch block?
false
finally block (if it exists)
code after try

56. Propagating Exceptions
 When a method may throw an exception, either
directly or indirectly, we call the method an
exception thrower.
 Every exception thrower must be one of two
types:
 a catcher, or
 a propagator.

57. Propagating Exceptions
 An exception catcher is an exception thrower that
includes a matching catch block for the thrown
exception.
 An exception propagator does not contain a
matching catch block.
 A method may be a catcher of one exception
and a propagator of another.

58. Example
 The following figure shows a sequence of method calls
among the exception throwers.
 Method D throws an instance of Exception. The green
arrows indicate the direction of calls. The red arrows
show the reversing of call sequence, looking for a
matching catcher. Method B is the catcher.
 The call sequence is traced by using a stack.

59. Example

60. Propagating Exceptions
 If a method is an exception propagator, we need to
modify its header to declare the type of exceptions the
method propagates.
 We use the reserved word throws for this declaration.
public void C( ) throws Exception {
...
}
public void D( ) throws Exception {
...
}

61. Propagating Exceptions
 Without the required throws Exception clause,
the program will not compile.
 However, for exceptions of the type called
runtime exceptions, the throws clause is optional.

62. Propagating vs. Catching
 Do not catch an exception that is thrown as a
result of violating a condition set by the client
programmer.
 Instead, propagate the exception back to the
client programmer’s code and let him or her
handle it.

63. Exceptions in a Constructor
 We can thrown an exception in the constructor
if a condition is violated
 For example, we may throw an
IllegalArgumentException if a parameter value
is invalid