Exception Handling & Event Handling

 

Q. Define exception, exception handler, and built-in exception.  

Exception: An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions. Any unusual event, erroneous or not, that is detectable by either hardware or software and that may require special processing is called exception.

Exception Handler: An exception handler processes a raised exception. The exception can be either predefined or user-defined. Predefined exceptions are raised implicitly (automatically) by the run-time system.  

Built-in Exception: Built-in exceptions are the exceptions which are available in libraries. These exceptions are suitable to explain certain error situations. Below is the list of important built-in exceptions in Java.

1. Arithmetic Exception
2. ArrayIndexOutOfBoundException
3. ClassNotFoundException
4. FileNotFoundException
5. IOException.

Q. What are the advantages of having support for exception handling built-in to a language?

Without built-in exception handling, the code to detect error condition can be a clutter to the program. The existence of built-in exception handling would simplify a source program.

A language that supports exception handling encourages its users to consider all of the events that could occur during program execution and how they can be handled. This approach is far better than not considering such possibilities and simply hoping nothing will go wrong.

Exception handling encourages programmers to consider many different possible errors.

Exception propagation allows a high level of reuse of exception handling code.

Q. What are the design issues of exception handling? 

The goals of exception handling mechanisms are to make programs more reliable and robust.

1. How are user-defined exceptions specified?
2. Should there be default exception handlers for programs that do not provide their own?
3. Can built-in exceptions be explicitly raised?
4. Are hardware-detectable errors treated as exceptions that can be handled?
5. Are there any built-in exceptions?
6. How can exceptions be disabled?
7. How and where are exception handlers specified and what is their scope?
8. How is an exception occurrence bound to an exception handler?
9. Can information about the exception be passed to the handler?
10. Where does execution continue,  after an exception handler completes its execution?
11. Is some form of finalization provided?

Q. What does it mean for an exception to be bound to an exception handler?

A specific exception is to be handled by a specific exception handler also, because different exceptions are to be treated differently.

Q. In a language without exception-handling facilities, we could send an error-handling procedure as a parameter to each procedure that can detect errors that must be handled. What disadvantages are there to this method?

There are several disadvantages of sending error handling subprograms to other subprograms.  One is that it may be necessary to send several error handlers to some subprograms, greatly complicating both the writing and execution of calls.

Another is that there is no method of propagating exceptions, meaning that they must all be handled locally.  This complicates exception handling, because it requires more attention to handling in more places.

Q. Give an example of hardware-detectable execution.

Division by zero.

Q. What is the difference between checked and unchecked exceptions in Java?

Read Ravi Raja Reddy's answer to What is difference between a checked and unchecked exception? on Quora

Q. Why do we use finally block?

• What happens if an exception you're not handling gets thrown? (I hope you're not catching Throwable...)
• What happens if you return from inside the try block?
• What happens if the catch block throws an exception?

A finally block makes sure that however you exit that block, it will get executed. That's important for deterministic cleanup of resources.