2. Objectives
On completion of this period, you would be able to
know:
• Exception handling mechanism in Java
• How to deal with exceptions
2
3. Recap
• In the previous class, you have studied about the
features and advantages of exceptions
• Advantages are
• Exception is a built-in class in Java
• Separation of logic of program for error handling
section
• Classification of exceptions possible
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4. Exception Handling Mechanism In Java
• As soon as on exception occurs, the Java runtime
generates an exception object of the required type
• This is known as creating and throwing exception
• It then checks the method, where the exception is
generated, for any code to handle the exception
• This method is known as exception handling code
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5. Exception Handling Mechanism In Java
Contd . . .
• Then, the runtime system checks for each block in
that method for catch block
• Catch block is known as the exception handler
• If there is no catch block
• The system checks for catch block, in the calling method of
the current method
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6. Exception Handling Mechanism In Java
Contd . . .
• The system continuously checks nesting methods
until main method in checked
• If the current method is main method, then the
exception message is shown on the error output
• If there is a catch block in any method, the exception
will be handled
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7. What The Java Run Time Does When An
Exception Occurs ? Contd . . .
• The method call sequence is Method3 without an
exception handler
shown
• The call sequence starts from Method2 without an Method call
the main method exception handler
• main calls Method1
Method1 with an Method call
• Method1 calls Method2
exception handler
• Method2 calls Method3
Method call
main
Fig. 43.1 Call stack
8. Exception Handling Mechanism In Java
Contd . . .
• Here, in Method3 Throws Method3 without an
exception occurs exception exception handler
Looking for
• Method3 does not appropriate
contain the exception handler
Forwards Method2 without an
handler
exception exception handler
• Method3 is called by
method2 which has no Looking for
exception handler Catches Method1 with an appropriate
some exception handler handler
• Method2 is called by exception
method1
• Method1 has exception
main
handler
• In Method1 exception is
handled
Fig. 43.2 Searching the call stack for
exception handler
9. How To Deal With Exceptions
• Java exception handling is managed via five
keywords
• try
• catch
• throw
• throws
• finally
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10. How To Deal With Exceptions Contd...
• Program statements that you want to monitor for
exceptions are put within a try block
• If an exception occurs within the try block, it is
thrown
• Your code can catch this exception using catch
and handle it in some rational manner
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11. How To Deal With Exceptions Contd..
• System-generated exceptions are automatically
thrown by the Java run-time system
• To manually throw an exception, use the keyword
throw
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12. How To Deal With Exceptions Contd..
• Any exception that is thrown out of a method must
be specified as such by a throws clause
• Any code that absolutely must be executed before a
method returns is put in a finally block
• finally block is useful – for writing clean up code
e.g :
To close an opened for file
To close database connection
To re-initialize variable. etc
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13. Format Of An Exception-Handling Block
try {
// block of code to monitor for errors
}
catch (ExceptionType1 exOb) {
// exception handler for ExceptionType1
}
catch (ExceptionType2 exOb) {
// exception handler for ExceptionType2
}
finally {
// block of code to be executed before try block ends
}
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14. Discussion
• How many try blocks can be there in a method?
• It depends on the type of exceptions we fore-
see
• How many catch blocks can be there for a try
block?
• Any number of catch blocks
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15. Summary
• In this class we have discussed
• Exception handling mechanism in java
• The keywords necessary for handling
exceptions
• try, catch, throw, throws and finally
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16. Quiz
1. Program statements that you want to monitor for
exceptions are put within a try block
A. True
B. False
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