Questions on Exceptional Handling

Questions on  Exceptional Handling :

Q: What are the different ways to handle exceptions?

A:  There are two ways to handle exceptions,

1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and

2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.

 

Q: If I write return at the end of the try block, will the finally block still execute?

A: Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.

Q: How to create custom exceptions?

A: Your class should extend class Exception, or some more specific type thereof.

 

Q: If I want an object of my class to be thrown as an exception object, what should I do?

A: The class should extend from Exception class. Or you can extend your class from some more precise exception type also.

 

 Q: If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?

A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.

 

 

Q:How does an exception permeate through the code?

A:An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.

 

Q: What is the basic difference between the 2 approaches to exception handling.

1  try catch block and

2  specifying the candidate exceptions in the throws clause

 

When should you use which approach?

A:      In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

 

 

Q: Is it necessary that each try block must be followed by a catch block?

A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

 

 

Q:What are Checked and UnChecked Exception?

A:  A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

Q:     What are runtime exceptions?

A: Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

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